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.
History

Thu, 01 May 2025 16:15:00 +0000

Type Values Removed Values Added
Metrics ssvc

{'options': {'Automatable': 'no', 'Exploitation': 'none', 'Technical Impact': 'partial'}, 'version': '2.0.3'}


Thu, 01 May 2025 01:00:00 +0000

Type Values Removed Values Added
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.
Title PKCE bypass via downgrade attack
Weaknesses CWE-287
References
Metrics cvssV4_0

{'score': 5.3, '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'}


cve-icon MITRE

Status: PUBLISHED

Assigner: cloudflare

Published: 2025-05-01T00:50:27.543Z

Updated: 2025-05-01T15:33:23.092Z

Reserved: 2025-04-30T16:39:00.801Z

Link: CVE-2025-4144

cve-icon Vulnrichment

Updated: 2025-05-01T14:51:45.618Z

cve-icon NVD

Status : Awaiting Analysis

Published: 2025-05-01T01:15:54.267

Modified: 2025-05-02T13:53:40.163

Link: CVE-2025-4144

cve-icon Redhat

No data.