rules:
  - id: auth.go.jwt.unchecked-method
    languages:
      - go
    severity: ERROR
    message: |
      A JWT `Keyfunc` returns the verification key without checking `token.Method`, enabling algorithm confusion.
      It is passed to `jwt.Parse`/`jwt.ParseWithClaims` and hands back the key
      without first asserting the signing algorithm. If the server verifies
      RS256 tokens with an RSA public key, an attacker can forge an HS256 token
      using that public key as the HMAC secret, and the library will accept it:
      a complete authentication bypass (CWE-347).

      Always assert the signing method inside the keyfunc before returning the
      key, e.g.:
      `if _, ok := t.Method.(*jwt.SigningMethodHMAC); !ok { return nil, err }`
      (or `*jwt.SigningMethodRSA` / `*jwt.SigningMethodECDSA` as appropriate),
      so a token signed with an unexpected algorithm is rejected.
    # Matches a keyfunc literal that returns the key directly (`return $KEY, nil`)
    # and whose body does NOT reference `.Method` (neither a type assertion
    # `t.Method.(*jwt.SigningMethod...)` nor a comparison `t.Method != ...`).
    # The `pattern-not` with the `<... $T.Method ...>` deep-expression operator
    # excludes any keyfunc that inspects the signing method, keeping correct
    # method-checking keyfuncs from being flagged.
    patterns:
      - pattern-either:
          - pattern: |
              jwt.Parse($TOK, func($T *jwt.Token) (interface{}, error) {
                ...
                return $KEY, nil
              })
          - pattern: |
              jwt.ParseWithClaims($TOK, $CLAIMS, func($T *jwt.Token) (interface{}, error) {
                ...
                return $KEY, nil
              })
      - pattern-not: |
          jwt.Parse($TOK, func($T *jwt.Token) (interface{}, error) {
            ...
            $T.Method
            ...
          })
      - pattern-not: |
          jwt.ParseWithClaims($TOK, $CLAIMS, func($T *jwt.Token) (interface{}, error) {
            ...
            $T.Method
            ...
          })
      - pattern-not: |
          jwt.Parse($TOK, func($T *jwt.Token) (interface{}, error) {
            ...
            if <... $T.Method ...> { ... }
            ...
          })
      - pattern-not: |
          jwt.ParseWithClaims($TOK, $CLAIMS, func($T *jwt.Token) (interface{}, error) {
            ...
            if <... $T.Method ...> { ... }
            ...
          })
    metadata:
      oauthlint-rule-id: AUTH-GO-JWT-004
      oauthlint-doc-url: https://oauthlint.dev/rules/go-jwt-unchecked-method
      category: security
      cwe: CWE-347
      owasp: API2:2023
      llm-prevalence: HIGH
      technology:
        - golang-jwt
      references:
        - https://pkg.go.dev/github.com/golang-jwt/jwt/v5#Keyfunc
        - https://cwe.mitre.org/data/definitions/347.html