CVE-2025-4144: Cloudflare Workers-OAuth-Provider

Critical severity, CVSS 9.8. EPSS: 0.6% chance of exploitation in the next 30 days.

PKCE was implemented in the OAuth implementation in workers-oauth-provider that is part of MCP framework https://github.com/cloudflare/workers-mcp . However, it was found that an attacker could cause the check to be skipped. Fixed in: https://github.com/cloudflare/workers-oauth-provider/pull/27 https://github.com/cloudflare/workers-oauth-provider/pull/27 Impact: PKCE is a defense-in-depth mechanism against certain kinds of attacks and was an optional extension in OAuth 2.0 which became required in the OAuth 2.1 draft. (Note that the MCP specification requires OAuth 2.1.). This bug completely bypasses PKCE protection.

Affected products

• Cloudflare Workers-OAuth-Provider: version 0.0.5 only

Published 2025-05-01. Last modified 2026-06-17.