JWT verification with explicit algorithm pin — defense against alg-confusion

What it is

JWT alg-confusion: if your verify function accepts both RS256 and HS256, an attacker re-signs the token using the public key as an HS256 secret + your verify accepts it.

Vulnerable example

// vulnerable: accepts any algorithm
const payload = jwt.verify(token, publicKey);

Fixed example

// pin algorithm + issuer + audience
const payload = jwt.verify(token, publicKey, {
  algorithms: ["RS256"],          // pin algorithm
  issuer: "https://your-issuer.com",
  audience: "your-app-id",
  clockTolerance: 30,
});

How Securie catches it

// pin algorithm + issuer + audience
const payload = jwt.verify(token, publicKey, {
  algorithms: ["RS256"],          // pin algorithm
  issuer: "https://your-issuer.com",
  audience: "your-app-id",
  clockTolerance: 30,
});

Checklist

• algorithms[] explicitly set
• issuer + audience verified
• clockTolerance set
• Signing key from secure source (not request)

FAQ

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