Description
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.
References10
- github.com/microsoft/msquic/commit/0591586443cc73a2d2cfb679527d…
- github.com/microsoft/msquic/commit/508e811370df93e2ad848f5c7834…
- github.com/microsoft/msquic/commit/90fd45498bf9b506b8556fb40708…
- github.com/microsoft/msquic/commit/a01333cf7c2659cce0ff03ef3f21…
- github.com/microsoft/msquic/pull/6274
- github.com/microsoft/msquic/pull/6275
- github.com/microsoft/msquic/pull/6276
- github.com/microsoft/msquic/pull/6277
- github.com/microsoft/msquic/releases/tag/v2.4.20
- github.com/microsoft/msquic/releases/tag/v2.5.11
Metrics
- CVSS 4.0
-
9.1
CRITICAL
Score source: CNA
CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:H/VI:H/VA:N/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X - EPSS — exploit probability
- 1% percentile: 63 · as of 10 November 2026
- CWE
- CWE-295
Could this be exploited in your environment?
A patch closes the known hole. A penetration test shows what an attacker can still reach — with proof.