Researchers Release PoC Exploit for Windows CryptoAPI Bug Discovered by NSA

Jan 26, 2023Ravie LakshmananEncryption / Windows Security

Proof-of-concept (Poc) code has been released for a now-patched
high-severity security flaw in the Windows CryptoAPI that the U.S.
National Security Agency (NSA) and the U.K. National Cyber Security
Centre (NCSC) reported to Microsoft last year.

Tracked as CVE-2022-34689 (CVSS score: 7.5),
the spoofing vulnerability was addressed by the tech giant as part
of Patch Tuesday updates released in August 2022, but was only
publicly disclosed two months later on October 11, 2022.

“An attacker could manipulate an existing public x.509
certificate^[1] to spoof their identity
and perform actions such as authentication or code signing as the
targeted certificate,” Microsoft said^[2]
in an advisory released at the time.

The Windows CryptoAPI^[3]
offers an interface for developers to add cryptographic services
such as encryption/decryption of data and authentication using
digital certificates to their applications.

Web security company Akamai, which released^[4]
the PoC, said^[5]
CVE-2022-34689 is rooted in the fact that the vulnerable piece of
code that’s designed to accept an x.509 certificate carried out a
check that solely relied on the certificate’s MD5 fingerprint.

MD5, a message-digest algorithm used for hashing, is essentially
cryptographically broken^[6] as of December 2008
owing to the risk of birthday attacks^[7], a cryptanalytic method
used to find collisions in a hash function.

The net effect of this shortcoming is that it opens the door for
a bad actor to serve a modified version of a legitimate certificate
to a victim app, and then create a new certificate whose MD5 hash
collides with the rigged certificate and use it to masquerade as
the original entity.

In other words, the flaw could be weaponized by a rogue
interloper to stage a mallory-in-the-middle (MitM^[8]) attack and redirect
users relying on an old version of Google Chrome (version 48 and
earlier) to an arbitrary website of the actor’s choosing simply
because the susceptible version of the web browser trusts the
malicious certificate.

“Certificates play a major role in identity verification online,
making this vulnerability lucrative for attackers,” Akamai
said.

Although the flaw has a limited scope, the
Massachusetts-headquartered firm pointed out “there is still a lot
of code that uses this API and might be exposed to this
vulnerability, warranting a patch even for discontinued versions of
Windows, like Windows 7.”