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CVE-2025-4143 — @cloudflare/workers-oauth…

Fix: cloudflare/workers-oauth-provider#26

CVE-2025-4143 is a Open Redirect vulnerability in @cloudflare/workers-oauth-provider. A fix is available for @cloudflare/workers-oauth-provider — see the affected versions and patch details below.

@cloudflare/workers-oauth-provider missing validation of redirect_uri on authorize endpoint

Published
Updated
Affected
1 pkg
Patched
1 / 1
Exploits
None indexed
Exploitation data as of Nov 5, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

Exploitation Status

No confirmed exploitation observed yet

  • CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.

Exploitation and automatability from CISA’s SSVC triage for CVE-2025-4143.

EPSS Exploitation Probability

via FIRST.org ↗
0.3%probability of exploitation in next 30 days
Lower Risk0.00%
Lower risk than most CVEs26th percentile — riskier than 26% of all scored CVEsHighest risk
0.00%0.28%0.55%0.83%0.3%0.3%May 26Aug 26Nov 26

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

Real-World Exposure

1 pkg affected

How broadly this vulnerability is actually deployed: weekly install volume shows current usage, and reverse-dependency count shows how many other packages break if it stays unpatched.

69other npm packages depend on this — each one inherits the vulnerability until it's patched upstream
@cloudflare/workers-oauth-providernpm
1.1Mdownloads / week

Description

Summary

The OAuth implementation failed to check that redirect_uri was among the allowed set for the client_id.

Impact

Under certain circumstances (see below), if a victim had previously authorized with a server built on workers-oath-provider, and an attacker could later trick the victim into visiting a malicious web site, then attacker could potentially steal the victim's credentials to the same OAuth server and subsequently impersonate them.

In order for the attack to be possible, the OAuth server's authorized callback must be designed to auto-approve authorizations that appear to come from an OAuth client that the victim has authorized previously. The authorization flow is not implemented by workers-oauth-provider; it is up to the application built on top to decide whether to implement such automatic re-authorization. However, many applications do implement such logic.

Patches

Fixed in: https://github.com/cloudflare/workers-oauth-provider/pull/26

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

Note

It is a basic, well-known requirement that OAuth servers should verify that the redirect URI is among the allowed list for the client, both during the authorization flow and subsequently when exchanging the authorization code for an access token. workers-oauth-provider implemented only the latter check, not the former. Unfortunately, the former is the much more important check.

Readers who are familiar with OAuth may recognize that failing to check redirect URIs against the allowed list is a well-known, basic mistake, covered extensively in the RFC and elsewhere. The author of this library would like everyone to know that he was, in fact, well-aware of this requirement, thought about it a lot while designing the library, and then, somehow, forgot to actually make sure the check was in the code. That is, it's not that he didn't know what he was doing, it's that he knew what he was doing but flubbed it.

Affected Packages

1 total 1 fixed
EcosystemPackageVulnerable rangeFix
📦npm@cloudflare/workers-oauth-providerall versions0.0.5npm install @cloudflare/workers-oauth-provider@0.0.5

Affected Products

1 product · 1 configurations
Application
workers-oauth-providercloudflare
1 version
0.0.5

Detection & mitigation playbook

Open-source dependency

  1. Detect

    Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for @cloudflare/workers-oauth-provider, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

  2. Fix

    Update @cloudflare/workers-oauth-provider to 0.0.5 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2025-4143 is resolved across your whole dependency graph.

  3. Workarounds

    Stop reflecting attacker-controlled destinations: resolve every redirect target against an allowlist of paths or hosts you own, prefer a server-side key over a full URL in the request, and reject absolute URLs entirely where the flow only ever needs a relative one.

Frequently Asked Questions

### Summary The OAuth implementation failed to check that redirect_uri was among the allowed set for the client_id. ### Impact Under certain circumstances (see below), if a victim had previously authorized with a server built on workers-oath-provider, and an attacker could later trick the victim into visiting a malicious web site, then attacker could potentially steal the victim's credentials to the same OAuth server and subsequently impersonate them. In order for the attack to be possible, the OAuth server's authorized callback must be designed to auto-approve authorizations that appear to
O3 Security · Impact-Aware SCA

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CVE-2025-4143: @cloudflare/workers — Fixed in 0.0.5