CVE-2025-4144: PKCE bypass via downgrade attack
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.
Classification
CVE ID: CVE-2025-4144
CVSS Base Severity: MEDIUM
CVSS Base Score: 5.3
CVSS 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
Problem Types
CWE-287 Improper AuthenticationAffected Products
Vendor:
Product:
Exploit Prediction Scoring System (EPSS)
EPSS Score: 0.05% (probability of being exploited)
EPSS Percentile: 15.62% (scored less or equal to compared to others)
EPSS Date: 2025-05-29 (when was this score calculated)
References
https://nvd.nist.gov/vuln/detail/CVE-2025-4144https://github.com/cloudflare/workers-oauth-provider/pull/27