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 without appropriate security headers, allowing an attacker who registers as a publisher to upload a VSIX package with malicious HTML. When an authenticated user accesses the crafted URL, the malicious HTML executes in the context of the open-vsx.org origin, enabling session token exfiltration, persistent Personal Access Token generation, and unauthorized publication of malicious extension versions. This can lead to a supply chain attack affecting all downstream users of Open VSX extensions distributed to VS Code, VSCodium, Cursor, Windsurf, and other compatible editors. The CVSS v3.1 base score is 4.1, indicating medium severity. No official patch or remediation is currently documented.

Successful exploitation allows an unauthenticated attacker to execute malicious HTML in the context of the open-vsx.org origin for authenticated users. This can result in exfiltration of session tokens, creation of persistent Personal Access Tokens, and unauthorized publication of malicious extension updates. Consequently, this vulnerability enables supply chain attacks impacting all users of Open VSX extensions, potentially compromising their development environments and trust in extension integrity.

Patch status is not yet confirmed — check the vendor advisory for current remediation guidance. Until an official fix is available, users and administrators should exercise caution when installing or updating extensions from Open VSX, especially from newly registered publishers. Monitoring official Eclipse Foundation communications for updates is recommended.