OAuth server workers-oauth-provider up to 0.0.4 redirect_uri

Summaryinfo

A vulnerability categorized as problematic has been discovered in OAuth server workers-oauth-provider up to 0.0.4. Affected is the function redirect_uri of the component OAuth. Such manipulation leads to redirect. This vulnerability is traded as CVE-2025-4143. The attack may be launched remotely. There is no exploit available. It is advisable to upgrade the affected component.

Detailsinfo

A vulnerability was found in OAuth server workers-oauth-provider up to 0.0.4 and classified as problematic. Affected by this issue is the function redirect_uri of the component OAuth. The manipulation with an unknown input leads to a redirect vulnerability. Using CWE to declare the problem leads to CWE-601. A web application accepts a user-controlled input that specifies a link to an external site, and uses that link in a Redirect. This simplifies phishing attacks. Impacted is integrity. CVE summarizes:

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 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. 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.

The advisory is available at github.com. This vulnerability is handled as CVE-2025-4143 since 04/30/2025. The exploitation is known to be easy. The attack may be launched remotely. No form of authentication is required for exploitation. Successful exploitation requires user interaction by the victim. Technical details are known, but there is no available exploit. This vulnerability is assigned to T1204.001 by the MITRE ATT&CK project.

Upgrading to version 0.0.5 eliminates this vulnerability. Applying a patch is able to eliminate this problem. The bugfix is ready for download at github.com. The best possible mitigation is suggested to be upgrading to the latest version.

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Productinfo

Vendor

• OAuth server

Name

• workers-oauth-provider

Version

• 0.0.1
• 0.0.2
• 0.0.3
• 0.0.4

CPE 2.3info

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CPE 2.2info

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