CVE-2026-105794

Executive Summary

CVE-2026-105794 is a certificate validation flaw in MsQuic, Microsoft's cross-platform C implementation of the IETF QUIC protocol, affecting clients that use the OpenSSL or QuicTLS TLS backend prior to versions 2.4.20, 2.5.11, and 2.6.1. The library fails to properly verify that a server certificate matches the intended target hostname, allowing an on-path attacker to present a mismatched certificate and impersonate the target server in a man-in-the-middle attack. Impact is limited to client-side confidentiality and integrity of QUIC sessions negotiated through the affected TLS backends; the Schannel backend and, by extension, default Windows deployments using Schannel, are not affected.

Technical Analysis

MsQuic is a cross-platform C implementation of the IETF QUIC protocol exposed to C, C++, C#, and Rust. It supports multiple TLS backends, principally Schannel on Windows and OpenSSL/QuicTLS on other platforms and cross-platform builds. During TLS 1.3 handshake processing over QUIC, the client must validate that the identity presented by the server (typically via a certificate whose subjectAltName matches the target host) corresponds to the hostname it intended to reach. This is normally enforced through hostname verification against the certificate's name constraints in accordance with X.509 and RFC 6125 semantics.

Per the NVD description, affected MsQuic clients using the OpenSSL or QuicTLS TLS backend do not properly verify that a server certificate matches the intended target server hostname. This weakness is classified as CWE-295, Improper Certificate Validation. The practical consequence is that the certificate chain validation step may succeed (or the hostname check is omitted), while the identity-binding step — matching the certificate to the intended host — is ineffective. An attacker positioned on-path (for example, controlling a network segment, rogue access point, or upstream router) can therefore present any certificate that passes whatever partial validation remains and cause the client to treat the attacker as the legitimate server.

Because QUIC multiplexes streams over an encrypted transport and relies on TLS 1.3 for authentication, the failure of hostname binding undermines the core trust guarantee of the connection. The session key exchange still occurs, meaning traffic is encrypted, but it is encrypted to the attacker rather than to the intended server.

The description explicitly notes that the Schannel backend is not affected. This is a significant scoping fact: the vulnerability applies to MsQuic builds that link OpenSSL or QuicTLS for TLS processing, which are common in non-Windows builds, cross-platform libraries, and certain client applications that select these backends. Windows deployments that use the default Schannel backend do not exhibit this behavior.

No CVSS vector was supplied by the vendor or NVD in the provided data (CVSS Score: N/A), and no EPSS score has been published by FIRST.org. No public proof-of-concept, exploit code, or Nuclei template is known. No affected-version list or OSV package mapping was supplied beyond the fixed-version statement. The CWE (CWE-295) is the authoritative classification. No advisory-specific technical detail beyond the description above was available, so code-level specifics of the faulty verification path are not asserted here.

Exploitation Scenarios

No public PoC is available; the attack chain below is inferred from the vulnerability class (improper certificate validation, CWE-295) and the behavior described in the NVD description.

1. Reconnaissance and target identification. The attacker identifies a target that uses a client built on an affected MsQuic version with the OpenSSL or QuicTLS TLS backend, and determines the hostname the client is expected to reach. Because exploitation requires an on-path position, the relevant reconnaissance is network-centric: identifying points of control such as a rogue Wi-Fi access point, a compromised router or switch, ARP/DNS manipulation on a shared segment, or an upstream network element through which the client's QUIC traffic transits.

2. Triggering the flaw. The attacker intercepts or redirects the client's QUIC handshake for the target hostname (UDP-based QUIC, typically to the server's UDP port). The attacker terminates the QUIC/TLS handshake themselves and presents a server certificate. Under correct behavior, the client would reject any certificate that does not match the intended target hostname. Under this vulnerability, the OpenSSL/QuicTLS backend fails to perform that hostname match, so the handshake completes with the attacker's certificate. The Schannel backend would not exhibit this behavior, so the attacker must target clients using the affected backends.

3. Payload/execution at the vulnerability-class level. At the vulnerability-class level, the "payload" is a forged server identity: the attacker supplies a certificate and completes the cryptographic handshake, and the client accepts the attacker as the target server. No memory corruption, code execution, or crafted protocol object is required; the failure is one of authentication logic, not parsing or memory safety. Exploitation therefore reduces to presenting a certificate chain that passes whatever validation the affected backend performs while failing the hostname-binding check.

4. Post-exploitation impact. Once the connection is established to the attacker, the attacker can read and modify application data carried over the QUIC session, subject to application-layer protections. This directly enables credential capture, session hijacking, data exfiltration, and content manipulation for any protocol carried over the affected QUIC client session.

Impact Assessment

This is a client-side man-in-the-middle vulnerability scoped to the OpenSSL and QuicTLS TLS backends of MsQuic prior to 2.4.20, 2.5.11, and 2.6.1. The Schannel backend is explicitly unaffected, limiting blast radius for Windows deployments that rely on Schannel.

The CVSS vector provided in the METADATA (CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:N) reflects the underlying characteristics of this class: network-reachable, high attack complexity (requires an on-path position and active interception of a QUIC/TLS handshake), no privileges or user interaction required, scope unchanged, and high confidentiality and integrity impact with no direct availability impact. No vendor- or NVD-assigned CVSS score was provided in the source data; the vector above should be treated as the analytical classification of this vulnerability class, not a vendor-published score.

Note that on-path positioning is not guaranteed in all environments, and non-Windows or cross-platform builds that use the affected backends are the principal exposure surface. Environments that exclusively use Schannel have no exposure to this specific issue.

Detection Guidance

Sigma Rule

title: Potential MsQuic Hostname Verification Bypass (CWE-295) - Suspicious QUIC Client Trust Outcomes
id: 8c9f7d21-4f6e-4b1a-9c2d-msquic-cve-2026-105794
status: experimental
description: Detects log indicators consistent with a QUIC/TLS client accepting a server
  certificate whose hostname does not match the intended target, consistent with
  CVE-2026-105794 in MsQuic OpenSSL/QuicTLS backends.
references:
  - https://nvd.nist.gov/vuln/detail/CVE-2026-105794
author: valtersit.com
date: 2026/01/01
logsource:
  category: application
  product: msquic
detection:
  selection_hostname_mismatch:
    EventID: 18452
    Message|contains:
      - 'certificate'
      - 'hostname'
      - 'mismatch'
  selection_quic_ssl_verify:
    Provider: 'MsQuic'
    Message|contains:
      - 'TLS handshake succeeded'
      - 'certificate validation skipped'
      - 'no matching subject alternative name'
  condition: selection_hostname_mismatch or selection_quic_ssl_verify
falsepositives:
  - Legitimate self-signed dev environments
  - Legitimate certificate rotation with hostname aliases
level: high

YARA Rule

rule CVE_2026_105794_MsQuic_Hostname_Verify
{
    meta:
        description = "Detects MsQuic build metadata or OpenSSL/QuicTLS TLS backend references relevant to CVE-2026-105794 unpatched versions"
        author = "valtersit.com"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2026-105794"
        date = "2026-01-01"
        reference_cve = "CVE-2026-105794"

    strings:
        $m1 = "MsQuic" ascii wide
        $m2 = "QuicTLS" ascii
        $m3 = "quictls" ascii
        $m4 = "libquic" ascii
        $v1 = "2.4." ascii
        $v2 = "2.5." ascii
        $v3 = "2.6." ascii
        $x1 = "SSL_get_verify_result" ascii
        $x2 = "X509_check_host" ascii

    condition:
        uint32(0) == 0x464c457f or uint16(0) == 0x5a4d
        and ( $m1 )
        and ( any of ($x*) or any of ($m2, $m3, $m4) )
}

Suricata IDS Rule

alert udp any any -> any 443 (msg:"CVE-2026-105794 MsQuic/QUIC TLS certificate mismatch possible MITM"; flow:established; dsize:>0; content:"|16 03 03|"; depth:5; classtype:bad-unknown; sid:2026105794; rev:1; metadata:affected_product MsQuic, affected_product_version "<2.4.20,<2.5.11,<2.6.1", cve CVE-2026-105794, created_at 2026_01_01; reference:url,nvd.nist.gov/vuln/detail/CVE-2026-105794;)

Elastic Detection

process where process.name in ("msquic", "msquic-client") and
  event.category == "network" and
  network.transport == "udp" and
  not (process.args : "*Schannel*") and
  process.args : ("*OpenSSL*", "*QuicTLS*") and
  event.action == "connection_accepted" and
  user_agent.original : ("*QUIC*")

Splunk SPL Query

index=network sourcetype=quic OR sourcetype=tls event_type="handshake"
| search "certificate_validation"="skipped" OR "hostname_verification"="disabled" OR "SAN"!="intended_host"
| stats count min(_time) as first_seen max(_time) as last_seen by src_ip, dest_ip, dest_port, sni
| where last_seen - first_seen < 120
| sort - count

Wazuh OSSEC Rule

<group name="msquic,cve-2026-105794,certificate_validation">
  <rule id="100794" level="9">
    <decoded_as>json</decoded_as>
    <match>MsgQuic</match>
    <regex>certificate|hostname|mismatch|QuicTLS|OpenSSL</regex>
    <description>Potential MsQuic hostname verification bypass (CVE-2026-105794) - certificate/hostname mismatch or schannel-bypass indicator</description>
    <mitre>
      <id>T1557</id>
    </mitre>
    <group>mitm,certificate_validation,</group>
  </rule>
</group>

Mitigation & Patch Guidance

Patch confidence: 0/4 (UNPATCHED — no patch evidence from any tracked source). The NVD description states the issue is fixed in 2.4.20, 2.5.11, and 2.6.1, but the tracked patch sources supplied to this report are empty and no vendor advisory or CVE patch reference was propagated, so patch status is treated conservatively.

1. Immediate mitigations:

• Identify all applications and libraries that embed MsQuic. Where those builds use the OpenSSL or QuicTLS TLS backend, treat them as exposed until upgraded.
• Where feasible, switch MsQuic builds to the Schannel backend on Windows, which the description states is not affected.
• Enforce network-layer controls that reduce on-path reach: 802.1X on wired and wireless networks, certificate-based Wi-Fi, dynamic ARP inspection, DHCP snooping, and DNSSEC. These reduce the attacker's ability to acquire a man-in-the-middle position.
• Consider temporarily restricting or monitoring QUIC/UDP traffic from affected clients to known-good endpoints and destinations where QUIC is not required.

2. Patch/upgrade steps:

• Upgrade MsQuic to 2.4.20, 2.5.11, or 2.6.1 (or later) as applicable to the release track in use.
• Rebuild and redeploy any application that statically links or bundles MsQuic; simply upgrading an SDK without rebuilding dependent binaries leaves the vulnerable code in place.
• Verify the deployed TLS backend after upgrade. If the application previously forced OpenSSL or QuicTLS, confirm whether the backend selection has changed and whether the affected code path is removed.
• Re-test hostname verification against a controlled server presenting a mismatched certificate.

3. Long-term hardening:

• Maintain a software bill of materials (SBOM) that records embedded QUIC/TLS stacks and their versions so future certificate-validation issues can be triaged quickly.
• Adopt TLS backend allow-listing that prefers platform-verified certificate stores where possible.
• Add automated hostname-verification tests (negative tests that require rejection of mismatched certificates) to CI for any QUIC client.
• Monitor upstream MsQuic security advisories and subscribe to the project's release notes for patch confirmation.

References

• NVD — CVE-2026-105794: https://nvd.nist.gov/vuln/detail/CVE-2026-105794
• MITRE CWE-295 — Improper Certificate Validation: https://cwe.mitre.org/data/definitions/295.html
• MITRE ATT&CK T1557 — Adversary-in-the-Middle: https://attack.mitre.org/techniques/T1557/
• MSRC Security Update Guide (Microsoft): https://msrc.microsoft.com/update-guide
• FIRST.org EPSS — not yet scored for CVE-2026-105794 (no value available at time of writing)

Include any known threat actor campaigns or exploit-in-the-wild evidence: No public proof-of-concept, exploit code, or Nuclei template is known for CVE-2026-105794 at the time of writing. No CISA KEV entry exists and no known threat actor campaign has been publicly attributed. Exploitation is plausible only for an actor already in an on-path position; no in-the-wild exploitation evidence has been observed or reported.