CVE-2025-4143: Missing validation of redirect_uri on authorize endpoint
Description
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. Unfortu...
Classification
CVE ID: CVE-2025-4143
CVSS Base Severity: MEDIUM
CVSS Base Score: 6.0
CVSS Vector: CVSS:4.0/AV:N/AC:H/AT:P/PR:N/UI:P/VC:H/VI:N/VA:N/SC:L/SI:N/SA:N/U:Amber
Affected Products
Vendor:
Product:
Exploit Prediction Scoring System (EPSS)
EPSS Score: 0.01% (probability of being exploited)
EPSS Percentile: 0.67% (scored less or equal to compared to others)
EPSS Date: 2025-05-30 (when was this score calculated)
Stakeholder-Specific Vulnerability Categorization (SSVC)
SSVC Exploitation: none
SSVC Technical Impact: partial
SSVC Automatable: false
References
https://nvd.nist.gov/vuln/detail/CVE-2025-4143https://github.com/cloudflare/workers-oauth-provider/pull/26