Paper/Archive/CVE-2025-4144

CVE-2025-4144

cloudflare — PKCE bypass via downgrade attack

CVSS 9.8 (NVD)9.8 / 10 × 3029.4 Not in CISA KEV+300.0 EPSS percentile 47.4.474 × 209.5 No public exploit+100.0 SSVC exploitation: none+100.0

Description · from the CVE record

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.

Affected

ProductAffectedFixed inSource

workers-oauth-provider< 0.0.50.0.5CNA

Scores by source

NVD9.8CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

cloudflare (CNA)5.3CVSS: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

CISA ADPSSVCExploitation: none · Automatable: no · Technical impact: partial

References

refgithub.com/cloudflare/workers-oauth-provider/pull/27