Bug 2176406

Support for the PAC extended KDC signature (or "full PAC checksum") has recently been implemented upstream[1] for MIT Kerberos. The fix for bug 2169477 backports the krb5_kdc_sign_ticket() function[2] to krb5 1.18.2. This function is a replacement for krb5_pac_sign() and krb5_pac_sign_ext() functions[3], which are currently used in IPA 4.9.11[4]. These two functions do not generate the extended KDC signature, only krb5_kdc_sign_ticket() does. This is the reason why we are backporting it.

The krb5_kdc_sign_ticket() function was initially introduced[5] to support the new "ticket signature" which was introduced by Microsoft[6] to mitigate CVE-2020-17049 (bronze bit attack). This is the reason why this function expects a krb5_enc_tkt_part[7] parameter representing the encrypted part of the ticket out of which the "ticket signature" is generated.

So, the krb5_kdc_sign_ticket() function generates both the "ticket signature" and the "extended KDC signature", and needs the encrypted part of the ticket.

However, on IPA 4.9.11 the PAC is generated by setting the sign_authdata() callback[8][9]. This callback does not provide the ticket's encrypted part as parameter, making krb5_kdc_sign_ticket() not usable in practice.

To be mentioned that, according to the document[10] that made CVE-2022-37967 public, the ticket signature does not have to be present for the PAC to be accepted.


[1]  https://github.com/krb5/krb5/commit/4602a10dbe380d75d1ec00f7d34479ac9d503735
[2]  https://github.com/krb5/krb5/blob/e35b32f81f9defbcce4f2398d93a975ffb807ee7/src/include/krb5/krb5.hin#L8365-L8393 
[3]  https://github.com/krb5/krb5/blob/e35b32f81f9defbcce4f2398d93a975ffb807ee7/src/include/krb5/krb5.hin#L8351-L8363
[4]  https://pagure.io/freeipa/blob/ipa-4-9/f/daemons/ipa-kdb/ipa_kdb_mspac_v6.c#_179
[5]  https://github.com/krb5/krb5/commit/ee4e3c5c9eee061048d5b7393b8f3820d1a563a8
[6]  https://winprotocoldoc.blob.core.windows.net/productionwindowsarchives/MS-PAC/%5bMS-PAC%5d-201123-diff.pdf
     Page 26, section 2.8.3
[7]  https://github.com/krb5/krb5/blob/krb5-1.20.1-final/src/include/krb5/krb5.hin#L1927-L1938
[8]  https://github.com/krb5/krb5/blob/4c3063afb4a4b141b70d56aa381784f7efd637c9/src/include/kdb.h#L1277-L1360
[9]  https://pagure.io/freeipa/blob/ipa-4-9/f/daemons/ipa-kdb/ipa_kdb.c#_725
[10] https://i.blackhat.com/EU-22/Thursday-Briefings/EU-22-Tervoort-Breaking-Kerberos-RC4-Cipher-and-Spoofing-Windows-PACs-wp.pdf
     Page 6, 1st paragraph