MsQuic clients that use the OpenSSL or QuicTLS TLS backends failed to verify that a server certificate matches the intended target hostname, allowing an attacker to present a fraudulent certificate and impersonate the server. This flaw enables a man‑in‑the‑middle attack without the need for privileged access or code execution on the client machine.

Affected systems include Microsoft MsQuic prior to release 2.4.20, 2.5.11, and 2.6.1 when using OpenSSL or QuicTLS backends. The Schannel backend is not vulnerable. The issue is fixed in the three mentioned releases.

The CVSS base score of 9.1 indicates critical severity, while the EPSS score of 1% suggests that exploitation is currently rare but not impossible. The flaw is not listed in the CISA KEV catalog. An attacker who can position themselves as an on‑path entity can perform a man‑in‑the‑middle attack by presenting a fraudulent certificate that bypasses hostname validation. The vulnerability is exploitable remotely over any network that permits interception of the TLS handshake, making it highly relevant to public‑facing services using MsQuic without the latest patches.