# CVE-2025-4144: PKCE bypass via downgrade attack

> CVE-2025-4144: PKCE bypass via downgrade attack. CVSS 9.8 (CVSS 3.1).

Canonical: https://rewindex.com/cve/CVE-2025-4144
Last updated: 2026-06-17T09:32:35Z

- Published: 2025-05-01 00:50 UTC · Updated: 2026-06-17 09:32 UTC · Assigner: cloudflare
- CVSS: 9.8 (CVSS 3.1) · Severity: Critical · Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
- EPSS: 0.57%, 47th percentile (scored 2026-10-25)
- CISA KEV: no
- CWE: CWE-287

## 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.

## Affected vendors and products

_None listed._

## Tracked companies

- [Cloudflare](https://rewindex.com/company/cloudflare.md)

## References

- [CVE record on cve.org](https://www.cve.org/CVERecord?id=CVE-2025-4144)
- [NVD entry](https://nvd.nist.gov/vuln/detail/CVE-2025-4144)
- [https://github.com/cloudflare/workers-oauth-provider/pull/27](https://github.com/cloudflare/workers-oauth-provider/pull/27)

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_Sources: CVE records from the CVE Program (cve.org). CVE® is a registered trademark of The MITRE Corporation. (https://www.cve.org/) This product uses data from the NVD API but is not endorsed or certified by the NVD. (https://nvd.nist.gov/) Known Exploited Vulnerabilities catalog from CISA. (https://www.cisa.gov/known-exploited-vulnerabilities-catalog) EPSS scores from FIRST (Exploit Prediction Scoring System). (https://www.first.org/epss/)_


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Canonical: https://rewindex.com/cve/CVE-2025-4144 · Markdown: https://rewindex.com/cve/CVE-2025-4144.md · JSON: https://rewindex.com/api/v1/cves/CVE-2025-4144 · Data licensed CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/) · Generated 2026-10-26T02:51:43.734Z