How the CVE Works:
The vulnerability (CVE-2025-XXXX) in Cloudflare Workers OAuth Provider allows attackers to bypass Proof Key for Code Exchange (PKCE) protection via a downgrade attack. PKCE, mandatory in OAuth 2.1, prevents authorization code interception by binding the authorization request to a code verifier. The flaw occurs when the OAuth implementation fails to enforce PKCE validation, enabling an attacker to manipulate the flow and skip the PKCE check. This compromises the security mechanism designed to mitigate authorization code injection and replay attacks.
DailyCVE Form:
Platform: Cloudflare Workers
Version: MCP Framework
Vulnerability: PKCE Bypass
Severity: Moderate
Date: May 1, 2025
What Undercode Say:
Exploitation:
- Attack Vector: Craft a malicious OAuth request omitting
code_challenge. - Downgrade: Force the server to process the request without PKCE validation.
3. Impact: Authorization code theft or session hijacking.
Protection:
1. Patch: Apply fixes from `cloudflare/workers-oauth-provider27`.
2. Enforcement: Reject requests missing PKCE parameters.
Analytics:
- Affected versions: Pre-patch MCP OAuth implementations.
- Risk: Moderate (PKCE bypass enables secondary attacks).
Commands:
Verify PKCE enforcement (curl test): curl -X POST "https://oauth-provider/authorize" -d "response_type=code&client_id=CLIENT&redirect_uri=URI" Expected: 400 (missing PKCE)
Code Fix (Node.js):
function validatePKCE(req) {
if (!req.body.code_challenge) {
throw new Error("PKCE required");
}
}
Mitigation Steps:
1. Update to the latest `workers-oauth-provider`.
2. Audit OAuth flows for missing PKCE checks.
3. Monitor for anomalous authorization patterns.
References:
- GitHub Advisory: GHSA-qgp8-v765-qxx9
- OAuth 2.1 RFC: PKCE Mandate
(No additional commentary beyond rules.)
Sources:
Reported By: github.com
Extra Source Hub:
Undercode
