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Missing JWT signature check

ID: java/missing-jwt-signature-check
Kind: path-problem
Security severity: 7.8
Severity: error
Precision: high
Tags:
   - security
   - external/cwe/cwe-347
Query suites:
   - java-code-scanning.qls
   - java-security-extended.qls
   - java-security-and-quality.qls

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A JSON Web Token (JWT) consists of three parts: header, payload, and signature. The io.jsonwebtoken.jjwt library is one of many libraries used for working with JWTs. It offers different methods for parsing tokens like parse, parseClaimsJws, and parsePlaintextJws. The last two correctly verify that the JWT is properly signed. This is done by computing the signature of the combination of header and payload and comparing the locally computed signature with the signature part of the JWT.

Therefore it is necessary to provide the JwtParser with a key that is used for signature validation. Unfortunately the parse method accepts a JWT whose signature is empty although a signing key has been set for the parser. This means that an attacker can create arbitrary JWTs that will be accepted if this method is used.

Recommendation

Always verify the signature by using either the parseClaimsJws and parsePlaintextJws methods or by overriding the onPlaintextJws or onClaimsJws of JwtHandlerAdapter.

Example

The following example shows four cases where a signing key is set for a parser. In the first ‘BAD’ case the parse method is used, which will not validate the signature. The second ‘BAD’ case uses a JwtHandlerAdapter where the onPlaintextJwt method is overriden, so it will not validate the signature. The third and fourth ‘GOOD’ cases use parseClaimsJws method or override the onPlaintextJws method.

public void badJwt(String token) {
    Jwts.parserBuilder()
                .setSigningKey("someBase64EncodedKey").build()
                .parse(token); // BAD: Does not verify the signature
}

public void badJwtHandler(String token) {
    Jwts.parserBuilder()
                .setSigningKey("someBase64EncodedKey").build()
                .parse(plaintextJwt, new JwtHandlerAdapter<Jwt<Header, String>>() {
                    @Override
                    public Jwt<Header, String> onPlaintextJwt(Jwt<Header, String> jwt) {
                        return jwt;
                    }
                }); // BAD: The handler is called on an unverified JWT
}

public void goodJwt(String token) {
    Jwts.parserBuilder()
                .setSigningKey("someBase64EncodedKey").build()
                .parseClaimsJws(token) // GOOD: Verify the signature
                .getBody();
}

public void goodJwtHandler(String token) {
    Jwts.parserBuilder()
                .setSigningKey("someBase64EncodedKey").build()
                .parse(plaintextJwt, new JwtHandlerAdapter<Jws<String>>() {
                    @Override
                    public Jws<String> onPlaintextJws(Jws<String> jws) {
                        return jws;
                    }
                }); // GOOD: The handler is called on a verified JWS
}

References

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