UrlUtil.getBaseUrl builds the absolute URLs in a response — download links, icons, asset and API URLs — from the X-Forwarded-Host, X-Forwarded-Proto and X-Forwarded-Prefix request headers, with no check on whether the sender was a trusted proxy, falling back to the client-supplied Host header.

Those responses are cached under keys that do not include the host (extension.json since 0.6.0, namespace.details.json since 0.9.0, sitemap since 0.14.5, latest.extension.version.vscode since 0.34.2). A single request carrying a forged header therefore places attacker-chosen URLs into an entry served to every other client for the lifetime of that entry — one hour by default, and cluster-wide where ovsx.redis.enabled is set.

The VSIX download URL, its signature URL and the public key URL are all derived from the same base URL, so extension signing does not limit the impact: an attacker who poisons an entry supplies the package, the signature over it, and the key used to verify it.

Exploitability depends on deployment topology. A server reachable directly by clients, or fronted by a proxy that relays the client's X-Forwarded-Ho…