Vulnerability · Published May 1, 2025 · Assigned by cloudflare
CVE-2025-4144PKCE bypass via downgrade attack
Medium 5.3
In plain English
A medium-severity improper authentication in workers-oauth-provider. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.6% (higher than 48% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.
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.
Affected packages & fixed versions
| Ecosystem | Package | Fixed in | Advisory |
|---|---|---|---|
| npm | @cloudflare/workers-oauth-provider | 0.0.5 | GHSA-qgp8-v765-qxx9 |
Affected products (as reported)
| Vendor | Product | Versions |
|---|---|---|
| workers-oauth-provider | 0 – before 0.0.5 |
Weakness: CWE-287 · Vector: CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:P/VC:N/VI:L/VA:N/SC:N/SI:N/SA:N/U:Amber