Clatter, a Rust implementation of the Noise protocol, contains a protocol compliance flaw in versions before 2.2.0. The library accepted post‑quantum handshake patterns that violate the PSK validity rule, allowing PSK‑derived keys to be combined with self‑chosen nonce values without proper randomization. This undermines the integrity of encryption and can enable catastrophic key reuse, effectively erasing the confidentiality guarantees of the protocol.

The vulnerability affects the "jmlepisto:clatter" library in all releases older than 2.2.0. Default post‑quantum handshake patterns, such as noise_pqkk_psk0, noise_pqkn_psk0, noise_pqnk_psk0, noise_pqnn_psk0 and several hybrid variants, are impacted. Users incorporating these patterns in their applications are exposed.

The CVSS score of 8 indicates a high severity, while the EPSS score of less than 1 % suggests a low likelihood of exploitation in the wild. The flaw is not listed in the CISA KEV catalog. Exploitation requires an application to use the vulnerable library in a handshake that contains a *psk0* pattern; an attacker who can influence the handshake parameter selection could obtain a weak key and conduct cryptographic attacks. The likely attack vector is through libraries used in server or client software that opts for post‑quantum Noise patterns without verifying the PSK validity rule.