CVE-2026-105794: MsQuic: Improper Certificate Validation in Microsoft.Native.Quic.MsQuic.OpenSSL

Should you act?

Critical and remotely reachable

No known exploitation yet, but it is network-reachable without credentials. This signal is computed from CISA KEV, EPSS and CVSS data; it is not an editorial verdict.

• No confirmed exploitation. It is not in CISA’s KEV catalog and CISA’s SSVC assessment records no known exploitation. That can change quickly after a public write-up; re-check if you are affected.
• EPSS 1.0%, percentile 62: FIRST’s model estimates a 1.0% chance of exploitation activity in the next 30 days, higher than 62% of all scored CVEs (moderate). EPSS is a probability, not a verdict.
• Remotely reachable with no login and no user interaction. If the vulnerable service is exposed to the internet, prioritize it.
• Only applies if your dependency tree includes Microsoft.Native.Quic.MsQuic.OpenSSL (NuGet) < 2.4.20, fixed in 2.4.20; Microsoft.Native.Quic.MsQuic.OpenSSL (NuGet) >= 2.5.0, < 2.5.11, fixed in 2.5.11; Microsoft.Native.Quic.MsQuic.OpenSSL (NuGet) >= 2.6.0, < 2.6.1, fixed in 2.6.1. Check your lockfile, including transitive dependencies.

What it is

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.

From the GitHub advisory

Summary

Improper TLS hostname verification allows a man-in-the-middle (MITM) attack on MsQuic.

Details

Only MsQuic with the OpenSSL and QuicTLS TLS backends is affected (the Schannel backend is not affected).

Patches

Weakness type: CWE-295.

Product: Microsoft.Native.Quic.MsQuic.OpenSSL (NuGet) · Assigned by: GitHub_M · Published: 2026-10-06

Affected and fixed versions

Severity scores

• Attack vector: network (remotely reachable)
• Attack complexity: low
• Attack requirements: present (depends on deployment conditions)
• Privileges required: none (no login)
• User interaction: none

CVSS measures how bad a flaw is if exploited, not how likely exploitation is. Read it next to the exploitation data below.

Exploitation and urgency

• CISA KEV: not listed (no confirmed in-the-wild exploitation known to CISA).
• CISA SSVC assessment: exploitation none, automatable no, technical impact partial.
• FIRST EPSS (model of 2026-10-21): 1.00% probability of exploitation activity in the next 30 days; percentile 62.

Timeline

• 2026-10-06 — CVE record last updated
• 2026-10-06 — CVE record published by GitHub_M
• 2026-10-06 — GitHub-reviewed advisory GHSA-w5f4-fx9m-m4q7 published (critical)

Sources and references

Original sources first. Version and fix details should always be confirmed against the vendor or project advisory.

Vulnerability records

1. GitHub Advisory GHSA-w5f4-fx9m-m4q7 github.com
2. CVE.org record CVE-2026-105794 cve.org
3. NVD entry CVE-2026-105794 nvd.nist.gov

Other references from the CVE record and advisory

1. github.com github.com
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