-----BEGIN PGP SIGNED MESSAGE----- MIT krb5 Security Advisory 2005-002 Original release: 2005-07-12 Topic: buffer overflow, heap corruption in KDC Severity: CRITICAL SUMMARY ======= The MIT krb5 Key Distribution Center (KDC) implementation can corrupt the heap by attempting to free memory at a random address when it receives a certain unlikely (but valid) request via a TCP connection. This attempt to free unallocated memory can result in a KDC crash and consequent denial of service. [CAN-2005-1174, VU#259798] Additionally, the same request, when received by the KDC via either TCP or UDP, can trigger a bug in the krb5 library which results in a single-byte overflow of a heap buffer. Application servers are vulnerable to a highly improbable attack, provided that the attacker controls a realm sharing a cross-realm key with the target realm. [CAN-2005-1175, VU#885830] An unauthenticated attacker may be able to use these vulnerabilities to execute arbitrary code on the KDC host, potentially compromising an entire Kerberos realm. No exploit code is known to exist at this time. Exploitation of these vulnerabilities is believed to be difficult. IMPACT ====== An unauthenticated attacker may be able to execute arbitrary code on the KDC host, potentially compromising an entire Kerberos realm. An unsuccessful attack against the heap corruption vulnerability may result in a denial of service by crashing the KDC process. AFFECTED SOFTWARE ================= * [CAN-2005-1174] affects the KDC implementation in all MIT krb5 releases supporting TCP client connections to the KDC. This includes krb5-1.3 and later releases, up to and including krb5-1.4.1. * [CAN-2005-1175] affects KDC implementations and application servers in all MIT krb5 releases, up to and including krb5-1.4.1. Third-party application servers which use MIT krb5 are also affected. FIXES ===== * The upcoming krb5-1.4.2 release will have fixes for these vulnerabilities. * WORKAROUNDS: Disabling TCP support in the KDC avoids one vulnerability [CAN-2005-1174]. The single-byte overflow [CAN-2005-1175] is still possible even without KDC TCP support enabled. Running the KDC from init or from some similar automatic respawning facility may reduce the durations of denials of service, but this approach may make it difficult to detect deliberate attacks targeted at code execution. * Apply the patch at: http://web.mit.edu/kerberos/advisories/2005-002-patch_1.4.1.txt The associated detached PGP signature is at: http://web.mit.edu/kerberos/advisories/2005-002-patch_1.4.1.txt.asc The patch was generated against the krb5-1.4.1 release. It may apply, with some offset, to earlier releases. On releases prior to krb5-1.3, only the patch to lib/krb5/krb/unparse.c should be necessary. REFERENCES ========== This announcement and related security advisories may be found on the MIT Kerberos security advisory page at: http://web.mit.edu/kerberos/advisories/index.html The main MIT Kerberos web page is at: http://web.mit.edu/kerberos/index.html CVE: CAN-2005-1174 http://cve.mitre.org/cgi-bin/cvename.cgi?name=CAN-2005-1174 CERT: VU#259798 http://www.kb.cert.org/vuls/id/259798 CVE: CAN-2005-1175 http://cve.mitre.org/cgi-bin/cvename.cgi?name=CAN-2005-1175 CERT: VU#885830 http://www.kb.cert.org/vuls/id/885830 ACKNOWLEDGMENTS =============== Thanks to Daniel Wachdorf for reporting these vulnerabilities. DETAILS ======= Kerberos 5 principal names may have an arbitrary number of components. The krb5_unparse_name() function in the MIT krb5 library converts an internal representation of a Kerberos principal name into a human-readable string. The internal representation might have originated from the decoding of a Kerberos protocol message. The single-byte overflow occurs whenever the krb5_unparse_name() function is called on a principal name having zero components. The function writes a null byte to an address one beyond the end of a buffer allocated my malloc(). The corresponding krb5_parse_name() function never generates an internal representation having zero components; instead, it generates at least one zero-length component. The current string representation form of Kerberos principal names has some ambiguity between a zero-component principal name and a one-component principal name having a zero-length single component. Application servers which call krb5_unparse_name(), directly or indirectly, are vulnerable to the single-byte overflow in krb5_unparse_name(), provided that the attacker controls a realm which shares a cross-realm key with the target realm. This enables the attacker to use a cross-realm ticket for a zero-component client principal name, which the application server will then pass to krb5_unparse_name(), triggering the single-byte overflow. For this attack to succeed, the attacker needs access to a KDC in the target realm which will create a ticket for a zero-component client principal name. Since the current MIT krb5 KDC implementation will refuse to create such a ticket, the attack is unlikely to succeed unless the implementation has been altered to allow the issuance of tickets for zero-component client principal names. When the KDC fails to find the principal with a zero-component name in its database (such a principal is very unlikely to exist in most databases, as there are extremely few uses for such a principal), it attempts to encode an error packet containing the offending principal name, using prepare_error_as() or prepare_error_tgs(). This encoding attempt fails inside encode_krb5_error(), since the ASN.1 encoder function asn1_encode_principal_name() interprets the internal representation of a zero-component principal name as an error condition. encode_krb5_error() does not allocate an output buffer when it encounters an error condition. While the UDP request handling code in kdc/network.c:process_packet() does not attempt to free the output buffer containing the encoded message when it encounters an error, the TCP request handling code in process does free the buffer inside kill_tcp_connection(), which attempts to free unallocated memory pointed to by an uninitialized pointer. REVISION HISTORY ================ 2005-05-12 original release Copyright (C) 2005 Massachusetts Institute of Technology -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.2.5 (SunOS) iQCVAwUBQtMbCabDgE/zdoE9AQFo9QP5AZMbr0YGmyzYbARTqFq+Lt+FYbfQ7XC/ c1hqTfsTkN0Mfh1I5d6dTjhXQT6kfN+EdNYfPhY+4LANB5CW9xe9BARPcW9i2ftt xSTIODrD6LdNtOCCut1ha3T5tcV5GodvXzj7dSClde29j0IJR6dBcigfvR4mAygw /U7r46obgM0= =SnqK -----END PGP SIGNATURE-----