CVE-2025-4143: The OAuth implementation in workers-oauth-provider that is part of MCP framework https://github.com/cloudflar

Routine

MEDIUM 6.1DOWNGRADE

CVE-2025-4143 is a medium-severity flaw (CVSS 6.1) affecting workers-oauth-provider. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

The OAuth implementation in workers-oauth-provider that is part of MCP framework https://github.com/cloudflare/workers-mcp , did not correctly validate that redirect_uri was on the allowed list of redirect URIs for the given client registration. Fixed in: https://github.com/cloudflare/workers-oauth-provider/pull/26 https://github.com/cloudflare/workers-oauth-provider/pull/26 Impact: Under certain circumstances (see below), if a victim ha

Why this priority

No elevated risk signals yet

No fix is published yet: follow the downgrade or mitigation steps.

Risk facts

CVSS score	6.1 (Medium)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	Required
EPSS exploit chance	0.3% (top 75%)
CISA exploited list	No
Weakness	CWE-601
Published	2025-05-01
Last modified	2026-06-17

Publisher (CNA): Cloudflare, Inc. · Vendor: cloudflare

Last updated 2026-06-17 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2025-05-01 01:15 UTCPublished CVE record published
2026-06-17 09:32 UTCUpdated Record last modified

cloudflare workers-oauth-provider DOWNGRADE

Affected: 0.0.5

Fixed in: no fixed release known

Safe to downgrade to: a release before 0.0.5

1. Check installed version

apt-cache policy workers-oauth-provider || rpm -q workers-oauth-provider || workers-oauth-provider --version

2. No fixed release: DOWNGRADE to a release before 0.0.5 (Debian / Ubuntu)

apt-cache madison workers-oauth-provider sudo apt install workers-oauth-provider=<VERSION_BEFORE_0.0.5> sudo apt-mark hold workers-oauth-provider

2. No fixed release: DOWNGRADE (RHEL / Amazon Linux / Fedora)

dnf --showduplicates list workers-oauth-provider sudo dnf downgrade workers-oauth-provider-<VERSION_BEFORE_0.0.5> sudo dnf versionlock add workers-oauth-provider

Downgrade risk

Downgrading can break dependent software and may reopen older bugs. Read the distro advisory first and test on a non-production host.

After downgrade

Restart the service, and keep the package held until a fixed release ships.

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2025-4143?

workers-oauth-provider: downgrade or apply the listed mitigation. Exact commands for each OS are on this page.

Is CVE-2025-4143 being exploited?

CVE-2025-4143 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2025-4143 affect?

CVE-2025-4143 affects workers-oauth-provider. Severity is medium (CVSS 6.1). The OAuth implementation in workers-oauth-provider that is part of MCP framework https://github.com/cloudflare/workers-mcp , did not correctly validate that redirect_uri was on the allowed list of redirect URIs for the given client registr

How urgent is CVE-2025-4143?

Priority 28/100. No elevated risk signals yet.

Software pages

GitHub vulnerabilities · Cloudflare vulnerabilities

Related CVEs

CVE-2025-4144 CRITICAL 9.8 DOWNGRADE PKCE was implemented in the OAuth implementation in workers-oauth-provider that is part of MCP framework http

References

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