rules:
  - id: auth.go.flow.weak-rand
    languages:
      - go
    severity: ERROR
    message: |
      A security-sensitive value is being generated with the `math/rand`
      package. Its name indicates a token, secret, key, password, nonce, OTP,
      or salt. `math/rand` is a deterministic PRNG: its output is predictable
      and an attacker who observes enough values can recover the seed and
      forecast every future token. For OAuth/OIDC this means forgeable
      `state` values, guessable authorization codes, and predictable refresh
      tokens.

      Use `crypto/rand` instead: allocate a byte slice and fill it with
      `rand.Read(b)` (from `crypto/rand`), then hex- or base64url-encode it.
      Never derive a credential from `math/rand`.
    # We match `rand.$F(...)` where $F is one of the math/rand-EXCLUSIVE
    # generators (Intn/Int/Int31/Int63/Float64/Perm). These do not exist in
    # crypto/rand, so a match is unambiguous. We deliberately do NOT match
    # `rand.Read(...)`, which exists in BOTH packages. The crypto/rand one is
    # the safe path.
    #
    # The result must be assigned (`:=` or `=`) to a secret-shaped identifier;
    # `i := rand.Intn(10)` for a loop or `delay := rand.Intn(500)` for jitter
    # are not flagged. The name constraint uses `metavariable-pattern` +
    # `pattern-regex` (rather than `metavariable-regex`) so that $VAR is
    # recorded as a distinguishing binding. Otherwise Semgrep's Go frontend
    # leaves the short-var-decl LHS unbound and collapses two genuinely
    # distinct secrets into a single finding.
    pattern-either:
      - patterns:
          - pattern: '$VAR := rand.$F(...)'
          - metavariable-regex:
              metavariable: $F
              regex: ^(Intn|Int|Int31|Int63|Float64|Perm)$
          - metavariable-pattern:
              metavariable: $VAR
              patterns:
                - pattern-regex: (?i)(token|secret|key|password|nonce|otp|salt)
      - patterns:
          - pattern: '$VAR = rand.$F(...)'
          - metavariable-regex:
              metavariable: $F
              regex: ^(Intn|Int|Int31|Int63|Float64|Perm)$
          - metavariable-pattern:
              metavariable: $VAR
              patterns:
                - pattern-regex: (?i)(token|secret|key|password|nonce|otp|salt)
    metadata:
      oauthlint-rule-id: AUTH-GO-FLOW-001
      oauthlint-doc-url: https://oauthlint.dev/rules/go-flow-weak-rand
      category: security
      cwe: CWE-330
      owasp: A02:2021
      llm-prevalence: HIGH
      technology:
        - math/rand
      references:
        - https://pkg.go.dev/crypto/rand#Read
        - https://cwe.mitre.org/data/definitions/330.html