Security Experts:

Enterprises Warned of New PetitPotam Attack Exposing Windows Domains

Enterprises have been warned of a new attack method that can be used by malicious actors to take complete control of a Windows domain.

The technique was disclosed last week after France-based security researcher Lionel Gilles (aka Topotam) posted a proof-of-concept (PoC) exploitation tool named PetitPotam on GitHub.

PetitPotam abuses the Encrypting File System Remote (MS-EFSRPC) protocol, which is designed for performing maintenance and management operations on encrypted data that is stored remotely and accessed over a network. An unauthenticated attacker can use PetitPotam to get a targeted server to connect to their server and perform NTLM authentication.

PetitPotam can be chained with an exploit targeting Active Directory Certificate Services (AD CS), which provides public key infrastructure (PKI) functionality.

“An attacker can target a Domain Controller to send its credentials by using the MS-EFSRPC protocol and then relaying the DC [domain controller] NTLM credentials to the Active Directory Certificate Services AD CS Web Enrollment pages to enroll a DC certificate,” cybersecurity firm Truesec explained in a blog post. “This will effectively give the attacker an authentication certificate that can be used to access domain services as a DC and compromise the entire domain.”

The SANS Institute’s Internet Storm Center has published a step-by-step description of the attack.

After the PoC was released, Microsoft published an advisory saying that PetitPotam “is a classic NTLM Relay Attack,” noting that the company previously provided numerous mitigation options for such attacks.

“To prevent NTLM Relay Attacks on networks with NTLM enabled, domain administrators must ensure that services that permit NTLM authentication make use of protections such as Extended Protection for Authentication (EPA) or signing features such as SMB signing,” Microsoft said. “PetitPotam takes advantage of servers where Active Directory Certificate Services (AD CS) is not configured with protections for NTLM Relay Attacks.”

Mitigations recommended by Microsoft include disabling NTLM authentication on Windows domain controllers or enabling the Extended Protection for Authentication (EPA) feature on AD CS services.

However, some members of the cybersecurity industry are not happy with Microsoft’s response.

SANS’s Bojan Zdrnjan noted, “What [Microsoft’s advisory] missed is the fact that the PetitPotam vulnerability ... is a completely separate issue — it allows an attacker to provoke a server to authenticate to an arbitrary machine. Abusing AD CS is just one way to use this — any service that allows NTLM authentication can probably be abused similarly (Print Spooler could be a candidate).”

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Eduard Kovacs (@EduardKovacs) is a contributing editor at SecurityWeek. He worked as a high school IT teacher for two years before starting a career in journalism as Softpedia’s security news reporter. Eduard holds a bachelor’s degree in industrial informatics and a master’s degree in computer techniques applied in electrical engineering.