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.

Category

5.3
CVSS
Severity: Medium
CVSS 4.0 •
CVSS 3.1 •
EPSS 0.05%
Published at:
Updated at:

References

Link Tags
https://github.com/cloudflare/workers-oauth-provider/pull/27 patch issue tracking

Frequently Asked Questions

What is the severity of CVE-2025-4144?
CVE-2025-4144 has been scored as a medium severity vulnerability.
How to fix CVE-2025-4144?
To fix CVE-2025-4144, make sure you are using an up-to-date version of the affected component(s) by checking the vendor release notes. As for now, there are no other specific guidelines available.
Is CVE-2025-4144 being actively exploited in the wild?
As for now, there are no information to confirm that CVE-2025-4144 is being actively exploited. According to its EPSS score, there is a ~0% probability that this vulnerability will be exploited by malicious actors in the next 30 days.
What software or system is affected by CVE-2025-4144?
CVE-2025-4144 affects .
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