GMS-2023-1826: github.com/lestrrat-go/jwx vulnerable to Potential Padding Oracle Attack

Summary

Decrypting AES-CBC encrypted JWE has Potential Padding Oracle Attack Vulnerability.

Details

On v2.0.10, decrypting AES-CBC encrypted JWE may return an error “failed to generate plaintext from decrypted blocks: invalid padding”:

https://github.com/lestrrat-go/jwx/blob/8840ffd4afc5839f591ff0e9ba9034af52b1643e/jwe/internal/aescbc/aescbc.go#L210-L213

Reporting padding error causes Padding Oracle Attack Vulnerability. RFC 7516 JSON Web Encryption (JWE) says that we MUST NOT do this.

11.5. Timing Attacks To mitigate the attacks described in RFC 3218 [RFC3218], the recipient MUST NOT distinguish between format, padding, and length errors of encrypted keys. It is strongly recommended, in the event of receiving an improperly formatted key, that the recipient substitute a randomly generated CEK and proceed to the next step, to mitigate timing attacks.

In addition, the time to remove padding depends on the length of the padding. It may leak the length of the padding by Timing Attacks.

https://github.com/lestrrat-go/jwx/blob/796b2a9101cf7e7cb66455e4d97f3c158ee10904/jwe/internal/aescbc/aescbc.go#L33-L66

To mitigate Timing Attacks, it MUST be done in constant time.

Impact

The authentication tag is verified, so it is not an immediate attack.