# MIT Kerberos Documentation

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# MIT Kerberos features

https://web.mit.edu/kerberos

## Quick facts

License - MIT Kerberos License information

Releases:

  - Latest stable: https://web.mit.edu/kerberos/krb5-1.22/

  - Supported: https://web.mit.edu/kerberos/krb5-1.21/

  - Release cycle: approximately 12 months

Supported platforms / OS distributions:

  - Windows (KfW 4.0): Windows 7, Vista, XP

  - Solaris: SPARC, x86_64/x86

  - GNU/Linux: Debian x86_64/x86, Ubuntu x86_64/x86, RedHat x86_64/x86

  - BSD: NetBSD x86_64/x86

Crypto backends:

  - builtin - MIT Kerberos native crypto library

  - OpenSSL (1.0+) - https://www.openssl.org

Database backends: LDAP, DB2, LMDB

krb4 support: Kerberos 5 release < 1.8

DES support: Kerberos 5 release < 1.18 (See Retiring DES)

## Interoperability

Microsoft

Starting from release 1.7:

- Follow client principal referrals in the client library when obtaining initial tickets.

- KDC can issue realm referrals for service principals based on domain names.

- Extensions supporting DCE RPC, including three-leg GSS context setup and unencapsulated GSS tokens inside SPNEGO.

- Microsoft GSS_WrapEX, implemented using the gss_iov API, which is similar to the equivalent SSPI functionality. This is needed to support some instances of DCE RPC.

- NTLM recognition support in GSS-API, to facilitate dropping in an NTLM implementation for improved compatibility with older releases of Microsoft Windows.

- KDC support for principal aliases, if the back end supports them. Currently, only the LDAP back end supports aliases.

- Support Microsoft set/change password (RFC 3244) protocol in kadmind.

- Implement client and KDC support for GSS_C_DELEG_POLICY_FLAG, which allows a GSS application to request credential delegation only if permitted by KDC policy.

Starting from release 1.8:

- Microsoft Services for User (S4U) compatibility

Heimdal

- Support for KCM credential cache starting from release 1.13

## Feature list

For more information on the specific project see https://k5wiki.kerberos.org/wiki/Projects

Release 1.7

  - Credentials delegation RFC 5896

  - Cross-realm authentication and referrals RFC 6806

  - Master key migration

  - PKINIT RFC 4556 PKINIT configuration

Release 1.8

  - Anonymous PKINIT RFC 6112 Anonymous PKINIT

  - Constrained delegation

  - IAKERB https://tools.ietf.org/html/draft-ietf-krb-wg-iakerb-02

  - Heimdal bridge plugin for KDC backend

  - GSS-API S4U extensions https://msdn.microsoft.com/en-us/library/cc246071

  - GSS-API naming extensions RFC 6680

  - GSS-API extensions for storing delegated credentials RFC 5588

Release 1.9

  - Advance warning on password expiry

  - Camellia encryption (CTS-CMAC mode) RFC 6803

  - KDC support for SecurID preauthentication

  - kadmin over IPv6

  - Trace logging Trace logging

  - GSSAPI/KRB5 multi-realm support

  - Plugin to test password quality Password quality interface (pwqual)

  - Plugin to synchronize password changes KADM5 hook interface (kadm5_hook)

  - Parallel KDC

  - GSS-API extensions for SASL GS2 bridge RFC 5801 RFC 5587

  - Purging old keys

  - Naming extensions for delegation chain

  - Password expiration API

  - Windows client support (build-only)

  - IPv6 support in iprop

Release 1.10

  - Plugin interface for configuration Configuration interface (profile)

  - Credentials for multiple identities Credential cache selection interface (ccselect)

Release 1.11

  - Client support for FAST OTP RFC 6560

  - GSS-API extensions for credential locations

  - Responder mechanism

Release 1.12

  - Plugin to control krb5_aname_to_localname and krb5_kuserok behavior Local authorization interface (localauth)

  - Plugin to control hostname-to-realm mappings and the default realm Host-to-realm interface (hostrealm)

  - GSSAPI extensions for constructing MIC tokens using IOV lists IOV MIC tokens

  - Principal may refer to nonexistent policies Policy Refcount project

  - Support for having no long-term keys for a principal Principals Without Keys project

  - Collection support to the KEYRING credential cache type on Linux Credential cache

  - FAST OTP preauthentication module for the KDC which uses RADIUS to validate OTP token values OTP Preauthentication

  - Experimental Audit plugin for KDC processing Audit project

Release 1.13

> - Add support for accessing KDCs via an HTTPS proxy server using the MS-KKDCP protocol.

> - Add support for hierarchical incremental propagation, where replicas can act as intermediates between an upstream primary and other downstream replicas.

> - Add support for configuring GSS mechanisms using `/etc/gss/mech.d/*.conf` files in addition to `/etc/gss/mech`.

> - Add support to the LDAP KDB module for binding to the LDAP server using SASL.

> - The KDC listens for TCP connections by default.

> - Fix a minor key disclosure vulnerability where using the “keepold” option to the kadmin randkey operation could return the old keys. [CVE-2014-5351]

> - Add client support for the Kerberos Cache Manager protocol. If the host is running a Heimdal kcm daemon, caches served by the daemon can be accessed with the KCM: cache type.

> - When built on macOS 10.7 and higher, use “KCM:” as the default cachetype, unless overridden by command-line options or krb5-config values.

> - Add support for doing unlocked database dumps for the DB2 KDC back end, which would allow the KDC and kadmind to continue accessing the database during lengthy database dumps.

Release 1.14

> - Administrator experience

>   - Add a new kdb5_util tabdump command to provide reporting-friendly tabular dump formats (tab-separated or CSV) for the KDC database. Unlike the normal dump format, each output table has a fixed number of fields. Some tables include human-readable forms of data that are opaque in ordinary dump files. This format is also suitable for importing into relational databases for complex queries.

>   - Add support to kadmin and kadmin.local for specifying a single command line following any global options, where the command arguments are split by the shell–for example, “kadmin getprinc principalname”. Commands issued this way do not prompt for confirmation or display warning messages, and exit with non-zero status if the operation fails.

>   - Accept the same principal flag names in kadmin as we do for the default_principal_flags kdc.conf variable, and vice versa. Also accept flag specifiers in the form that kadmin prints, as well as hexadecimal numbers.

>   - Remove the triple-DES and RC4 encryption types from the default value of supported_enctypes, which determines the default key and salt types for new password-derived keys. By default, keys will only created only for AES128 and AES256. This mitigates some types of password guessing attacks.

>   - Add support for directory names in the KRB5_CONFIG and KRB5_KDC_PROFILE environment variables.

>   - Add support for authentication indicators, which are ticket annotations to indicate the strength of the initial authentication. Add support for the “require_auth” string attribute, which can be set on server principal entries to require an indicator when authenticating to the server.

>   - Add support for key version numbers larger than 255 in keytab files, and for version numbers up to 65535 in KDC databases.

>   - Transmit only one ETYPE-INFO and/or ETYPE-INFO2 entry from the KDC during pre-authentication, corresponding to the client’s most preferred encryption type.

>   - Add support for server name identification (SNI) when proxying KDC requests over HTTPS.

>   - Add support for the err_fmt profile parameter, which can be used to generate custom-formatted error messages.

> - Developer experience:

>   - Change gss_acquire_cred_with_password() to acquire credentials into a private memory credential cache. Applications can use gss_store_cred() to make the resulting credentials visible to other processes.

>   - Change gss_acquire_cred() and SPNEGO not to acquire credentials for IAKERB or for non-standard variants of the krb5 mechanism OID unless explicitly requested. (SPNEGO will still accept the Microsoft variant of the krb5 mechanism OID during negotiation.)

>   - Change gss_accept_sec_context() not to accept tokens for IAKERB or for non-standard variants of the krb5 mechanism OID unless an acceptor credential is acquired for those mechanisms.

>   - Change gss_acquire_cred() to immediately resolve credentials if the time_rec parameter is not NULL, so that a correct expiration time can be returned. Normally credential resolution is delayed until the target name is known.

>   - Add krb5_prepend_error_message() and krb5_wrap_error_message() APIs, which can be used by plugin modules or applications to add prefixes to existing detailed error messages.

>   - Add krb5_c_prfplus() and krb5_c_derive_prfplus() APIs, which implement the RFC 6113 PRF+ operation and key derivation using PRF+.

>   - Add support for pre-authentication mechanisms which use multiple round trips, using the the KDC_ERR_MORE_PREAUTH_DATA_REQUIRED error code. Add get_cookie() and set_cookie() callbacks to the kdcpreauth interface; these callbacks can be used to save marshalled state information in an encrypted cookie for the next request.

>   - Add a client_key() callback to the kdcpreauth interface to retrieve the chosen client key, corresponding to the ETYPE-INFO2 entry sent by the KDC.

>   - Add an add_auth_indicator() callback to the kdcpreauth interface, allowing pre-authentication modules to assert authentication indicators.

>   - Add support for the GSS_KRB5_CRED_NO_CI_FLAGS_X cred option to suppress sending the confidentiality and integrity flags in GSS initiator tokens unless they are requested by the caller. These flags control the negotiated SASL security layer for the Microsoft GSS-SPNEGO SASL mechanism.

>   - Make the FILE credential cache implementation less prone to corruption issues in multi-threaded programs, especially on platforms with support for open file description locks.

> - Performance:

>   - On replica KDCs, poll the primary KDC immediately after processing a full resync, and do not require two full resyncs after the primary KDC’s log file is reset.

Release 1.15

- Administrator experience:

  - Add support to kadmin for remote extraction of current keys without changing them (requires a special kadmin permission that is excluded from the wildcard permission), with the exception of highly protected keys.

  - Add a lockdown_keys principal attribute to prevent retrieval of the principal’s keys (old or new) via the kadmin protocol. In newly created databases, this attribute is set on the krbtgt and kadmin principals.

  - Restore recursive dump capability for DB2 back end, so sites can more easily recover from database corruption resulting from power failure events.

  - Add DNS auto-discovery of KDC and kpasswd servers from URI records, in addition to SRV records. URI records can convey TCP and UDP servers and primary KDC status in a single DNS lookup, and can also point to HTTPS proxy servers.

  - Add support for password history to the LDAP back end.

  - Add support for principal renaming to the LDAP back end.

  - Use the getrandom system call on supported Linux kernels to avoid blocking problems when getting entropy from the operating system.

- Code quality:

  - Clean up numerous compilation warnings.

  - Remove various infrequently built modules, including some preauth modules that were not built by default.

- Developer experience:

  - Add support for building with OpenSSL 1.1.

  - Use SHA-256 instead of MD5 for (non-cryptographic) hashing of authenticators in the replay cache. This helps sites that must build with FIPS 140 conformant libraries that lack MD5.

- Protocol evolution:

  - Add support for the AES-SHA2 enctypes, which allows sites to conform to Suite B crypto requirements.

Release 1.16

- Administrator experience:

  - The KDC can match PKINIT client certificates against the “pkinit_cert_match” string attribute on the client principal entry, using the same syntax as the existing “pkinit_cert_match” profile option.

  - The ktutil addent command supports the “-k 0” option to ignore the key version, and the “-s” option to use a non-default salt string.

  - kpropd supports a –pid-file option to write a pid file at startup, when it is run in standalone mode.

  - The “encrypted_challenge_indicator” realm option can be used to attach an authentication indicator to tickets obtained using FAST encrypted challenge pre-authentication.

  - Localization support can be disabled at build time with the –disable-nls configure option.

- Developer experience:

  - The kdcpolicy pluggable interface allows modules control whether tickets are issued by the KDC.

  - The kadm5_auth pluggable interface allows modules to control whether kadmind grants access to a kadmin request.

  - The certauth pluggable interface allows modules to control which PKINIT client certificates can authenticate to which client principals.

  - KDB modules can use the client and KDC interface IP addresses to determine whether to allow an AS request.

  - GSS applications can query the bit strength of a krb5 GSS context using the GSS_C_SEC_CONTEXT_SASL_SSF OID with gss_inquire_sec_context_by_oid().

  - GSS applications can query the impersonator name of a krb5 GSS credential using the GSS_KRB5_GET_CRED_IMPERSONATOR OID with gss_inquire_cred_by_oid().

  - kdcpreauth modules can query the KDC for the canonicalized requested client principal name, or match a principal name against the requested client principal name with canonicalization.

- Protocol evolution:

  - The client library will continue to try pre-authentication mechanisms after most failure conditions.

  - The KDC will issue trivially renewable tickets (where the renewable lifetime is equal to or less than the ticket lifetime) if requested by the client, to be friendlier to scripts.

  - The client library will use a random nonce for TGS requests instead of the current system time.

  - For the RC4 string-to-key or PAC operations, UTF-16 is supported (previously only UCS-2 was supported).

  - When matching PKINIT client certificates, UPN SANs will be matched correctly as UPNs, with canonicalization.

- User experience:

  - Dates after the year 2038 are accepted (provided that the platform time facilities support them), through the year 2106.

  - Automatic credential cache selection based on the client realm will take into account the fallback realm and the service hostname.

  - Referral and alternate cross-realm TGTs will not be cached, avoiding some scenarios where they can be added to the credential cache multiple times.

  - A German translation has been added.

- Code quality:

  - The build is warning-clean under clang with the configured warning options.

  - The automated test suite runs cleanly under AddressSanitizer.

Release 1.17

- Administrator experience:

  - A new Kerberos database module using the Lightning Memory-Mapped Database library (LMDB) has been added. The LMDB KDB module should be more performant and more robust than the DB2 module, and may become the default module for new databases in a future release.

  - “kdb5_util dump” will no longer dump policy entries when specific principal names are requested.

- Developer experience:

  - The new krb5_get_etype_info() API can be used to retrieve enctype, salt, and string-to-key parameters from the KDC for a client principal.

  - The new GSS_KRB5_NT_ENTERPRISE_NAME name type allows enterprise principal names to be used with GSS-API functions.

  - KDC and kadmind modules which call com_err() will now write to the log file in a format more consistent with other log messages.

  - Programs which use large numbers of memory credential caches should perform better.

- Protocol evolution:

  - The SPAKE pre-authentication mechanism is now supported. This mechanism protects against password dictionary attacks without requiring any additional infrastructure such as certificates. SPAKE is enabled by default on clients, but must be manually enabled on the KDC for this release.

  - PKINIT freshness tokens are now supported. Freshness tokens can protect against scenarios where an attacker uses temporary access to a smart card to generate authentication requests for the future.

  - Password change operations now prefer TCP over UDP, to avoid spurious error messages about replays when a response packet is dropped.

  - The KDC now supports cross-realm S4U2Self requests when used with a third-party KDB module such as Samba’s. The client code for cross-realm S4U2Self requests is also now more robust.

- User experience:

  - The new ktutil addent -f flag can be used to fetch salt information from the KDC for password-based keys.

  - The new kdestroy -p option can be used to destroy a credential cache within a collection by client principal name.

  - The Kerberos man page has been restored, and documents the environment variables that affect programs using the Kerberos library.

- Code quality:

  - Python test scripts now use Python 3.

  - Python test scripts now display markers in verbose output, making it easier to find where a failure occurred within the scripts.

  - The Windows build system has been simplified and updated to work with more recent versions of Visual Studio. A large volume of unused Windows-specific code has been removed. Visual Studio 2013 or later is now required.

Release 1.18

- Administrator experience:

  - Remove support for single-DES encryption types.

  - Change the replay cache format to be more efficient and robust. Replay cache filenames using the new format end with `.rcache2` by default.

  - setuid programs will automatically ignore environment variables that normally affect krb5 API functions, even if the caller does not use krb5_init_secure_context().

  - Add an `enforce_ok_as_delegate` krb5.conf relation to disable credential forwarding during GSSAPI authentication unless the KDC sets the ok-as-delegate bit in the service ticket.

- Developer experience:

  - Implement krb5_cc_remove_cred() for all credential cache types.

  - Add the krb5_pac_get_client_info() API to get the client account name from a PAC.

- Protocol evolution:

  - Add KDC support for S4U2Self requests where the user is identified by X.509 certificate. (Requires support for certificate lookup from a third-party KDB module.)

  - Remove support for an old (“draft 9”) variant of PKINIT.

  - Add support for Microsoft NegoEx. (Requires one or more third-party GSS modules implementing NegoEx mechanisms.)

- User experience:

  - Add support for `dns_canonicalize_hostname=fallback`, causing host-based principal names to be tried first without DNS canonicalization, and again with DNS canonicalization if the un-canonicalized server is not found.

  - Expand single-component hostnames in hhost-based principal names when DNS canonicalization is not used, adding the system’s first DNS search path as a suffix. Add a `qualify_shortname` krb5.conf relation to override this suffix or disable expansion.

- Code quality:

  - The libkrb5 serialization code (used to export and import krb5 GSS security contexts) has been simplified and made type-safe.

  - The libkrb5 code for creating KRB-PRIV, KRB-SAFE, and KRB-CRED messages has been revised to conform to current coding practices.

  - The test suite has been modified to work with macOS System Integrity Protection enabled.

  - The test suite incorporates soft-pkcs11 so that PKINIT PKCS11 support can always be tested.

Release 1.19

- Administrator experience:

  - When a client keytab is present, the GSSAPI krb5 mech will refresh credentials even if the current credentials were acquired manually.

  - It is now harder to accidentally delete the K/M entry from a KDB.

- Developer experience:

  - gss_acquire_cred_from() now supports the “password” and “verify” options, allowing credentials to be acquired via password and verified using a keytab key.

  - When an application accepts a GSS security context, the new GSS_C_CHANNEL_BOUND_FLAG will be set if the initiator and acceptor both provided matching channel bindings.

  - Added the GSS_KRB5_NT_X509_CERT name type, allowing S4U2Self requests to identify the desired client principal by certificate.

  - PKINIT certauth modules can now cause the hw-authent flag to be set in issued tickets.

  - The krb5_init_creds_step() API will now issue the same password expiration warnings as krb5_get_init_creds_password().

- Protocol evolution:

  - Added client and KDC support for Microsoft’s Resource-Based Constrained Delegation, which allows cross-realm S4U2Proxy requests. A third-party database module is required for KDC support.

  - kadmin/admin is now the preferred server principal name for kadmin connections, and the host-based form is no longer created by default. The client will still try the host-based form as a fallback.

  - Added client and server support for Microsoft’s KERB_AP_OPTIONS_CBT extension, which causes channel bindings to be required for the initiator if the acceptor provided them. The client will send this option if the client_aware_gss_bindings profile option is set.

User experience:

> - The default setting of dns_canonicalize_realm is now “fallback”. Hostnames provided from applications will be tried in principal names as given (possibly with shortname qualification), falling back to the canonicalized name.

> - kinit will now issue a warning if the des3-cbc-sha1 encryption type is used in the reply. This encryption type will be deprecated and removed in future releases.

> - Added kvno flags –out-cache, –no-store, and –cached-only (inspired by Heimdal’s kgetcred).

Release 1.20

- Administrator experience:

  - Added a “disable_pac” realm relation to suppress adding PAC authdata to tickets, for realms which do not need to support S4U requests.

  - Most credential cache types will use atomic replacement when a cache is reinitialized using kinit or refreshed from the client keytab.

  - kprop can now propagate databases with a dump size larger than 4GB, if both the client and server are upgraded.

  - kprop can now work over NATs that change the destination IP address, if the client is upgraded.

- Developer experience:

  - Updated the KDB interface. The sign_authdata() method is replaced with the issue_pac() method, allowing KDB modules to add logon info and other buffers to the PAC issued by the KDC.

  - Host-based initiator names are better supported in the GSS krb5 mechanism.

- Protocol evolution:

  - Replaced AD-SIGNEDPATH authdata with minimal PACs.

  - To avoid spurious replay errors, password change requests will not be attempted over UDP until the attempt over TCP fails.

  - PKINIT will sign its CMS messages with SHA-256 instead of SHA-1.

- Code quality:

  - Updated all code using OpenSSL to be compatible with OpenSSL 3.

  - Reorganized the libk5crypto build system to allow the OpenSSL back-end to pull in material from the builtin back-end depending on the OpenSSL version.

  - Simplified the PRNG logic to always use the platform PRNG.

  - Converted the remaining Tcl tests to Python.

Release 1.21

- User experience:

  - Added a credential cache type providing compatibility with the macOS 11 native credential cache.

- Developer experience:

  - libkadm5 will use the provided krb5_context object to read configuration values, instead of creating its own.

  - Added an interface to retrieve the ticket session key from a GSS context.

- Protocol evolution:

  - The KDC will no longer issue tickets with RC4 or triple-DES session keys unless explicitly configured with the new allow_rc4 or allow_des3 variables respectively.

  - The KDC will assume that all services can handle aes256-sha1 session keys unless the service principal has a session_enctypes string attribute.

  - Support for PAC full KDC checksums has been added to mitigate an S4U2Proxy privilege escalation attack.

  - The PKINIT client will advertise a more modern set of supported CMS algorithms.

- Code quality:

  - Removed unused code in libkrb5, libkrb5support, and the PKINIT module.

  - Modernized the KDC code for processing TGS requests, the code for encrypting and decrypting key data, the PAC handling code, and the GSS library packet parsing and composition code.

  - Improved the test framework’s detection of memory errors in daemon processes when used with asan.

Release 1.22

- User experience:

  - The libdefaults configuration variable “request_timeout” can be set to limit the total timeout for KDC requests. When making a KDC request, the client will now wait indefinitely (or until the request timeout has elapsed) on a KDC which accepts a TCP connection, without contacting any additional KDCs. Clients will make fewer DNS queries in some configurations.

  - The realm configuration variable “sitename” can be set to cause the client to query site-specific DNS records when making KDC requests.

- Administrator experience:

  - Principal aliases are supported in the DB2 and LMDB KDB modules and in the kadmin protocol. (The LDAP KDB module has supported aliases since release 1.7.)

  - UNIX domain sockets are supported for the Kerberos and kpasswd protocols.

  - systemd socket activation is supported for krb5kdc and kadmind.

- Developer experience:

  - KDB modules can be be implemented in terms of other modules using the new krb5_db_load_module() function.

  - The profile library supports the modification of empty profiles and the copying of modified profiles, making it possible to construct an in-memory profile and pass it to krb5_init_context_profile().

  - GSS-API applications can pass the GSS_C_CHANNEL_BOUND flag to gss_init_sec_context() to request strict enforcement of channel bindings by the acceptor.

- Protocol evolution:

  - The PKINIT preauth module supports elliptic curve client certificates, ECDH key exchange, and the Microsoft paChecksum2 field.

  - The IAKERB implementation has been changed to comply with the most recent draft standard and to support realm discovery.

  - Message-Authenticator is supported in the RADIUS implementation used by the OTP kdcpreauth module.

- Code quality:

  - Removed old-style function declarations, to accomodate compilers which have removed support for them.

  - Added OSS-Fuzz to the project’s continuous integration infrastructure.

  - Rewrote the GSS per-message token parsing code for improved safety.

Pre-authentication mechanisms

- PW-SALT RFC 4120 Section 5.2.7.3

- ENC-TIMESTAMP RFC 4120 Section 5.2.7.2

- SAM-2

- FAST negotiation framework (release 1.8) RFC 6113

- PKINIT with FAST on client (release 1.10) RFC 6113

- PKINIT RFC 4556

- FX-COOKIE RFC 6113 Section 5.2

- S4U-X509-USER (release 1.8) https://msdn.microsoft.com/en-us/library/cc246091

- OTP (release 1.12) OTP Preauthentication

- SPAKE (release 1.17) SPAKE Preauthentication