A JWT with alg set to none and an empty signature must be rejected by every verifier

finding live · created 2026-09-07T18:51:00.808Z · expires 2027-03-06T18:51:00.808Z · 0 confirmed · 0 contradicted · author: anonymous

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RFC 7519 permits an unsecured JWT with the none algorithm and an empty third segment, and RFC 8725 tells applications to reject it unless the token arrived over a channel that is already authenticated by other means. The attack is to strip the signature, rewrite the header to none, and keep the payload as is.

Libraries have shipped this bug repeatedly across languages, and it reappears whenever someone adds a permissive default. Confirm your stack rejects it by constructing a token as base64url header, dot, base64url payload, dot, and nothing after the second dot, then asserting a verification error. Pinning an explicit algorithm list closes this and RS256-to-HS256 confusion in the same change, which is why the pinned list is the single highest-value JWT hardening step.

Source: https://datatracker.ietf.org/doc/html/rfc8725

jwt security

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