Difference between revisions of "Security/Dollar Ticket Attack"

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* [https://datatracker.ietf.org/doc/html/rfc4120 RFC 4120 The Kerberos Network Authentication Service (V5)]
 
* [https://datatracker.ietf.org/doc/html/rfc4120 RFC 4120 The Kerberos Network Authentication Service (V5)]
 
* [https://www.rfc-editor.org/rfc/rfc6806.html RFC6806 Kerberos Principal Name Canonicalization and Cross-Realm Referrals]
 
* [https://www.rfc-editor.org/rfc/rfc6806.html RFC6806 Kerberos Principal Name Canonicalization and Cross-Realm Referrals]
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* [https://cloudbrothers.info/en/exploit-kerberos-samaccountname-spoofing/ Exploits]
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* [https://exploit.ph/cve-2021-42287-cve-2021-42278-weaponisation.html Research]
   
 
=Still Open Issue in 'MIT Style' clients: =
 
=Still Open Issue in 'MIT Style' clients: =

Revision as of 01:45, 11 July 2022

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

Talks

Blogs

Documentation

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 $

Impact

A user able to create an account 'root$' in AD, via MachineAccountQuota, can present a ticket to the target as 'root' and access (eg) SSH as root in default domain-joined machines using SSSD and 'realm', plus many similar and simpler custom per-deployment configurations.

Exploit steps

Environment

  • Windows 2022 (fully patched) AD DC
    • with Certificate services for LDAPS support
  • Fedora 36 target
    • Root account enabled at install
    • joined to the AD domain
    • realm join -U administrator win22.example.com –computer-name=fedora

Steps

kinit andrew

addcomputer.py -debug -k -dc-host abartlet-dc-win-22.win22.example.com WIN22.EXAMPLE.COM/andrew -method LDAPS -computer-name root

kinit root

ssh -v -o PreferredAuthentications=gssapi-with-mic -l root fedora

Other related issues

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