CVE-2025-4144 PKCE bypass via downgrade attack
- Exploited in the wild
- Not known
- Fix
- Available
What is CVE-2025-4144?
CVE-2025-4144 is a medium-severity vulnerability in cloudflare:workers-oauth-provider, published on 1 May 2025.
Official description
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.
Weakness: CWE-287
Which versions are affected by CVE-2025-4144?
2 open source packages ship an affected version, including @cloudflare/workers-oauth-provider. It affects 1 product.
| Package | Affected | Fixed in |
|---|---|---|
@cloudflare/workers-oauth-provider npm | All versions | 0.0.5 |
@cloudflare/workers-oauth-provider npm | All versions | 0.0.5 |
Products include Cloudflare Workers-Oauth-Provider.
How do I fix CVE-2025-4144?
Upgrade @cloudflare/workers-oauth-provider to 0.0.5 or later.
Is CVE-2025-4144 being exploited?
There is no known exploitation of CVE-2025-4144 in the wild and no public exploit code.
Other vulnerabilities in cloudflare:workers-oauth-provider
Sources
Also known as GHSA-qgp8-v765-qxx9.