CVE-2025-4143: The OAuth implementation in workers-oauth-provider that is part of MCP framework https://github.com/cloudflare/workers-mcp , did not correctly validate that re

The OAuth implementation in workers-oauth-provider that is part of MCP framework https://github.com/cloudflare/workers-mcp , did not correctly validate that re

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 requi

Description from the National Vulnerability Database, published by cna@cloudflare.com

What it means for a small business

If your business uses Workers-oauth-provider, install the vendor's update. If you're not sure whether you use it, ask whoever manages your computers.

How likely is an attack? The Exploit Prediction Scoring System, run by FIRST (the global association of security response teams), puts the odds of this one being exploited in the next 30 days at 0.32%, higher than 25% of CVEs. That’s low, but low isn’t zero, and the odds jump the day someone publishes an exploit. Scored October 29, 2026, updated daily.

Check your own setup

Website check: reads what your site runs from the outside (WordPress version, plugins it can see, old server software) and matches it against this database.
Server check: what your web server says about itself, and whether that version has known flaws.

Sources

Part of the Huttenlocher Technologies vulnerability database, refreshed twice a day from CISA and the NVD. The title, category and plain-language guidance are ours; check the linked record before acting on a detail. This is general guidance, not a review of your systems.

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