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CVE-2025-4144: @cloudflare/workers-oauth-provider PKCE bypass via downgrade attack

mediumCVSS 4.0 5.3GHSA-qgp8-v765-qxx9

Summary PKCE was implemented in the OAuth implementation in workers-oauth-provider that is part of MCP framework. However, it was found that an attacker could cause the check to be skipped. 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.

Published May 1, 2025

Affected packages

@cloudflare/workers-oauth-provider

Ecosystem: npm
Affected: < 0.0.5
Fixed in: 0.0.5
Check: npm ls @cloudflare/workers-oauth-provider

Advisory details

From the GitHub Advisory Database (CC BY 4.0).

Summary

PKCE was implemented in the OAuth implementation in workers-oauth-provider that is part of MCP framework. However, it was found that an attacker could cause the check to be skipped.

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.

Patches

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

We patched up the vulnerabilities in the latest version, v 0.0.5 of the Workers OAuth provider (https://www.npmjs.com/package/@cloudflare/workers-oauth-provider). You'll need to update your MCP servers to use that version to resolve the vulnerability.

Workarounds

None

References

How this page is made: the data panel comes from the GitHub Advisory Database, CISA’s KEV catalog, the NVD and FIRST EPSS, and updates twice a day. No analysis has been written for this entry yet. Methodology

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