Software Engineering Institute

A vulnerability in the way the GSS-API library provided with MIT krb5 handles messages with an invalid direction encoding may result in a double free. According to MIT krb5 Security Advisory MITKRB5-SA-2007-003:

The kg_unseal_v1() function in src/lib/gssapi/krb5/k5unseal.c frees memory allocated for the "message_buffer" gss_buffer_t when it detects an invalid direction encoding on the message. It does not set the pointer to NULL, nor does it set the length to zero. An application subsequently calling gss_release_buffer() on this gss_buffer_t will cause memory to be freed twice.

Much code provided with MIT krb5 does not attempt to call gss_release_buffer() when gss_unseal() or gss_unwrap() fails, even though the GSS-API C-bindings specification permits it to do so. The RPCSEC_GSS authentication flavor for the RPC library, introduced in krb5-1.4, does call gss_release_buffer() when gss_unwrap() fails. This allows an authenticated attacker to trigger a double-free situation.

Note that this issue affects all releases of MIT krb5 up to and including krb5-1.6. Other server applications that utilize the RPC library or the MIT GSS-API library provided with MIT krb5 may also be affected.

This vulnerability can be triggered by sending a specially crafted Kerberos message to a vulnerable system.

A remote, authenticated user may be able to execute arbitrary code on an affected system or cause the affected program to crash, resulting in a denial of service. Secondary impacts of code execution include complete compromise of the Kerberos key database.

Apply Patch
A patch can be obtained from MIT krb5 Security Advisory MITKRB5-SA-2007-003. MIT also states that this will be addressed in the upcoming krb5-1.6.1 release.