Vulnerability-Lookup

GHSA-JHM7-29PJ-4XVF

Vulnerability from github – Published: 2026-04-16 21:09 – Updated: 2026-04-24 20:53

Summary

@node-oauth/oauth2-server: PKCE code_verifier ABNF not enforced in token exchange allows brute-force redemption of intercepted authorization codes

Details

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

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).
lib/grant-types/authorization-code-grant-type.js compares hash(code_verifier) to stored codeChallenge without validating verifier format/length.
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

Send repeated token requests with guessed code_verifier values:

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>

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

Confirmed PoC output

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

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

Severity

5.9 (Medium)


                  
                    CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:N/A:N

Show details on source website

JSON

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{
  "affected": [
    {
      "database_specific": {
        "last_known_affected_version_range": "\u003c= 5.2.1"
      },
      "package": {
        "ecosystem": "npm",
        "name": "@node-oauth/oauth2-server"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "5.3.0"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2026-41213"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-1289",
      "CWE-307"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-04-16T21:09:50Z",
    "nvd_published_at": "2026-04-23T19:17:29Z",
    "severity": "MODERATE"
  },
  "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\u0026\nclient_id=client1\u0026\nclient_secret=s3cret\u0026\ncode=stolen-auth-code\u0026\nredirect_uri=https://client.example/callback\u0026\ncode_verifier=\u003cguess\u003e\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: \u0027z\u0027, 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\u2019s 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.",
  "id": "GHSA-jhm7-29pj-4xvf",
  "modified": "2026-04-24T20:53:49Z",
  "published": "2026-04-16T21:09:50Z",
  "references": [
    {
      "type": "WEB",
      "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"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:N/A:N",
      "type": "CVSS_V3"
    }
  ],
  "summary": "@node-oauth/oauth2-server: PKCE code_verifier ABNF not enforced in token exchange allows brute-force redemption of intercepted authorization codes"
}

CVE-2026-41213 (GCVE-0-2026-41213)

Vulnerability from cvelistv5 – Published: 2026-04-23 18:33 – Updated: 2026-04-25 01:23

Title

@node-oauth/oauth2-server: PKCE code_verifier ABNF not enforced in token exchange allows brute-force redemption of intercepted authorization codes

Summary

@node-oauth/oauth2-server is a module for implementing an OAuth2 server in Node.js. 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.

Severity

5.9 (Medium)


                        
                          CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:N/A:N

SSVC

Exploitation: poc Automatable: no Technical Impact: partial

CISA Coordinator · CISA-ADP (v2.0.3)

Decision recorded 2026-04-25 01:23 UTC

CWE

CWE-307 - Improper Restriction of Excessive Authentication Attempts
CWE-1289 - Improper Validation of Unsafe Equivalence in Input

Assigner

GitHub_M mitre root

References

1 reference

URL	Tags
https://github.com/node-oauth/node-oauth2-server/…	x_refsource_CONFIRM

Impacted products

1 product

Vendor	Product	Version
node-oauth	node-oauth2-server	Affected: < 5.3.0

Show details on NVD website

JSON

To clipboard

{
  "containers": {
    "adp": [
      {
        "metrics": [
          {
            "other": {
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                "id": "CVE-2026-41213",
                "options": [
                  {
                    "Exploitation": "poc"
                  },
                  {
                    "Automatable": "no"
                  },
                  {
                    "Technical Impact": "partial"
                  }
                ],
                "role": "CISA Coordinator",
                "timestamp": "2026-04-25T01:23:23.060024Z",
                "version": "2.0.3"
              },
              "type": "ssvc"
            }
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        ],
        "providerMetadata": {
          "dateUpdated": "2026-04-25T01:23:58.715Z",
          "orgId": "134c704f-9b21-4f2e-91b3-4a467353bcc0",
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        },
        "references": [
          {
            "tags": [
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            "url": "https://github.com/node-oauth/node-oauth2-server/security/advisories/GHSA-jhm7-29pj-4xvf"
          }
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        "title": "CISA ADP Vulnrichment"
      }
    ],
    "cna": {
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          "product": "node-oauth2-server",
          "vendor": "node-oauth",
          "versions": [
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            }
          ]
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      "descriptions": [
        {
          "lang": "en",
          "value": "@node-oauth/oauth2-server is a module for implementing an OAuth2 server in Node.js. 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."
        }
      ],
      "metrics": [
        {
          "cvssV3_1": {
            "attackComplexity": "HIGH",
            "attackVector": "NETWORK",
            "availabilityImpact": "NONE",
            "baseScore": 5.9,
            "baseSeverity": "MEDIUM",
            "confidentialityImpact": "HIGH",
            "integrityImpact": "NONE",
            "privilegesRequired": "NONE",
            "scope": "UNCHANGED",
            "userInteraction": "NONE",
            "vectorString": "CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:N/A:N",
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      "providerMetadata": {
        "dateUpdated": "2026-04-23T18:33:42.365Z",
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          "name": "https://github.com/node-oauth/node-oauth2-server/security/advisories/GHSA-jhm7-29pj-4xvf",
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        "discovery": "UNKNOWN"
      },
      "title": "@node-oauth/oauth2-server: PKCE code_verifier ABNF not enforced in token exchange allows brute-force redemption of intercepted authorization codes"
    }
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  "cveMetadata": {
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    "datePublished": "2026-04-23T18:33:42.365Z",
    "dateReserved": "2026-04-18T02:51:52.975Z",
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    "state": "PUBLISHED"
  },
  "dataType": "CVE_RECORD",
  "dataVersion": "5.2"
}

Sightings

Author	Source	Type	Date	Other

Nomenclature

Seen: The vulnerability was mentioned, discussed, or observed by the user.
Confirmed: The vulnerability has been validated from an analyst's perspective.
Published Proof of Concept: A public proof of concept is available for this vulnerability.
Exploited: The vulnerability was observed as exploited by the user who reported the sighting.
Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
Not confirmed: The user expressed doubt about the validity of the vulnerability.
Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.

The MITRE ATT&CK techniques below are AI-generated suggestions, inferred from the description of the vulnerability by the CIRCL/vulnerability-attack-technique-classification-roberta-base model, served locally by ML-Gateway. They have not been verified by an analyst and are provided for guidance only.
The approach is described in our paper Mapping CVEs to MITRE ATT&CK Techniques: A Curated Gold-Set Classifier and the Limits of LLM-Assisted Label Expansion.
Browse all ATT&CK techniques and the vulnerabilities related to each.

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Action not permitted

GHSA-JHM7-29PJ-4XVF

Summary

Root cause

Steps to Reproduce

Setup

Reproduction

Confirmed PoC output

Impact

Recommended Fix

CVE-2026-41213 (GCVE-0-2026-41213)

Tags

Sightings

Nomenclature

Related by attack behaviour