CVE-2026-105794 in MsQuic
Summary
by MITRE • 10/06/2026
MsQuic is a cross-platform C implementation of the IETF QUIC protocol exposed to C, C++, C#, and Rust. Prior to 2.4.20, 2.5.11, and 2.6.1, MsQuic clients using the OpenSSL or QuicTLS TLS backend do not properly verify that a server certificate matches the intended target server hostname. An on-path attacker can therefore present a certificate that does not match the intended target hostname and spoof the server in a man-in-the-middle attack. The Schannel backend is not affected. This issue is fixed in versions 2.4.20, 2.5.11, and 2.6.1.
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Analysis
by VulDB Data Team • 10/06/2026
The MsQuic library serves as a critical cross-platform implementation of the IETF QUIC protocol, providing essential networking capabilities for applications developed in C, C++, C#, and Rust environments. A significant security deficiency was identified within specific versions of this library, namely those prior to 2.4.20, 2.5.11, and 2.6.1, when utilizing the OpenSSL or QuicTLS TLS backends. The core technical flaw resides in the certificate validation logic during the handshake process. Specifically, these implementations failed to properly verify that the server presented a X.509 certificate containing a Subject Alternative Name (SAN) or Common Name (CN) that matches the intended target hostname established by the client application. This omission represents a fundamental breakdown in identity verification mechanisms required for secure transport layer communications.
This vulnerability creates a severe risk of man-in-the-middle attacks where an adversary positioned on the network path can intercept and manipulate traffic between the client and server. By presenting a valid but mismatched certificate, typically obtained from a Certificate Authority or self-signed without proper hostname constraints, the attacker can successfully spoof the target server. Because the MsQuic implementation does not enforce strict hostname matching for these specific backends, the client application will accept the fraudulent connection as legitimate. This allows the attacker to decrypt, read, and potentially modify sensitive data transmitted over the QUIC protocol, effectively bypassing encryption protections intended to ensure confidentiality and integrity.
The operational impact of this flaw is substantial, particularly in enterprise environments where automated services or applications rely on MsQuic for high-performance network communication. An adversary exploiting this vulnerability can perform session hijacking, credential theft, or data exfiltration without triggering standard security alerts associated with certificate errors. It is important to note that the Schannel backend, which relies on Windows native cryptographic APIs, remains unaffected by this specific issue due to its different implementation of TLS validation logic. However, organizations utilizing OpenSSL or QuicTLS backends are exposed to significant risk until an upgrade is performed.
To mitigate this vulnerability, it is imperative for all affected systems and applications to update the MsQuic library to version 2.4.20, 2.5.11, or later, depending on the specific branch in use. These updated versions correct the certificate verification logic to ensure that hostname validation is strictly enforced regardless of the TLS backend employed. In addition to software updates, security teams should audit their dependency management processes to prevent the inclusion of vulnerable library versions in production environments. Monitoring network traffic for anomalies related to QUIC connections and ensuring that internal Certificate Authorities enforce strict Subject Alternative Name policies can provide additional layers of defense against exploitation attempts.
From a classification perspective, this vulnerability aligns with CWE-295 Improper Certificate Validation, as it involves the failure to correctly validate an X.509 certificate presented by a remote entity. Furthermore, in the context of the MITRE ATT&CK framework, this flaw facilitates techniques associated with Man-in-the-Middle attacks under T1557, allowing adversaries to intercept and potentially alter communications between two parties who believe they are directly communicating with each other. The lack of hostname verification is also related to CWE-346 Origin Validation Error, highlighting the critical importance of verifying that a cryptographic identity corresponds to the expected network endpoint.