CVE-2025-4144

01.05.2025, 01:15

PKCE was implemented in the OAuth implementation in workers-oauth-provider that is part of MCP framework https://github.com/cloudflare/workers-mcp . However, itwas 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.

Provider	Type	Base Score	Atk. Vector	Atk. Complexity	Priv. Required	Vector
NIST	NIST	9.8 CRITICAL	NETWORK	LOW	NONE	CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
cloudflare	CNA	---				---
CISA-ADP	ADP	---				---

Base Score

CVSS 3.x

EPSS Score

Percentile: 15%

Vendor	Product	Version
cloudflare	workers-oauth-provider	0.0.5

𝑥

= Vulnerable software versions

Common Weakness Enumeration

CWE-287 - Improper Authentication
When an actor claims to have a given identity, the software does not prove or insufficiently proves that the claim is correct.

References

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