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Security advisories — Jul 26, 2021

PetitPotam NTLM Relay Attack

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THE THREAT

PetitPotam is a variant of NTLM Relay attacks discovered by security researcher Gilles Lionel. Proof of Concept code released last week [1] relies on the Encrypting File System Remote (EFSRPC) protocol to provoke a Windows host into performing an NTLM authentication request against an attacker-controlled server, exposing NTLM authentication details or authentication certificates. This information can be used by an attacker to elevate their privileges in a domain environment.

Microsoft has released mitigations for the attack. Security updates are not available at time of writing. In the wild attacks abusing PetitPotam have not been observed at this time.

What we’re doing about it

What you should do about it

In response to PetitPotam, Microsoft has released mitigations for this attack [2].

These include:

  1. Disabling NTLM authentication in the environment
  2. Disabling NTLM on Active Directory Certificate Services (AD CS) servers
  3. Enable Extended Protection for Authentication (EPA) on AD CS servers if NTLM cannot be disabled.

Detailed mitigation steps are available in Microsoft’s knowledge base article [2].

Additional information

PetitPotam is a form of NTLM Relay attack which abuses the EFSRPC protocol. In an attack scenario, an attacker with local network access would abuse this protocol to coerce an NTLM authentication request to a server under an attacker’s control. Researchers have successfully relayed NTLM requests retrieved using PetitPotam to Active Directory Certificate Services (AD CS) servers to sign a certificate using the relayed NTLM credentials. An attacker can then leverage this signed certificate to elevate their access and compromise the domain. This attack method is described in detail by SANS [3].

Known attack methods abusing PetitPotam require the following conditions to be met:

  1. NTLM Authentication is enabled on your domain
  2. Active Directory Certificate Service is active on your network
  3. Either of the following services are enabled on AD CS:
    1. Certificate Authority Web Enrollment
    2. Certificate Enrollment Web Service

References:

[1] https://github.com/topotam/PetitPotam
[2] https://support.microsoft.com/en-gb/topic/kb5005413-mitigating-ntlm-relay-attacks-on-active-directory-certificate-services-ad-cs-3612b773-4043-4aa9-b23d-b87910cd3429
[3] https://isc.sans.edu/diary/27668

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