{"id":"CVE-2026-41213","aliases":["GHSA-jhm7-29pj-4xvf"],"url":"https://o3.security/vulnerability/CVE-2026-41213","summary":"@node-oauth/oauth2-server: PKCE code_verifier ABNF not enforced in token exchange allows brute-force redemption of intercepted authorization codes","details":"## Summary\n\nThe token exchange path accepts RFC7636-invalid `code_verifier` values (including one-character strings) for `S256` PKCE flows.  \nBecause 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.\n\n\n\n### Root cause\n\n1. `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).\n2. `lib/grant-types/authorization-code-grant-type.js` compares `hash(code_verifier)` to stored `codeChallenge` without validating verifier format/length.\n3. In `AuthorizationCodeGrantType.handle`, authorization code revocation happens **after** verifier validation. Invalid guesses fail before revoke, so the same code can be retried repeatedly.\n\n## Steps to Reproduce\n\n### Setup\n\n- PKCE authorization code exists with:\n  - `codeChallengeMethod = \"S256\"`\n  - `codeChallenge = BASE64URL(SHA256(\"z\"))` (verifier is one character, RFC-invalid)\n- Attacker has intercepted the authorization code value.\n\n### Reproduction\n\n1. Send repeated token requests with guessed `code_verifier` values:\n\n```http\nPOST /token HTTP/1.1\nHost: oauth.example\nContent-Type: application/x-www-form-urlencoded\n\ngrant_type=authorization_code&\nclient_id=client1&\nclient_secret=s3cret&\ncode=stolen-auth-code&\nredirect_uri=https://client.example/callback&\ncode_verifier=<guess>\n```\n\n2. Observe invalid guesses return `invalid_grant`.\n3. Continue guessing (`a`..`z`).\n4. When `code_verifier=z`, token issuance succeeds and returns bearer tokens.\n\n### Confirmed PoC output\n\n```text\nBRUTE_FORCE_SUCCESS { tries: 26, guess: 'z', status: 200, tokenIssued: true }\n```\n\n## Impact\n\nAn intercepted authorization code can be redeemed by brute-forcing low-entropy verifiers that the server should have rejected under RFC7636.  \nThis weakens PKCE’s protection goal and allows token theft when clients generate short/predictable verifiers.\n\n## Recommended Fix\n\n1. Enforce `pkce.codeChallengeMatchesABNF(request.body.code_verifier)` in authorization code token exchange before hashing/comparison.\n2. Reject verifier values outside RFC7636 charset/length (`43..128` unreserved).\n3. Invalidate authorization codes on failed verifier attempts (or add strict retry limits) to prevent online guessing.","published":"2026-04-23T18:33:42.365Z","modified":"2026-10-28T02:30:52.485791448Z","cvss":{"score":5.9,"severity":"MEDIUM","vector":"CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:N/A:N"},"epss":{"score":0.00259,"percentile":0.17572,"asOf":"2026-08-12"},"cisaKev":null,"exploitsKnown":0,"affectedPackages":[{"ecosystem":"npm","name":"@node-oauth/oauth2-server","fixedVersion":"5.3.0"}],"fix":null,"references":[{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/41xxx/CVE-2026-41213.json"},{"type":"ADVISORY","url":"https://github.com/node-oauth/node-oauth2-server/security/advisories/GHSA-jhm7-29pj-4xvf"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-41213"},{"type":"PACKAGE","url":"https://github.com/node-oauth/node-oauth2-server"}],"provenance":{"sources":["OSV.dev","FIRST.org (EPSS)"],"lastVerified":"2026-10-28T02:30:52.485791448Z"}}