CVE-2025-4144 — PKCE bypass via downgrade attack

Description (CNA)

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 (CNA)

Vendor / product	Versions
Not supplied / Not supplied	affected 0 before 0.0.5

CWE

CWE-287

References

https://github.com/cloudflare/workers-oauth-provider/pull/27

ADP enrichment

CISA-ADP

No English description supplied.

No references supplied.