Summary
@cloudflare/workers-oauth-provider missing validation of redirect_uri on authorize endpoint
The OAuth implementation failed to check that redirect_uri was among the allowed set for the client_id.
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.
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.
Untrusted input controls a URL used for redirection, which can forward users to attacker-controlled sites. Typical impact: phishing and credential harvesting via a trusted domain.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
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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.
Frequently Asked Questions
- What is CVE-2025-4143? CVE-2025-4143 is a medium-severity open redirect vulnerability in @cloudflare/workers-oauth-provider (npm), affecting versions < 0.0.5. It is fixed in 0.0.5. Untrusted input controls a URL used for redirection, which can forward users to attacker-controlled sites.
- Which versions of @cloudflare/workers-oauth-provider are affected by CVE-2025-4143? @cloudflare/workers-oauth-provider (npm) versions < 0.0.5 is affected.
- Is there a fix for CVE-2025-4143? Yes. CVE-2025-4143 is fixed in 0.0.5. Upgrade to this version or later.
- Is CVE-2025-4143 exploitable, and should I be worried? Whether CVE-2025-4143 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2025-4143 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2025-4143? Upgrade
@cloudflare/workers-oauth-providerto 0.0.5 or later.