Executive brief
Open VSX, an extension marketplace for VS Code, was configured to reflect arbitrary website origins in its CORS headers while allowing credentialed requests. This allowed attackers to create a malicious webpage that steals sensitive user data like login tokens and extension information from logged-in users, or create new API tokens with permission to publish and delete extensions.
Technical details
The open-vsx.org CDN/edge layer incorrectly reflected the requesting origin in Access-Control-Allow-Origin headers together with Access-Control-Allow-Credentials: true on authenticated /user/* endpoints, violating CORS same-origin policy. An attacker could craft a malicious webpage to make credentialed cross-origin requests to user profile endpoints (/user, /user/tokens, /user/namespaces), read CSRF tokens from /user/csrf, and chain these to create new personal access tokens via /user/token/create with publish and delete permissions. The vulnerability existed in the CDN/edge configuration rather than the Open VSX application itself, which correctly configured a single exact origin.
Affected products
- Eclipse Foundation Open VSX affected from 2026-04-29 to 2026-09-07
Timeline
- 2026-09-22: disclosed: Vulnerability published