When Stronger Crypto Breaks Auth: The FreshRSS Bcrypt Truncation Bypass

The article explains an authentication bypass in FreshRSS caused by an unintended interaction between a longer SHA-256 nonce and bcrypt’s 72-byte input limit. Because bcrypt truncates input beyond 72 bytes, the system ended up hashing only non–password-dependent data, allowing any password to succeed. The issue came from a well-intentioned crypto “upgrade” that increased nonce length, breaking assumptions in a custom challenge-response flow. The fix was simply reordering inputs so the password-dependent hash is included. The case highlights how combining secure primitives incorrectly can introduce critical vulnerabilities.

https://pentesterlab.com/blog/freshrss-bcrypt-truncation-auth-bypass

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