Vulnerability Details CVE-2025-4144
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.
Exploit prediction scoring system (EPSS) score
EPSS Score 0.001
EPSS Ranking 16.9%
CVSS Severity
CVSS v3 Score 9.8
Products affected by CVE-2025-4144
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cpe:2.3:a:cloudflare:workers-oauth-provider:0.0.5