Security/Dollar Ticket Attack

The "Dollar Ticket Attack" / Name confusion in Kerberos

This page attempts to document the "Dollar Ticket Attack" on Active Directory servers and clients.

Many of these attacks are worse in Windows domains as MachineAccountQuota is still enabled by default.

CVEs and Announcements of the Nov 2021 issues

• Samba CVE-2020-25717 A user in an AD Domain could become root on domain members

• Samba CVE-2020-25719 Samba AD DC did not always rely on the SID and PAC in Kerberos tickets.

• Microsoft CVE-2021-42287 KB5008380—Authentication updates

• CERT NZ Announcement of AD DC security issue and patch

Talks

• SambaXP: The inside story on the dollar ticket attack - Youtube Video PDF (longer)

• Kawaiicon: Live stream replay @ Youtube PDF without embedded video (shorter, more focus on the remaining still open issues)

Blogs

• Stay Curious: Lessons from the Dollar-ticket security issue
• Catalyst Samba team fixes critical Microsoft security issue

Documentation

• MS-KILE 3.3.5.6.1 Client Principal Lookup
• RFC 4120 The Kerberos Network Authentication Service (V5)
• RFC6806 Kerberos Principal Name Canonicalization and Cross-Realm Referrals

=Still Open Issue in 'MIT Style' clients:

Fallback to 'bare' users

Just as Samba saw with CVE-2020-25717, the default principal to username mapping in MIT Kerberos is unsafe if user creation on the realm is not privileged.

Even if (eg) sssd or Samba provides a more complex account to local name plugin, the default behaviour remains to to map a user in the default realm to the matching user locally. It helps to avoid (eg) sssd mapping to an unprivileged account if that account is taken away before the ticket is presented.

The [https://web.mit.edu/kerberos/krb5-1.20/doc/admin/host_config.html MIT Kerberos Host configuration documentation describes the defaults as:

[realms]
   DEFAULT.REALM = {

...

       # Also allow principals from the default realm.  Omit this line
       # to only allow access to principals in OTHER.REALM.
       auth_to_local = DEFAULT
   }

MIT-style servers don't know a $ is missing

Unless the MIT-style kerberos acceptor enforces the presence of the PAC, and can read the PAC, using the extra information provided by Windows and Samba, it is not possible to know that the username has been stripped of the trailing $. This allows more opportunity for attack as there is no need for permission to delete the account, and still operates after Machine Accounts were restricted to keeping their trailing $

Other related issues

There are many other security issues, some related, that come form the same tree, including:

• Certifried: Active Directory Domain Privilege Escalation (CVE-2022–26923)
• Microsoft CVE-2021-42282 KB5008382—Verification of uniqueness for user principal name, service principal name, and the service principal name alias
• Samba AD DC did not do suffienct access and conformance checking of data stored (SPN/UPN uniqueness)