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Hello all!
While doing research on LDAP client certificate authentication, I realize that the LDAP implementation of Active Directory supports the StartTLS mechanism, which has interesting implications on relay attacks.
TL;DR: Active Directory LDAP implements StartTLS and it can be used to bypass the Channel Binding requirement of LDAPS for some relay attacks such as the creation of a machine account if LDAP signing is not required by the domain controller.
More info in the accompanying blog post.
Example
Relaying to LDAPS with Channel Binding:
With this PR, ntlmrelayx uses StartTLS and the attack works:
Keep in mind that this bypass only works on domain controller with LDAP signing not required.
🌻