cdoco:php-jwt.md

cdoco/php-jwt exists insecure signature verification

0x01 Affected version

vendor: https://github.com/cdoco/php-jwt

version: 1.0.0

0x02 Vulnerability description

The jwt_verify_sha_hmac function of openssl.c implements the function of comparing the HS256 algorithm signature.

However, the function uses strcmp to compare the calculated signature with the JWT signature we provided, and strcmp is not a time-safe comparison function.

https://github.com/GlitchedPolygons/l8w8jwt/blob/b24083d920c93a2f46f30d3d3d7a2663ac19ca09/src/decode.c#L488

This means that the more similar the prefix of the signature we provide to the actual calculated signature, the longer the comparison will take, which can lead to temporal side-channel attacks. An attacker can learn the correct signature and bypass verification.