Software Engineering Institute

The gssrpc__svcauth_gssapi() function used by the Kerberos administration daemon can free an uninitialized pointer when receiving a specially crafted RPC request. This vulnerability may cause memory corruption that could allow a remote, unauthenticated user to execute arbitrary code. According to MIT krb5 Security Advisory MITKRB5-SA-2007-004:

The function gssrpc__svcauth_gssapi() in src/lib/rpc/svc_auth_gssapi.c declares an automatic variable "creds" of type auth_gssapi_creds. This type includes a gss_buffer_desc (which includes a pointer to void used as a pointer to a buffer of bytes). If gssrpc__svcauth_gssapi() receives an RPC credential with a length of zero, it jumps to the label "error", which executes some cleanup code. At this point, the gss_buffer_desc in "creds" is not yet initialized, and the cleanup code calls xdr_free() on "creds", which then attempts to free the memory pointed to by the uninitialized "value" member of the gss_buffer_desc.

Exploitation of freeing of invalid pointers is believed to be difficult, and depends on a variety of factors specific to a given malloc implementation.
Note that this issue affects all releases of MIT krb5 up to and including krb5-1.6.1. MIT has been provided with proof-of-concept exploit code that causes a denial of service, but it's not clear whether the exploit code is publicly available yet.

This vulnerability occurred as a result of failing to comply with rule EXP33-C of the CERT C Programming Language Secure Coding Standard.

A remote, unauthenticated 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 a patch
A patch can be obtained from MIT krb5 Security Advisory MITKRB5-SA-2007-004. MIT also states that this will be addressed in the upcoming krb5-1.6.2 and krb5-1.5.4 releases.