Eclipse Open VSX Registry served user-supplied HTML files from the "/vscode/unpkg/" endpoint without a Content‑Security‑Policy or a proper Content‑Disposition header. This flaw is a CWE-79 cross‑site scripting vulnerability. An unauthenticated attacker can register a publisher account, upload a VSIX containing a malicious HTML payload, and force an authenticated user to visit the resulting URL. The browser then renders the file inline within the open‑vsx.org origin context, allowing the attacker to exfiltrate the user’s session token, generate permanent Personal Access Tokens, and publish malicious extension versions. This cross‑site scripting flaw therefore enables a supply‑chain attack that can spread malware to all editors that consume Open VSX extensions.

The vulnerable product is Eclipse Foundation’s Eclipse Open VSX Registry, affecting all releases before version 1.0.2. Any downstream editor that installs extensions from Open VSX – such as VS Code, VSCodium, Cursor, Windsurf, and other compatible editors – is impacted when they receive an update that contains a malicious extension uploaded via this route.

The CVSS score of 4.1 indicates a low severity level. The EPSS score is below 1 %, reflecting a very low probability of exploitation at the time of analysis. The vulnerability is not listed in the CISA KEV catalog. Nevertheless, the flaw can be exploited with minimal effort: an attacker needs to create a publisher account, upload a malicious VSIX, and wait for an authenticated user to load the extension. No special network privileges are required, so the exploit is straightforward in environments where users routinely install or update extensions from Open VSX. The likely attack vector is a supply‑chain attack that involves an authenticated user visiting a maliciously crafted URL.