CVE-2025-4144
PKCE bypass via downgrade attack
CVE-2025-4144 is a medium vulnerability (CVSS 5.3) in Cloudflare Workers-oauth-provider, published May 1, 2025. No source lists public exploit code. EPSS puts the chance of exploitation in the next 30 days at 0.58%, the 47th percentile. Fixed in @cloudflare/workers-oauth-provider 0.0.5.
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.
Attack profile
How easy it is to exploit
- Attack vector
- Network
- Complexity
- Low
- Privileges needed
- None
- User interaction
- Passive
What an attacker can affect
- Confidentiality
- None
- Integrity
- Low
- Availability
- None
Exploit likelihood
What each source reports
CVE List Score used
PKCE bypass via downgrade attack
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.
- Record
CVE-2025-4144- CVSS 4.0
- 5.3 Medium
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 - Weakness
- CWE-287
- Updated
- 2026-11-02
EUVDEUVD-2025-12788
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.
- Record
EUVD-2025-12788- CVSS 4.0
- 5.3 Medium
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 - Updated
- 2026-11-02
GitHub AdvisoriesGHSA-qgp8-v765-qxx9
@cloudflare/workers-oauth-provider PKCE bypass via downgrade attack
Summary
PKCE was implemented in the OAuth implementation in workers-oauth-provider that is part of MCP framework. 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
- Record
GHSA-qgp8-v765-qxx9- CVSS 4.0
- 5.3 Medium
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 - Weakness
- CWE-287
- Updated
- 2026-11-02
GitHub AdvisoriesGHSA-vh4h-fvqf-q9wv
Duplicate Advisory: @cloudflare/workers-oauth-provider PKCE bypass via downgrade attack
Duplicate Advisory
This advisory has been withdrawn because it is a duplicate of GHSA-qgp8-v765-qxx9. This link is maintained to preserve external references.
Original 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.
- Record
GHSA-vh4h-fvqf-q9wv- CVSS 4.0
- 5.3 Medium
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/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:Amber - Weakness
- CWE-287
- Updated
- 2026-11-02
NVD
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.
- Record
CVE-2025-4144- CVSS 4.0
- 5.3 Medium
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/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:Amber - CVSS 3.1
- 9.8 Critical
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H - Weakness
- CWE-287
- Updated
- 2026-11-02
OSVGHSA-qgp8-v765-qxx9
@cloudflare/workers-oauth-provider PKCE bypass via downgrade attack
Summary
PKCE was implemented in the OAuth implementation in workers-oauth-provider that is part of MCP framework. 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
- Record
GHSA-qgp8-v765-qxx9- CVSS 4.0
- 5.3 Medium
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 - Weakness
- CWE-287
- Updated
- 2026-11-02
OSVGHSA-vh4h-fvqf-q9wv
Duplicate Advisory: @cloudflare/workers-oauth-provider PKCE bypass via downgrade attack
Duplicate Advisory
This advisory has been withdrawn because it is a duplicate of GHSA-qgp8-v765-qxx9. This link is maintained to preserve external references.
Original 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.
- Record
GHSA-vh4h-fvqf-q9wv- CVSS 4.0
- 5.3 Medium
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/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:Amber - Weakness
- CWE-287
- Updated
- 2026-11-02
Affected products
Affected packages
| Package | Affected | Fixed |
|---|---|---|
@cloudflare/workers-oauth-providernpm |
before 0.0.5 | 0.0.5 |
History
| When | Changed | Sources |
|---|---|---|
| 2026-11-03 | affected | |
| 2026-11-02 | created | CVE List, EUVD, GitHub Advisories, NVD, OSV |
References
github.com/cloudflare/workers-oauth-provider/pull/27
nvd.nist.gov/vuln/detail/CVE-2025-4144