JWT alg Allowlist Policy Checker

A sloppily configured JWT library trusts whatever algorithm the token declares. That hands the attacker the choice of how their own token gets verified, which is where the two classic attacks come from: switch alg to none and drop the signature entirely, or switch RS256 to HS256 and sign with the public key, which is, by definition, public. The fix is a fixed list of algorithms the server accepts.

What happens here: the first segment is decoded, the alg field of the header is read, and it is matched against the comma separated list you type. The match is exact and case sensitive, so rs256 will not match RS256, and it pays to write the list exactly the way your library spells it. The alg=none case gets its own message, with the CVE-2015-9235 reference attached.

Worth stating what this does not do: it never verifies the signature, never fetches a key from a JWKS endpoint, and knows nothing about JWE, where alg describes key management and the content algorithm lives in enc. It is a policy check, not an authenticity check. Use it during review to confirm that the tokens reaching your service line up with the allowlist you configured. The header is decoded in the browser.