GHSA-jhm7-29pj-4xvf

## Summary

The token exchange path accepts RFC7636-invalid `code_verifier` values (including one-character strings) for `S256` PKCE flows. Because short/weak verifiers are accepted and failed verifier attempts do not consume the authorization code, an attacker who intercepts an authorization code can brute-force `code_verifier` guesses online until token issuance succeeds.

### Root cause

1. `lib/pkce/pkce.js` (`getHashForCodeChallenge`) only checks that `verifier` is a non-empty string before hashing for `S256`; it does not enforce RFC7636 ABNF (`43..128` unreserved chars). 2. `lib/grant-types/authorization-code-grant-type.js` compares `hash(code_verifier)` to stored `codeChallenge` without validating verifier format/length. 3. In `AuthorizationCodeGrantType.handle`, authorization code revocation happens **after** verifier validation. Invalid guesses fail before revoke, so the same code can be retried repeatedly.

## Steps to Reproduce

### Setup

- PKCE authorization code exists with: - `codeChallengeMethod = "S256"` - `codeChallenge = BASE64URL(SHA256("z"))` (verifier is one character, RFC-invalid) - Attacker has intercepted the authorization code value.

### Reproduction

1. Send repeated token requests with guessed `code_verifier` values:

```http POST /token HTTP/1.1 Host: oauth.example Content-Type: application/x-www-form-urlencoded

grant_type=authorization_code& client_id=client1& client_secret=s3cret& code=stolen-auth-code& redirect_uri=https://client.example/callback& code_verifier=<guess> ```

2. Observe invalid guesses return `invalid_grant`. 3. Continue guessing (`a`..`z`). 4. When `code_verifier=z`, token issuance succeeds and returns bearer tokens.

### Confirmed PoC output

```text BRUTE_FORCE_SUCCESS { tries: 26, guess: 'z', status: 200, tokenIssued: true } ```

## Impact

An intercepted authorization code can be redeemed by brute-forcing low-entropy verifiers that the server should have rejected under RFC7636. This weakens PKCE’s protection goal and allows token theft when clients generate short/predictable verifiers.

## Recommended Fix

1. Enforce `pkce.codeChallengeMatchesABNF(request.body.code_verifier)` in authorization code token exchange before hashing/comparison. 2. Reject verifier values outside RFC7636 charset/length (`43..128` unreserved). 3. Invalidate authorization codes on failed verifier attempts (or add strict retry limits) to prevent online guessing.