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Q & A Security · September 11, 2026

What is the 'alg: none' JWT vulnerability?

JWT libraries historically accepted ‘alg: none’ as a valid signing algorithm — meaning no signature was required at all. Attackers could strip the signature, set alg=none in the header, and forge any claims. Modern libraries reject it by default. Always allowlist the expected algorithm when verifying — do not trust the alg field from the token.

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What Is a JWT? JSON Web Token Structure, Signing, and Security Explained
JWT (JSON Web Token) is the dominant format for API authentication, OAuth access tokens, and stateless session management. Three base64-encoded segments carry the header, claims, and signature — anyone can read them, but only the key holder can produce a valid token.
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