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EUVD-2026-236326

fkie_cve-2025-4144

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.

GHSA-qgp8-v765-qxx9 — @cloudflare/workers-oauth-provider PKCE bypass via downgrade attack

Affected: npm: @cloudflare/workers-oauth-provider

### Summary 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.

### 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