Analysis of the X Protocol for Security Concerns

David P. Wiggins

X Consortium

X Version 11, Release 7.7

Draft Version 2

Copyright © 1996 X Consortium

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May 10, 1996

This paper attempts to list all instances of certain types of security problems
in the X Protocol. Issues with authorization are not addressed. We assume that
a malicious client has already succeeded in connecting, and try to assess what
harm it can then do. We propose modifications to the semantics of the X
Protocol to reduce these risks.

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Table of Contents

1. Definition of Threats
2. General security concerns and remedies
    Access to Server Resources
    Denial of Service
        Memory Exhaustion
        CPU Monopolization
3. Security concerns with specific window attributes
    Background-pixmap
    ParentRelative and CopyFromParent
    Override-redirect
4. Security concerns with specific requests
    CreateWindow
    ChangeWindowAttributes
    GetWindowAttributes
    DestroyWindow, DestroySubwindows
    ChangeSaveSet
    MapWindow
    Window Operations
    GetGeometry
    QueryTree
    InternAtom
    GetAtomName
    ChangeProperty
    DeleteProperty
    GetProperty
    RotateProperties
    ListProperties
    SetSelectionOwner
    GetSelectionOwner
    ConvertSelection
    SendEvent
    Keyboard and Pointer Grabs
    ChangeActivePointerGrab
    GrabServer
    QueryPointer
    GetMotionEvents
    TranslateCoordinates
    WarpPointer
    SetInputFocus
    GetInputFocus
    QueryKeymap
    Font Requests
    CloseFont
    SetFontPath
    GetFontPath
    CreatePixmap
    FreePixmap
    CreateGC
    CopyGC
    ChangeGC, SetDashes, SetClipRectangles
    FreeGC
    Drawing Requests
    GetImage
    CreateColormap
    FreeColormap
    CopyColormapAndFree
    InstallColormap, UninstallColormap
    ListInstalledColormaps
    Color Allocation Requests
    FreeColors
    StoreColors, StoreNamedColor
    QueryColors, LookupColor
    CreateCursor, CreateGlyphCursor
    FreeCursor
    RecolorCursor
    QueryBestSize
    ListExtensions, QueryExtension
    Keyboard configuration requests
    Keyboard query requests
    ChangePointerControl, SetPointerMapping
    GetPointerControl, GetPointerMapping
    SetScreenSaver
    GetScreenSaver
    ForceScreenSaver
    ChangeHost
    ListHosts
    SetAccessControl
    SetCloseDownMode
    KillClient
    Clean Requests
5. Events
    KeymapNotify
    Expose
    GraphicsExposure
    VisibilityNotify
    ReparentNotify
    ConfigureNotify
    ConfigureRequest
    SelectionClear
    SelectionRequest
    MappingNotify
6. Errors
7. Future Work
8. References

Chapter 1. Definition of Threats

We analyze the X protocol for the following threats.

            occurs when a client gains access to information owned by another
            client without explicit permission from that other client. For this
            analysis, we take a broad view of ownership: any information that
            exists in the server due to the actions of a client is considered
            owned by that client. Furthermore, the client that has input focus
Theft       owns keyboard events, and the client that owns the window that the
            pointer is in owns mouse events. This view may reveal certain
            instances of "theft" that we don't care to stop, but we think it is
            better to identify all potential candidates up front and cull the
            list later than to do a partial analysis now and plan on
            reanalyzing for remaining holes later.

Denial of   occurs when a client causes another client or the user to lose the
service     ability to perform some operation.

            occurs when a client attempts to mimic another client with the hope
Spoofing    that the user will interact with it as if it really were the
            mimicked client. A wide variety of requests may be used in a
            spoofing attack; we will only point out a few likely candidates.

            occurs when a client causes another client to lose information in a
Destruction way that the client or user is likely to notice. (This does not
            count expected forms of destruction, e.g., exposures.)

            occurs when a client causes another client to lose information in a
Alteration  way that the client or user is unlikely to notice. e.g., changing
            one pixel in a drawable.

The line between alteration and destruction is subjective. Security literature
does often distinguish between them, though not always explicitly. Alteration
is often considered more insidious because its effects may not be realized
until long after it has occurred. In the intervening time, each time the
altered data is used, it can cause more damage.

Chapter 2. General security concerns and remedies

Table of Contents

Access to Server Resources
Denial of Service
    Memory Exhaustion
    CPU Monopolization

The following sections discuss security problems intrinsic to the X Protocol. A
statement of each problem is usually followed by potential remedies. A few
words here about possible remedies will help frame the specific ones described
below.

If a client attempts a threatening operation, the server may take one of the
following actions, listed roughly in order of severity:

 1. Execute the request normally. This is the right choice when we decide that
    a particular threat is not serious enough to worry about.

 2. Execute the request in some modified form, e.g., substitute different
    values for some of the request fields, or edit the reply.

 3. Arrange to ask the user what to do, given some subset of the other choices
    in this list. This must be used sparingly because of the performance
    impact.

 4. Treat the request as a no-op. If the client will probably not notice, or if
    it seems likely that the intent was benign, this is a good choice.

 5. Send a protocol error to the client. If the client will be confused enough
    by the other options that it will probably crash or become useless anyway,
    or if it seems likely that the intent was malicious, this is a good choice.

    Kill the client. This might be the right action if there is no doubt that
    the client is hostile.

In most cases we present the one option that seems most appropriate to counter
the threat, taking into account the seriousness of the threat, the
implementation difficulty, and the impact on applications. Our initial bias is
to err on the side of stronger security, with the accompanying tighter
restrictions. As we uncover important operations and applications that the new
restrictions interfere with, we can apply selective loosening to allow the
desired functionality.

In some cases we will suggest returning an Access error where the X protocol
does not explicitly allow one. These new Access errors arise when a client can
only perform a (non-empty) subset of the defined operations on a resource. The
disallowed operations cause Access errors. The resource at issue is usually a
root window.

Access to Server Resources

The X protocol allows clients to manipulate resources (objects) belonging to
other clients or to the server. Any request that specifies a resource ID is
vulnerable to some of the above threats. Such requests also provide a way for a
client to guess resource IDs of other clients. A client can make educated
guesses for possible resource IDs, and if the request succeeds, it knows it has
determined a valid resource ID. We call this "resource ID guessing" in the list
below.

One likely defense against these problems is to have the server send an
appropriate protocol error to deny the existence of any resource specified by a
client that doesn't belong to that client. A variation on this policy lets
cooperating groups of clients access each other's resources, but not those of
other groups. The Broadway project will initially use a less general form of
this idea by having two groups, trusted and untrusted. Trusted clients can do
everything that X clients can do today. They will be protected from untrusted
clients in ways described below. Untrusted clients will not be protected from
each other. Though this will be the initial design, we need to make sure there
is a growth path to multiple (more than two) groups.

Most of the time, applications never access server resources that aren't their
own, so the impact of disallowing such accesses should be minimal. There are a
few notable exceptions, most of which will be discussed under the relevant
protocol requests. They are: ICCCM selection transfer, Motif drag and drop, and
server-global resources like the root window and default colormap. Another
major exception is the window manager, which routinely manipulates windows of
other applications. The solution for window managers is to always run them as
trusted applications.

The implementation difficulty of limiting access to resources should not be
large. All resource accesses eventually funnel down to one of two functions in
<dix/resource.c>: LookupIDByType and LookupIDByClass. A few lines of checking
at the top of these functions will form the heart of this defense. There is a
small problem because these functions are not told which client is doing the
lookup, but that can be solved either by adding a client parameter (probably as
a new function to preserve compatibility), or by using the server global
requestingClient.

Note

ISSUE: are we really going to be able to get away with hiding trusted
resources, or will things like Motif drag and drop force us to expose them?
(Either way, the operations that untrusted clients can do to trusted resources
will have to be limited.) Is there something in Xt or the ICCCM that breaks if
you hide resources?

Denial of Service

Memory Exhaustion

Any request that causes the server to consume resources (particularly memory)
can be used in a denial of service attack. A client can use such requests
repeatedly until the server runs out of memory. When that happens, the server
will either crash or be forced to send Alloc errors. The most obvious
candidates are resource creation requests, e.g., CreatePixmap, but in reality a
large percentage of requests cause memory allocation, if only temporarily,
depending on the server implementation. For this reason, the list of requests
subject to this form of denial of service will be necessarily incomplete.

To address this form of denial of service, the server could set per-client
quotas on memory consumption. When the limit is surpassed, the server could
return Alloc errors. The application impact is minimal as long as the
application stays within quota. The implementation difficulty is another story.

Conceptually, it seems easy: simply have a way to set the limit, and on every
memory (de)allocation operation, update the client's current usage, and return
an error if the client is over the limit. The first problem is something we've
already touched on: the allocator functions aren't told which client the
allocation belongs to. Unlike resource lookups, allocations are done in too
many places to consider a new interface that passes the client, so using the
global requestingClient is practically mandatory.

The problems run deeper. The logical thing for the allocator to do if the
client is over its limit is to return NULL, indicating allocation failure.
Unfortunately, there are many places in the server that will react badly if
this happens. Most of these places, but not all, are "protected" by setting the
global variable Must_have_memory to True around the delicate code. We could
help the problem by skipping the limit check if Must_have_memory is True. The
best solution would be to bullet-proof the server against allocation failures,
but that is beyond the scope of Broadway. Another consideration is that the
additional checking may have a measurable performance impact, since the server
does frequent allocations.

A third problem is that there is no portable way to determine the size of a
chunk of allocated memory given just a pointer to the chunk, and that's all you
have inside Xrealloc and Xfree. The server could compensate by recording the
sizes itself somewhere, but that would be wasteful of memory, since the malloc
implementation also must be recording block sizes. On top of that, the
redundant bookkeeping would hurt performance. One solution is to use a custom
malloc that has the needed support, but that too seems beyond the scope of
Broadway.

Considering all of this, we think it is advisable to defer solving the memory
exhaustion problem to a future release. Keep this in mind when you see quotas
mentioned as a defense in the list below.

CPU Monopolization

Another general way that a client can cause denial of service is to flood the
server with requests. The server will spend a large percentage of its time
servicing those requests, possibly starving other clients and certainly hurting
performance. Every request can be used for flooding, so we will not bother to
list flooding on every request. A variation on this attack is to flood the
server with new connection attempts.

To reduce the effectiveness of flooding, the server could use a different
scheduling algorithm that throttles clients that are monopolizing the server,
or it could simply favor trusted clients over untrusted ones. Applications
cannot depend on a particular scheduling algorithm anyway, so changing it
should not affect them. The Synchronization extension specifies a way to set
client priorities, and a simple priority scheduler already exists in the server
to support it, so this should be simple to add.

Chapter 3. Security concerns with specific window attributes

Table of Contents

Background-pixmap
ParentRelative and CopyFromParent
Override-redirect

Background-pixmap

Clients can use windows with the background-pixmap attribute set to None
(hereafter "background none windows") to obtain images of other windows. A
background none window never paints its own background, so whatever happened to
be on the screen when the window was mapped can be read from the background
none window with GetImage. This may well contain data from other windows. The
CreateWindow and ChangeWindowAttributes requests can set the background-pixmap
attribute set to None, and many window operations can cause data from other
windows to be left in a background none window, including ReparentWindow,
MapWindow, MapSubwindows, ConfigureWindow, and CirculateWindow.

Background none windows can also be used to cause apparent alteration. A client
can create a window with background none and draw to it. The drawing will
appear to the user to be in the windows below the background none window.

To remedy these problems, the server could substitute a well-defined background
when a client specifies None. Ideally the substituted background would look
different enough from other windows that the user wouldn't be confused. A tile
depicting some appropriate international symbol might be reasonable. We believe
that there are few applications that actually rely on background none
semantics, and those that do will be easy for the user to identify because of
the distinctive tile. Implementation should not be a problem either. Luckily,
the window background cannot be retrieved through the X protocol, so we won't
have to maintain any illusions about its value.

Note

ISSUE: Some vendors have extensions to let you query the window background. Do
we need to accommodate that?

Note

ISSUE: Will this lead to unacceptable application breakage? Could the server be
smarter, only painting with the well-defined background when the window
actually contains bits from trusted windows?

ParentRelative and CopyFromParent

Several window attributes can take on special values that cause them to
reference (ParentRelative) or copy (CopyFromParent) the same attribute from the
window's parent. This fits our definition of theft. The window attributes are
class, background-pixmap, border-pixmap, and colormap. All of these can be set
with CreateWindow; all but class can be set with ChangeWindowAttributes.

These forms of theft aren't particularly serious, so sending an error doesn't
seem appropriate. Substitution of different attribute values seems to be the
only reasonable option, and even that is likely to cause trouble for clients.
Untrusted clients are already going to be prevented from creating windows that
are children of trusted clients (see CreateWindow below). We recommend that
nothing more be done to counter this threat.

Override-redirect

Windows with the override-redirect bit set to True are generally ignored by the
window manager. A client can map an override-redirect window that covers most
or all of the screen, causing denial of service since other applications won't
be visible.

To prevent this, the server could prevent more than a certain percentage
(configurable) the of screen area from being covered by override-redirect
windows of untrusted clients.

Override-redirect windows also make some spoofing attacks easier since the
client can more carefully control the presentation of the window to mimic
another client. Defenses against spoofing will be given under MapWindow .

Chapter 4. Security concerns with specific requests

Table of Contents

CreateWindow
ChangeWindowAttributes
GetWindowAttributes
DestroyWindow, DestroySubwindows
ChangeSaveSet
MapWindow
Window Operations
GetGeometry
QueryTree
InternAtom
GetAtomName
ChangeProperty
DeleteProperty
GetProperty
RotateProperties
ListProperties
SetSelectionOwner
GetSelectionOwner
ConvertSelection
SendEvent
Keyboard and Pointer Grabs
ChangeActivePointerGrab
GrabServer
QueryPointer
GetMotionEvents
TranslateCoordinates
WarpPointer
SetInputFocus
GetInputFocus
QueryKeymap
Font Requests
CloseFont
SetFontPath
GetFontPath
CreatePixmap
FreePixmap
CreateGC
CopyGC
ChangeGC, SetDashes, SetClipRectangles
FreeGC
Drawing Requests
GetImage
CreateColormap
FreeColormap
CopyColormapAndFree
InstallColormap, UninstallColormap
ListInstalledColormaps
Color Allocation Requests
FreeColors
StoreColors, StoreNamedColor
QueryColors, LookupColor
CreateCursor, CreateGlyphCursor
FreeCursor
RecolorCursor
QueryBestSize
ListExtensions, QueryExtension
Keyboard configuration requests
Keyboard query requests
ChangePointerControl, SetPointerMapping
GetPointerControl, GetPointerMapping
SetScreenSaver
GetScreenSaver
ForceScreenSaver
ChangeHost
ListHosts
SetAccessControl
SetCloseDownMode
KillClient
Clean Requests

To reduce the space needed to discuss 120 requests, most of the following
sections use a stylized format. A threat is given, followed by an imperative
statement. The implied subject is an untrusted client, and the object is
usually a trusted client. Following that, another statement starting with
"Defense:" recommends a countermeasure for the preceding threat(s).

Resources owned by the server, such as the root window and the default
colormap, are considered to be owned by a trusted client.

CreateWindow

Alteration: create a window as a child of another client's window, altering its
list of children.

Defense: send Window error. Specifying the root window as the parent will have
to be allowed, though.

Theft: create an InputOnly window or a window with background none on top of
other clients' windows, select for keyboard/mouse input on that window, and
steal the input. The input can be resent using SendEvent or an input synthesis
extension so that the snooped application continues to function, though this
won't work convincingly with the background none case because the drawing will
be clipped.

Defense: send an error if a top-level InputOnly window is created (or
reparented to the root). Countermeasures for background none and SendEvent are
discussed elsewhere.

Note

ISSUE: The Motif drag and drop protocol creates and maps such a window (at
-100, -100, size 10x10) to "cache frequently needed data on window properties
to reduce roundtrip server requests." Proposed solution: we could only send an
error if the window is visible, which would require checking in, MapWindow,
ConfigureWindow, and ReparentWindow.

Theft: resource ID guessing (parent, background-pixmap, border-pixmap,
colormap, and cursor).

Defense: send Window, Pixmap, Colormap, or Cursor error.

Denial of service: create windows until the server runs out of memory.

Defense: quotas.

Also Security concerns with specific window attributes

ChangeWindowAttributes

Alteration: change the attributes of another client's window.

Theft: select for events on another client's window.

Defense for both of the above: send Window error.

Note

ISSUE: The Motif drop protocol states that "the initiator should select for
DestroyNotify on the destination window such that it is aware of a potential
receiver crash." This will be a problem if the initiator is an untrusted window
and the destination is trusted. Can the server, perhaps with the help of the
security manager, recognize that a drop is in progress and allow the
DestroyNotify event selection in this limited case?

Note

ISSUE: The Motif pre-register drag protocol probably requires the initiator to
select for Enter/LeaveNotify on all top-level windows. Same problem as the
previous issue.

Theft: resource ID guessing (background-pixmap, border-pixmap, colormap, and
cursor).

Defense: send Pixmap, Colormap, or Cursor error.

Also Security concerns with specific window attributes

GetWindowAttributes

Theft: get the attributes of another client's window.

Theft: resource ID guessing (window).

Defense for both of the above: send Window error.

DestroyWindow, DestroySubwindows

Destruction: destroy another client's window.

Theft: resource ID guessing (window).

Defense for both of the above: send Window error.

ChangeSaveSet

Alteration: cause another client's windows to be reparented to the root when
this client disconnects (only if the other client's windows are subwindows of
this client's windows).

Defense: process the request normally. The trusted client gives away some of
its protection by creating a subwindow of an untrusted window.

Theft: resource ID guessing (window).

Defense: send Window error.

MapWindow

Spoofing: map a window that is designed to resemble a window of another client.
Additional requests will probably be needed to complete the illusion.

Defense:

We consider spoofing to be a significant danger only if the user is convinced
to interact with the spoof window. The defense centers on providing enough
information to enable the user to know where keyboard, mouse, and extension
device input is going. To accomplish this, the server will cooperate with the
security manager, an external process. The server will provide the following
facilities to the security manager:

1. A way to create a single window that is unobscurable by any window of any
other client, trusted or untrusted. It needs to be unobscurable so that it is
spoof-proof.

Note

ISSUE: is a weaker form of unobscurability better? Should the window be
obscurable by trusted windows, for example?

Note

ISSUE: does unobscurable mean that it is a child of the root that is always on
top in the stacking order?

2. A way to determine if a given window ID belongs to an untrusted client.

The security manager will need to select for the existing events FocusIn,
FocusOut, EnterNotify, LeaveNotify, DeviceFocusIn, and DeviceFocusOut on all
windows to track what window(s) the user's input is going to. Using the above
server facilities, it can reliably display the trusted/untrusted status of all
clients currently receiving input.

Note

ISSUE: is it too much to ask the security manager to select for all these
events on every window? Do we need to provide new events that you select for
*on the device* that tell where the device is focused?

None of this should have any application impact.

The unobscurable window may be tricky to implement. There is already some
machinery in the server to make an unobscurable window for the screen saver,
which may help but may also get in the way now that we have to deal with two
unobscurable windows.

Window Operations

Specifically, ReparentWindow, MapWindow, MapSubwindows, UnmapWindow,
UnmapSubwindows, ConfigureWindow, and CirculateWindow.

Alteration: manipulate another client's window.

Theft: resource ID guessing (window, sibling).

Defense for both of the above: send a Window error unless it is a root window,
in which case we should send an Access error.

GetGeometry

Theft: get the geometry of another client's drawable.

Theft: resource ID guessing (drawable).

Defense for both of the above: send Drawable error. However, root windows will
be allowed.

QueryTree

Theft: resource ID guessing (window).

Defense: send Window error.

Theft: discover window IDs that belong to other clients.

Defense: For the child windows, censor the reply by removing window IDs that
belong to trusted clients. Allow the root window to be returned. For the parent
window, if it belongs to a trusted client, return the closest ancestor window
that belongs to an untrusted client, or if such a window does not exist, return
the root window for the parent window.

Note

ISSUE: will some applications be confused if we filter out the window manager
frame window(s), or other windows between the queried window and the root
window?

Note

ISSUE: the Motif drag protocol (both preregister and dynamic) needs to be able
to locate other top-level windows for potential drop sites. See also Access to
Server Resources .

InternAtom

Theft: discover atom values of atoms interned by other clients. This lets you
determine if a specific set of atoms has been interned, which may lead to other
inferences.

Defense: This is a minor form of theft. Blocking it will interfere with many
types of inter-client communication. We propose to do nothing about this
threat.

Denial of service: intern atoms until the server runs out of memory.

Defense: quotas.

GetAtomName

Theft: discover atom names of atoms interned by other clients. This lets you
determine if a specific set of atoms has been interned, which may lead to other
inferences.

Defense: This is a minor form of theft. We propose to do nothing about this
threat.

ChangeProperty

Alteration: change a property on another client's window or one that was stored
by another client.

Theft: resource ID guessing (window).

Defense for both of the above: send Window error.

Note

ISSUE: Selection transfer requires the selection owner to change a property on
the requestor's window. Does the security manager get us out of this? Does the
server notice the property name and window passed in ConvertSelection and
temporarily allow that window property to be written?

Note

ISSUE: should certain root window properties be writable?

Denial of service: store additional property data until the server runs out of
memory.

Defense: quotas.

DeleteProperty

Destruction: delete a property stored by another client.

Theft: resource ID guessing (window).

Defense for both of the above: send Window error.

GetProperty

Theft: get a property stored by another client.

Theft: resource ID guessing (window).

Defense for both of the above: send Window error.

Note

ISSUE: should certain root window properties be readable? Proposed answer: yes,
some configurable list. Do those properties need to be polyinstantiated?

Note

ISSUE: Motif drag and drop needs to be able to read the following properties:
WM_STATE to identify top-level windows, _MOTIF_DRAG_WINDOW on the root window,
_MOTIF_DRAG_TARGETS on the window given in the _MOTIF_DRAG_WINDOW property, and
_MOTIF_DRAG_RECEIVER_INFO on windows with drop sites. Additionally, some
properties are needed that do not have fixed names.

RotateProperties

Alteration: rotate properties stored by another client.

Theft: resource ID guessing (window).

Defense for both of the above: send Window error.

ListProperties

Theft: list properties stored by another client.

Theft: resource ID guessing (window).

Defense for both of the above: send Window error.

Note

ISSUE: should certain root window properties be listable?

SetSelectionOwner

Theft: Steal ownership of a selection.

Denial of service: do this repeatedly so that no other client can own the
selection.

Defense for both of the above: have a configurable list of selections that
untrusted clients can own. For other selections, treat this request as a no-op.

Note

ISSUE: how does the security manager get involved here? Is it the one that has
the configurable list of selections instead of the server?

Theft: resource ID guessing (window).

Defense: send Window error.

GetSelectionOwner

Theft: discover the ID of another client's window via the owner field of the
reply.

Defense: if the selection is on the configurable list mentioned above, return
the root window ID, else return None.

Note

ISSUE: how does the security manager get involved here?

ConvertSelection

Theft: this initiates a selection transfer (see the ICCCM) which sends the
selection contents from the selection owner, which may be another client, to
the requesting client.

Defense: since in many cases ConvertSelection is done in direct response to
user interaction, it is probably best not to force it to fail, either silently
or with an error. The server should rely on the security manager to assist in
handling the selection transfer.

Theft: resource ID guessing (requestor).

Defense: send Window error.

SendEvent

A client can use SendEvent to cause events of any type to be sent to windows of
other clients. Similarly, a client could SendEvent to one of its own windows
with propagate set to True and arrange for the event to be propagated up to a
window it does not own. Clients can detect events generated by SendEvent, but
we cannot assume that they will.

Defense: ignore this request unless the event being sent is a ClientMessage
event, which should be sent normally so that selection transfer, Motif drag and
drop, and certain input methods have a chance at working.

Note

ISSUE: does allowing all ClientMessages open up too big a hole?

Theft: resource ID guessing (window).

Defense: send Window error.

Keyboard and Pointer Grabs

Specifically, GrabKeyboard, GrabPointer, GrabKey, and GrabButton.

Denial of service/Theft: take over the keyboard and pointer. This could be
viewed as denial of service since it prevents other clients from getting
keyboard or mouse input, or it could be viewed as theft since the user input
may not have been intended for the grabbing client.

Defense: provide a way to break grabs via some keystroke combination, and have
a status area that shows which client is getting input. (See MapWindow ).

Theft: resource ID guessing (grab-window, confine-to, cursor).

Defense: send Window or Cursor error.

ChangeActivePointerGrab

Theft: resource ID guessing (cursor).

Defense: send Cursor error.

GrabServer

Denial of service: a client can grab the server and not let go, locking out all
other clients.

Defense: provide a way to break grabs via some keystroke combination.

QueryPointer

Theft: A client can steal pointer motion and position, button input, modifier
key state, and possibly a window of another client with this request.

Defense: if the querying client doesn't have the pointer grabbed, and the
pointer is not in one of its windows, the information can be zeroed.

Theft: resource ID guessing (window).

Defense: send Window error.

GetMotionEvents

Theft: steal pointer motion input that went to other clients.

Defense: ideally, the server would return only pointer input that was not
delivered to any trusted client. The implementation effort to do that probably
outweighs the marginal benefits. Instead, we will always return an empty list
of motion events to untrusted clients.

Theft: resource ID guessing (window).

Defense: send Window error.

TranslateCoordinates

Theft: discover information about other clients' windows: position, screen, and
possibly the ID of one of their subwindows.

Defense: send an error if src-window or dst-window do not belong to the
requesting client.

Theft: resource ID guessing (src-window, dst-window).

Defense: send Window error.

WarpPointer

A client can cause pointer motion to occur in another client's window.

Denial of service: repeated pointer warping prevents the user from using the
mouse normally.

Defense for both of the above: if the querying client doesn't have the pointer
grabbed, and the pointer is not in one of its windows, treat the request as a
no-op.

Theft: resource ID guessing (src-window, dst-window).

Defense: send Window error.

SetInputFocus

Theft: a client can use this request to make one of its own windows have the
input focus (keyboard focus). The user may be unaware that keystrokes are now
going to a different window.

Denial of service: repeatedly setting input focus prevents normal use of the
keyboard.

Defense for both of the above: only allow untrusted clients to SetInputFocus if
input focus is currently held by another untrusted client.

Note

ISSUE: this will break clients using the Globally Active Input model described
in section 4.1.7 of the ICCCM.

Theft: resource ID guessing (focus).

Defense: send Window error.

GetInputFocus

Theft: the reply may contain the ID of another client's window.

Defense: return a focus window of None if a trusted client currently has the
input focus.

QueryKeymap

Theft: poll the keyboard with this to see which keys are being pressed.

Defense: zero the returned bit vector if a trusted client currently has the
input focus.

Font Requests

Specifically, OpenFont, QueryFont, ListFonts, ListFontsWithInfo, and
QueryTextExtents.

Theft: discover font name, glyph, and metric information about fonts that were
provided by another client (by setting the font path). Whether it is theft to
retrieve information about fonts from the server's initial font path depends on
whether or not you believe those fonts, by their existence in the initial font
path, are intended to be globally accessible by all clients.

Defense:

Maintain two separate font paths, one for trusted clients and one for untrusted
clients. They are both initialized to the default font path at server reset.
Subsequently, changes to one do not affect the other. Since untrusted clients
will not see font path elements added by trusted clients, they will not be able
to access any fonts provided by those font path elements.

Theft: resource ID guessing (font) (QueryFont and QueryTextExtents only).

Defense: send Font error.

Denial of service: open fonts until the server runs out of memory (OpenFont
only).

Defense: quotas.

CloseFont

Destruction: close another client's font.

Defense: send Font error.

SetFontPath

Denial of service: change the font path so that other clients cannot find their
fonts.

Alteration: change the font path so that other clients get different fonts than
they expected.

Defense for both of the above: separate font paths for trusted and untrusted
clients, as described in the Font Requests section.

Note

ISSUE: the printing project considered per-client font paths and concluded that
it was very difficult to do. We should look at this aspect of the print server
design to see if we can reuse the same scheme. We should also try to
reconstruct what was so difficult about this; it doesn't seem that hard on the
surface.

GetFontPath

Theft: retrieve font path elements that were set by other clients.

Use knowledge from font path elements to mount other attacks, e.g., attack a
font server found in the font path.

Defense for both of the above: separate font paths for trusted and untrusted
clients, as described in the Font Requests section.

CreatePixmap

Theft: resource ID guessing (drawable).

Defense: send Drawable error.

Denial of service: create pixmaps until the server runs out of memory.

Defense: quotas.

FreePixmap

Destruction: destroy another client's pixmap.

Defense: send Pixmap error.

CreateGC

Theft: resource ID guessing (drawable, tile, stipple, font, clip-mask).

Defense: send Drawable, Pixmap, or Font error.

Denial of service: create GCs until the server runs out of memory.

Defense: quotas.

CopyGC

Theft: copy GC values of another client's GC.

Alteration: copy GC values to another client's GC.

Defense for both of the above: send GC error.

ChangeGC, SetDashes, SetClipRectangles

Alteration: change values of another client's GC.

Theft: resource ID guessing (gc, tile, stipple, font, clip-mask) (last four for
ChangeGC only).

Defense for both of the above: send GC error.

FreeGC

Destruction: destroy another client's GC.

Defense: send GC error.

Drawing Requests

Specifically, ClearArea, CopyArea, CopyPlane, PolyPoint, PolyLine, PolySegment,
PolyRectangle, PolyArc, FillPoly, PolyFillRectangle, PolyFillArc, PutImage,
PolyText8, PolyText16, ImageText8, and ImageText16.

Alteration: draw to another client's drawable.

Theft: resource ID guessing: ClearArea - window; CopyArea, CopyPlane -
src-drawable, dst-drawable, gc; all others - drawable, gc.

Defense for both of the above: send appropriate error.

Note

ISSUE: The Motif preregister drag protocol requires clients to draw into
windows of other clients for drag-over/under effects.

Spoofing: draw to a window to make it resemble a window of another client.

Defense: see MapWindow .

GetImage

Theft: get the image of another client's drawable.

Theft: resource ID guessing (drawable).

Defense: send Drawable error.

Theft: get the image of your own window, which may contain pieces of other
overlapping windows.

Defense: censor returned images by blotting out areas that contain data from
trusted windows.

CreateColormap

Theft: resource ID guessing (window).

Defense: send Colormap error.

Denial of service: create colormaps with this request until the server runs out
of memory.

Defense: quotas.

FreeColormap

Destruction: destroy another client's colormap.

Defense: send Colormap error.

CopyColormapAndFree

Theft: resource ID guessing (src-map).

Defense: send Colormap error. However, default colormaps will be allowed.

Note

ISSUE: must untrusted applications be allowed to use standard colormaps? (Same
issue for ListInstalledColormaps, Color Allocation Requests, FreeColors,
StoreColors, StoreNamedColor, QueryColors, and LookupColor.)

Denial of service: create colormaps with this request until the server runs out
of memory.

Defense: quotas.

InstallColormap, UninstallColormap

Theft: resource ID guessing.

Defense: send Colormap error.

Denial of service: (un)install any colormap, potentially preventing windows
from displaying correct colors.

Defense: treat this request as a no-op. Section 4.1.8 of the ICCCM states that
(un)installing colormaps is the responsibility of the window manager alone.

Note

ISSUE: the ICCCM also allows clients to do colormap installs if the client has
the pointer grabbed. Do we need to allow that too?

ListInstalledColormaps

Theft: resource ID guessing (window).

Defense: send Colormap error.

Theft: discover the resource ID of another client's colormap from the reply.

Defense: remove the returned colormap IDs; only let through default colormaps
and colormaps of untrusted clients.

Color Allocation Requests

Specifically, AllocColor, AllocNamedColor, AllocColorCells, and
AllocColorPlanes.

Alteration/Denial of service: allocate colors in another client's colormap. It
is denial of service if the owning client's color allocations fail because
there are no cells available. Otherwise it is just alteration.

Theft: resource ID guessing (cmap).

Defense for both of the above: send Colormap error. However, default colormaps
will be allowed.

FreeColors

Theft: resource ID guessing (cmap).

Defense: send Colormap error. However, default colormaps will be allowed.

StoreColors, StoreNamedColor

Alteration: change the colors in another client's colormap.

Theft: resource ID guessing (cmap).

Defense for both of the above: send Colormap error. However, default colormaps
will be allowed.

QueryColors, LookupColor

Theft: retrieve information about the colors in another client's colormap.

Theft: resource ID guessing (cmap).

Defense for both of the above: send Colormap error. However, default colormaps
will be allowed.

CreateCursor, CreateGlyphCursor

Theft: resource ID guessing (source, mask or source-font, mask-font).

Defense: send Pixmap or Font error. However, the default font will be allowed.

Denial of service: create cursors until the server runs out of memory.

Defense: quotas.

FreeCursor

Destruction: free another client's cursor.

Defense: send Cursor error.

RecolorCursor

Alteration: recolor another client's cursor.

Theft: resource ID guessing (cursor).

Defense for both of the above: send Cursor error.

QueryBestSize

Theft: resource ID guessing (drawable).

Defense: send Drawable error.

ListExtensions, QueryExtension

Determine the extensions supported by the server, and use the list to choose
extension-specific attacks to attempt.

Defense: extensions will have a way to tell the server whether it is safe for
untrusted clients to use them. These requests will only return information
about extensions that claim to be safe.

Keyboard configuration requests

Specifically, ChangeKeyboardControl, ChangeKeyboardMapping, and
SetModifierMapping.

Alteration: change the keyboard parameters that were established by another
client.

Denial of service: with ChangeKeyboardControl, disable auto-repeat, key click,
or the bell. With ChangeKeyboardMapping or SetModifierMapping, change the key
mappings so that the keyboard is difficult or impossible to use.

Defense for both of the above: treat these requests as a no-op.

Keyboard query requests

Specifically, GetKeyboardControl, GetKeyboardMapping, and GetModifierMapping.

Theft: get keyboard information that was established by another client.

Defense: This is a minor form of theft. We propose to do nothing about this
threat.

ChangePointerControl, SetPointerMapping

Alteration: change the pointer parameters that were established by another
client.

Denial of service: set the pointer parameters so that the pointer is difficult
or impossible to use.

Defense for both of the above: treat these requests as a no-op.

GetPointerControl, GetPointerMapping

Theft: get pointer parameters that were established by another client.

Defense: This is a minor form of theft. We propose to do nothing about this
threat.

SetScreenSaver

Alteration: change the screen saver parameters that were established by another
client.

Denial of service: set the screen saver parameters so that the screen saver is
always on or always off.

Defense for both of the above: treat these requests as a no-op.

GetScreenSaver

Theft: get screen saver parameters that were established by another client.

Defense: This is a minor form of theft. We propose to do nothing about this
threat.

ForceScreenSaver

Denial of service: repeatedly activate the screen saver so that the user cannot
see the screen as it would look when the screen saver is off.

Denial of service: repeatedly reset the screen saver, preventing it from
activating.

Defense for both of the above: treat these requests as a no-op.

ChangeHost

Most servers already have some restrictions on which clients can use this
request, so whether the following list applies is implementation dependent.

Denial of service: remove a host from the list, preventing clients from
connecting from that host.

Add a host to the list. Clients from that host may then launch other attacks of
any type.

Defense for both of the above: return Access error.

ListHosts

Theft: steal host identities and possibly even user identities that are allowed
to connect.

Launch attacks of any type against the stolen host/user identities.

Defense for both of the above: return only untrusted hosts.

SetAccessControl

Most servers already have some restrictions on which clients can use this
request, so whether the following list applies is implementation dependent.

Alteration: change the access control value established by some other client.

Disable access control, allowing clients to connect who would normally not be
able to connect. Those clients may then launch other attacks of any type.

Defense for both of the above: return Access error.

SetCloseDownMode

Denial of service: set the close-down mode to RetainPermanent or
RetainTemporary, then disconnect. The server cannot reuse the resource-id-base
of the disconnected client, or the memory used by the retained resources,
unless another client issues an appropriate KillClient to cancel the
retainment. The server has a limited number of resource-id-bases, and when they
are exhausted, it will be unable to accept new client connections.

Defense: treat this request as a no-op.

KillClient

Destruction/Denial of service: kill another currently connected client.

Destruction: kill a client that has terminated with close-down mode of
RetainTemporary or RetainPermanent, destroying all its retained resources.

Destruction: specify AllTemporary as the resource, destroying all resources of
clients that have terminated with close-down mode RetainTemporary.

Defense for all of the above: return Value error.

Clean Requests

Other than denial of service caused by flooding, these requests have no known
security concerns: AllowEvents, UngrabPointer, UngrabButton, UngrabKeyboard,
UngrabKey, UngrabServer, NoOperation, and Bell.

Chapter 5. Events

Table of Contents

KeymapNotify
Expose
GraphicsExposure
VisibilityNotify
ReparentNotify
ConfigureNotify
ConfigureRequest
SelectionClear
SelectionRequest
MappingNotify

The only threat posed by events is theft. Selecting for events on another
client's resources is always theft. We restrict further analysis by assuming
that the client only selects for events on its own resources, then asking
whether the events provide information about other clients.

KeymapNotify

Theft: the state of the keyboard can be seen when the client does not have the
input focus. This is possible because a KeymapNotify is sent to a window after
every EnterNotify even if the window does not have input focus.

Defense: zero the returned bit vector if a trusted client currently has the
input focus.

Expose

Theft: discover where other clients' windows overlap your own. For instance,
map a full-screen window, lower it, then raise it. The resulting exposes tell
you where other windows are.

Defense: about the only thing you could do here is force backing store to be
used on untrusted windows, but that would probably use too much server memory.
We propose to do nothing about this threat.

GraphicsExposure

Theft: discover where other clients' windows overlap your own. For instance,
use CopyArea to copy the entire window's area exactly on top of itself. The
resulting GraphicsExposures tell you where the window was obscured.

Defense: see Expose above. We propose to do nothing about this threat.

VisibilityNotify

Theft: this event provides crude positional information about other clients,
though the receiver cannot tell which other clients.

Defense: The information content of this event is very low. We propose to do
nothing about this threat.

ReparentNotify

Theft: the parent window may belong to some other client (probably the window
manager).

Defense: If the parent window belongs to a trusted client, return the closest
ancestor window that belongs to an untrusted client, or if such a window does
not exist, return the root window for the parent window.

Note

ISSUE: what is the application impact?

ConfigureNotify

Theft: the above-sibling window may belong to some other client.

Defense: return None for the above-sibling window if it belongs to a trusted
client.

Note

ISSUE: what is the application impact?

ConfigureRequest

Theft: the sibling window may belong to some other client.

Defense: return None for the sibling window if it belongs to a trusted client.

ISSUE: what is the application impact?

SelectionClear

Theft: the owner window may belong to some other client.

Defense: return None for the owner window if it belongs to a trusted client.

SelectionRequest

Theft: the requestor window may belong to some other client.

Defense: Blocking this event or censoring the window would prevent selection
transfers from untrusted clients to trusted clients from working. We propose to
do nothing in the server about this threat. The security manager may reduce the
exposure of trusted window IDs by becoming the owner of all selections.

MappingNotify

Theft: discover keyboard, pointer, or modifier mapping information set by
another client.

Defense: Any tampering with this event will cause clients to have an
inconsistent view of the keyboard or pointer button configuration, which is
likely to confuse the user. We propose to do nothing about this threat.

Chapter 6. Errors

There appear to be no threats related to protocol errors.

Chapter 7. Future Work

The next steps are resolve the items marked ISSUE and to decide if the defenses
proposed are reasonable. Discussion on the security@x.org mailing list,
prototyping, and/or starting the implementation should help answer these
questions.

Chapter 8. References

Bellcore, "Framework Generic Requirements for X Window System Security,"
Technical Advisory FA-STS-001324, Issue 1, August 1992.

Dardailler, Daniel, "Motif Drag And Drop Protocol," unpublished design notes.

Kahn, Brian L., "Safe Use of X WINDOW SYSTEM protocol Across a Firewall",
unpublished draft, The MITRE Corporation, 1995.

Rosenthal, David S. H., "LINX - a Less INsecure X server," Sun Microsystems,
29th April 1989.

Rosenthal, David and Marks, Stuart W., "Inter-Client Communication Conventions
Manual Version 2.0," ftp://ftp.x.org/pub/R6.1/xc/doc/hardcopy/ICCCM/icccm.PS.Z

Scheifler, Robert W., "X Window System Protocol," ftp://ftp.x.org/pub/R6.1/xc/
doc/hardcopy/XProtocol/proto.PS.Z

Treese, G. Winfield and Wolman, Alec, "X Through the Firewall, and Other
Application Relays," Digital Equipment Corporation Cambridge Research Lab,
Technical Report Series, CRL 93/10, May 3, 1993.

