FreeRDP up to 3.28.0 certificate validation

Summaryinfo

A vulnerability was found in FreeRDP up to 3.28.0. It has been classified as problematic. Affected by this issue is the function tls_verify_certificate/tls_match_hostname/x509_utils_get_dns_names. This manipulation causes certificate validation. This vulnerability appears as CVE-2026-66402. The attack may be initiated remotely. There is no available exploit.

Detailsinfo

A vulnerability was found in FreeRDP up to 3.28.0. It has been declared as problematic. Affected by this vulnerability is the function tls_verify_certificate/tls_match_hostname/x509_utils_get_dns_names. The manipulation with an unknown input leads to a certificate validation vulnerability. The CWE definition for the vulnerability is CWE-295. The product does not validate, or incorrectly validates, a certificate. As an impact it is known to affect integrity. The summary by CVE is:

FreeRDP before 3.29.0 (affected versions <= 3.28.0) contains multiple TLS certificate identity validation weaknesses in tls_verify_certificate(), tls_match_hostname(), and x509_utils_get_dns_names(). Because FreeRDP performs custom Common Name and DNS SAN string matching instead of using OpenSSL's length-aware identity validation APIs, it (1) truncates DNS SAN values at embedded NUL bytes (accepting e.g. 'victim.example\0.attacker.example' as 'victim.example'), (2) accepts a matching Common Name even when non-matching DNS SAN entries are present, and (3) accepts IP-literal targets via DNS/CN matching without comparing iPAddress SANs. Under a trusted or misissued certificate chain, an attacker positioned to present such a certificate can bypass server identity verification, weakening TLS server authentication.

It is possible to read the advisory at github.com. This vulnerability is known as CVE-2026-66402 since 07/26/2026. The exploitation appears to be difficult. The attack can be launched remotely. The exploitation doesn't need any form of authentication. Technical details of the vulnerability are known, but there is no available exploit. The attack technique deployed by this issue is T1587.003 according to MITRE ATT&CK.

The vulnerability scanner Nessus provides a plugin with the ID 331610 (Linux Distros Unpatched Vulnerability : CVE-2026-66402), which helps to determine the existence of the flaw in a target environment.

Upgrading to version 3.29.0 eliminates this vulnerability.

The vulnerability is also documented in the databases at Tenable (331610) and EUVD (EUVD-2026-51851). Statistical analysis made it clear that VulDB provides the best quality for vulnerability data.

Productinfo

Type

• Remote Access Software

Name

• FreeRDP

Version

• 3.0
• 3.1
• 3.2
• 3.3
• 3.4
• 3.5
• 3.6
• 3.7
• 3.8
• 3.9
• 3.10
• 3.11
• 3.12
• 3.13
• 3.14
• 3.15
• 3.16
• 3.17
• 3.18
• 3.19
• 3.20
• 3.21
• 3.22
• 3.23
• 3.24
• 3.25
• 3.26
• 3.27
• 3.28.0

License

• open-source

Website

• Product: https://github.com/FreeRDP/FreeRDP/

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Statistical analysis made it clear that VulDB provides the best quality for vulnerability data.

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