Cryptographic Semantic Binding Flaw

ALTCHA libraries are vulnerable to a cryptographic semantic binding flaw. The vulnerability is due to ambiguous HMAC binding between challenge parameters and the nonce, which allows an attacker to splice or reinterpret a valid proof-of-work submission for example by modifying the expiration value...

EUVD-2025-203484

ALTCHA is privacy-first software for captcha and bot protection. A cryptographic semantic binding flaw in ALTCHA libraries allows challenge payload splicing, which may enable replay attacks. The HMAC signature does not unambiguously bind challenge parameters to the nonce, allowing an attacker to...