pgAdmin 4's Webserver authentication source is intended to accept an identity asserted by the web server or reverse proxy in front of pgAdmin, delivered through the WSGI/CGI environment. WebserverAuthentication.getuser() read config.WEBSERVERREMOTEUSER from request.environ and, when that returned nothing, fell back to reading the same name directly from the inbound HTTP request headers via request.headers.get(). An inbound HTTP header is written by whoever sends the request, so any client able to reach pgAdmin could supply that header itself and be authenticated as any username it named, including an existing Administrator, without presenting a password or any other credential. The environment lookup could also be satisfied by a client-supplied header whenever WEBSERVERREMOTEUSER was configured to an HTTP-prefixed or hyphenated name such as HTTPXFORWARDEDUSER or X-Forwarded-User, since WSGI servers place inbound headers into the environment under exactly those names. Deployments are affected only when 'webserver' is enabled in AUTHENTICATIONSOURCES.

The fix distinguishes a genuine CGI/WSGI variable from a header-derived one and implicitly trusts only the former. A header-asserted identity is now accepted only when the operator explicitly opts in via WEBSERVERREMOTEUSERFROMHEADER, the request arrives from a peer listed in WEBSERVERTRUSTEDPROXIES, and, when configured, a shared secret supplied in WEBSERVERSHAREDSECRETHEADER matches WEBSERVERSHAREDSECRET under a constant-time comparison. The trusted-peer check deliberately reads the real socket peer address rather than request.remoteaddr, because ProxyFix rewrites the latter from the client-controlled X-Forwarded-For header and would otherwise allow an attacker to claim to be the trusted proxy. As defence in depth, login() now refuses any account whose authsource is not 'webserver', so a misconfigured trust gate cannot be used to assume an internal or LDAP account.

This issue affects pgAdmin 4: from 6.2 before 9.18.