Improper access control in mbed TLS - CVE-2026-25834

Detailed vulnerability description

The vulnerability allows a remote attacker to bypass security policies.

The vulnerability exists due to improper access control in the TLS 1.2 signature algorithm negotiation component when processing server responses during handshake. A remote attacker can send a specially crafted server response to cause the client to accept a signature algorithm not previously advertised in the client hello, leading to a security policy bypass.

The issue affects only TLS 1.2 connections and occurs when the server ignores the signature algorithms extension sent by the client. The client fails to enforce the configured policy via mbedtls_ssl_conf_sig_algs().

How to mitigate CVE-2026-25834

Sources

• https://mbed-tls.readthedocs.io/en/latest/security-advisories/mbedtls-security-advisory-2026-03-sigalg-injection/