Key Exchange Downgrade Vulnerability in Rustls by Rust

CVE-2026-123611

What is CVE-2026-123611?

Rustls versions prior to 0.23.46 are susceptible to a key exchange downgrade vulnerability. This security flaw allows attackers to exploit the unauthenticated ServerHello mechanism, enabling them to manipulate the per-server-name key-exchange group record. By responding with a ServerHello that selects X25519, an attacker can influence subsequent ClientHellos to transmit only classical shares, thereby compromising post-quantum confidentiality and altering the expected secure communication.

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