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Security advisories — Jul 29, 2021

UPDATE: PetitPotam NTLM Relay Attack

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THE THREAT

PetitPotam is a variant of the NTLM Relay attack discovered by security researcher Gilles Lionel. It is tracked as an authentication bypass vulnerability in Active Directory (Certificate Services); Microsoft tracks this vulnerability as CVE-2021-36942. Proof of Concept (PoC) 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. Exploitation would require previous access to a vulnerable device.

In response to the release of PetitPotam PoC code, eSentire has deployed new detection content via eSentire's Managed Log service. Microsoft has released mitigations for the attack. Security patches were released on August 10th, 2021. In the wild attacks abusing PetitPotam have not been observed at this time.

What we’re doing about it

UPDATE:

What you should do about it

After performing a business impact review, apply the relevant security patches provided by Microsoft for CVE-2021-36942.

In cases where patching is not possible, Microsoft has released alternative 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 the Microsoft 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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