The vulnerability in Eclipse Open VSX (CVE-2026-13323) involves improper neutralization of input during web page generation (CWE-79). Specifically, the /vscode/unpkg/ endpoint serves user-uploaded HTML files with Content-Type: text/html but lacks Content-Security-Policy and Content-Disposition headers, allowing an attacker to craft malicious HTML payloads. An attacker who registers a publisher account can upload a VSIX containing this payload. When an authenticated user visits the crafted URL, the browser renders the malicious HTML inline within the open-vsx.org origin, enabling session token exfiltration, persistent Personal Access Token creation, and unauthorized publication of malicious extension versions. This creates a supply chain attack vector impacting all downstream users of Open VSX extensions, including VS Code, VSCodium, Cursor, Windsurf, and other compatible editors. The vulnerability affects version =0.1.0 of Open VSX. There is no vendor advisory or patch available at this time.

Exploitation of this vulnerability allows an attacker to execute malicious HTML in the security context of open-vsx.org, potentially leading to session token theft, creation of persistent Personal Access Tokens, and unauthorized publication of malicious extension updates. This compromises the integrity of the extension supply chain and can affect all downstream users relying on Open VSX extensions. The impact is limited to information disclosure and integrity violations but does not affect availability.

Patch status is not yet confirmed — check the vendor advisory for current remediation guidance. Until an official fix is released, users and administrators should exercise caution when installing or updating extensions from Open VSX, especially from newly registered publishers. Monitoring for suspicious extension updates and restricting publisher registrations may help reduce risk.