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.

Metrics

Attack Vector Network

Attack Complexity Low

Privileges Required None

Attack Requirements None

User Interaction Passive

Vulnerable System Confidentiality Impact None

Vulnerable System Integrity Impact Low

Vulnerable System Availability Impact None

Subsequent System Confidentiality Impact None

Subsequent System Integrity Impact None

Subsequent System Availability Impact None

Attack Vector Network

Attack Complexity Low

Privileges Required None

Scope Unchanged

Confidentiality Impact High

Integrity Impact High

Availability Impact High

User Interaction None

No CVSS v3.0

No CVSS v2

This CVE is not in the KEV list.

The EPSS score is 0.00068.

Exploitation none

Automatable no

Technical Impact partial

Affected Vendors & Products

Vendors Products
Cloudflare
  • Workers-oauth-provider

Configuration 1 [-]

cpe:2.3:a:cloudflare:workers-oauth-provider:0.0.5:*:*:*:*:*:*:*

No data.

No data.

Advisories
Source ID Title
EUVD EUVD EUVD-2025-12788 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.
Github GHSA Github GHSA GHSA-qgp8-v765-qxx9 @cloudflare/workers-oauth-provider PKCE bypass via downgrade attack
Fixes

Solution

No solution given by the vendor.

Workaround

No workaround given by the vendor.

References
Link Providers
https://github.com/cloudflare/workers-oauth-provider/pull/27 cve-icon cve-icon
History

Mon, 12 May 2025 20:00:00 +0000

Type Values Removed Values Added
First Time appeared Cloudflare
Cloudflare workers-oauth-provider
Weaknesses NVD-CWE-noinfo
CPEs cpe:2.3:a:cloudflare:workers-oauth-provider:0.0.5:*:*:*:*:*:*:*
Vendors & Products Cloudflare
Cloudflare workers-oauth-provider
Metrics cvssV3_1

{'score': 9.8, 'vector': 'CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H'}

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:

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 : Analyzed

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

Modified: 2025-05-12T19:39:31.053

Link: CVE-2025-4144

cve-icon Redhat

No data.

cve-icon OpenCVE Enrichment

No data.