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CVE-2025-4144— PKCE bypass via downgrade attack

Quick assessment

Affected
CVE-2025-4144
Exploitation
No confirmed in-the-wild exploitation; assess based on exposure
Recommended action
Check the vendor advisory and references for a fixed version. If immediate upgrade is impossible, restrict exposure and increase monitoring.

workers-oauth-provider是Cloudflare开源的一个Cloudflare Workers的OAuth提供程序库。 workers-oauth-provider存在安全漏洞，该漏洞源于PKCE检查被绕过，可能导致保护机制失效。

AI Predicted 7.5 Difficulty: Easy EPSS 0.55% · P44

I. Basic Information for CVE-2025-4144

Vulnerability Information
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Vulnerability Title
PKCE bypass via downgrade attack
Source: CVE Program / CVE List V5
Vulnerability 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.
Source: CVE Program / CVE List V5
CVSS Information
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
Source: CVE Program / CVE List V5
Vulnerability Type
认证机制不恰当
Source: CVE Program / CVE List V5
Vulnerability Title
workers-oauth-provider 安全漏洞
Source: CNNVD (China National Vulnerability Database)
Vulnerability Description
workers-oauth-provider是Cloudflare开源的一个Cloudflare Workers的OAuth提供程序库。 workers-oauth-provider存在安全漏洞，该漏洞源于PKCE检查被绕过，可能导致保护机制失效。
Source: CNNVD (China National Vulnerability Database)
CVSS Information
N/A
Source: CNNVD (China National Vulnerability Database)
Vulnerability Type
N/A
Source: CNNVD (China National Vulnerability Database)

II. Public POCs for CVE-2025-4144

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III. Intelligence Information for CVE-2025-4144

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Patches & Fixes for CVE-2025-4144 (1)

IV. Related Vulnerabilities

Same weakness: CWE-287

V. Comments for CVE-2025-4144

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