MIT Kerberos vulnerable to ticket splicing when using Kerberos4 triple DES service tickets
Vulnerability Note VU#442569
Original Release Date: 2003-03-20 | Last Revised: 2003-05-09
Several cryptographic vulnerabilities exist in the basic Kerberos version 4 protocol that could allow an attacker to impersonate any user in a Kerberos realm and gain any privilege authorized through that Kerberos realm.
The MIT Kerberos Development team has discovered a serious cryptographic flaw in the Kerberos version 4 protocol. This flaw could allow an attacker to compromise the entire affected Kerberos realm. In addition to the vulnerability described in VU#623217, an additional vulnerability was discovered in the MIT Kerberos implementation of triple-DES encryption of service tickets.
"As a result of concerns about single DES weaknesses, MIT implemented support for Kerberos 4 tickets encrypted in triple DES service keys. This support shares all the cryptographic weaknesses of single DES Kerberos 4. In addition, since it uses CBC mode rather than PCBC mode, it introduces new weaknesses not found in other Kerberos 4 implementations. When certain alignment constraints are met, it is possible to splice two tickets together, allowing an attacker to get a ticket with a known session key for a client without knowing that client's long term key. This attack does require sniffing a ticket for that client."
As a result, MIT implementations of Kerberos version 5 or derived implementations that include support for triple-DES keys in Kerberos version 4 are vulnerable.
In addition to the impacts described for VU#623217, an attacker may impersonate any principal to a service keyed with triple-DES Kerberos version 4 keys, given the ability to capture network traffic containing tickets for the target client principal.
Apply a patch from the vendor
The MIT Kerberos team has released MIT krb5 Security Advisory 2003-004 regarding this vulnerability. Sites are strongly encouraged to apply the patches referenced in the advisory.
In the absence of patching, the following workarounds have been proposed by the MIT Kerberos team:
1) V4 Cross Realm Considered Harmful
Kerberos implementations should gain an option to disable Kerberos 4 cross-realm authentication both in the KDC and in any implementations of the krb524 protocol. This configuration should be the default.
2) Application Migration
Application vendors and sites should migrate from Kerberos version 4 to Kerberos version 5. The OpenAFS community has introduced features that allow Kerberos 5 to be used for AFS in OpenAFS 1.2.8. Patches are available to add Kerberos 5 support to OpenSSH. Several other implementations of the SSH protocol also support Kerberos 5. Applications such as IMAP, POP and LDAP already support Kerberos 5.
3) TGT Key Separation
One motivation for the V4 triple DES support is that if a single DES key exists for the TGT principal then an attacker can attack that key both for v4 and v5 tickets. Kerberos implementations should gain support for a DES TGT key that is used for v4 requests but not v5 requests.
4) Remove Triple DES Kerberos 4 Support
The cut and paste attack is a critical failure in MIT's attempt at Kerberos 4 Triple DES. Even without cross-realm authentication, this can be exploited in real-world situations. As such the support for 3DES service keys should be disabled.
No statement is currently available from the vendor regarding this vulnerability.
The vendor has not provided us with any further information regarding this vulnerability.
WireX Communications, Inc. has released Immunix Secured OS Security Advisory IMNX-2003-7+-007-01 in response to this issue. Users are encouraged to review this advisory and apply the patches it refers to.
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