CVE-2025-4144
@cloudflare/workers-oauth-provider PKCE bypass via downgrade attack
Description
### Summary 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. ### 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. ### Patches Fixed in: https://github.com/cloudflare/workers-oauth-provider/pull/27 We patched up the vulnerabilities in the latest version, v 0.0.5 of the Workers OAuth provider (https://www.npmjs.com/package/@cloudflare/workers-oauth-provider). You'll need to update your MCP servers to use that version to resolve the vulnerability. ### Workarounds None
How to fix CVE-2025-4144
To remediate CVE-2025-4144, upgrade the affected package to a fixed version below.
- —upgrade to 0.0.5 or later
Is CVE-2025-4144 being exploited?
Low — EPSS is 0.4%, meaning exploitation activity has not been observed at scale.
Affected packages (1)
- from 0, < 0.0.5
CVSS scores
| Source | Version | Severity | Vector |
|---|---|---|---|
| osv | CVSS 4.0 | — | 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 |