core: Allow loopback hosts for admin endpoint (fix #5650) (#5664)

admin.go

@@ -318,7 +318,32 @@ func (admin AdminConfig) allowedOrigins(addr NetworkAddress) []*url.URL {
 				// messages. If the requested URI does not include an Internet host
 				// name for the service being requested, then the Host header field MUST
 				// be given with an empty value."
+				//
+				// UPDATE July 2023: Go broke this by patching a minor security bug in 1.20.6.
+				// Understandable, but frustrating. See:
+				// https://github.com/golang/go/issues/60374
+				// See also the discussion here:
+				// https://github.com/golang/go/issues/61431
+				//
+				// We can no longer conform to RFC 2616 Section 14.26 from either Go or curl
+				// in purity. (Curl allowed no host between 7.40 and 7.50, but now requires a
+				// bogus host; see https://superuser.com/a/925610.) If we disable Host/Origin
+				// security checks, the infosec community assures me that it is secure to do
+				// so, because:
+				// 1) Browsers do not allow access to unix sockets
+				// 2) DNS is irrelevant to unix sockets
+				//
+				// I am not quite ready to trust either of those external factors, so instead
+				// of disabling Host/Origin checks, we now allow specific Host values when
+				// accessing the admin endpoint over unix sockets. I definitely don't trust
+				// DNS (e.g. I don't trust 'localhost' to always resolve to the local host),
+				// and IP shouldn't even be used, but if it is for some reason, I think we can
+				// at least be reasonably assured that 127.0.0.1 and ::1 route to the local
+				// machine, meaning that a hypothetical browser origin would have to be on the
+				// local machine as well.
 				uniqueOrigins[""] = struct{}{}
+				uniqueOrigins["127.0.0.1"] = struct{}{}
+				uniqueOrigins["::1"] = struct{}{}
 			} else {
 				uniqueOrigins[net.JoinHostPort("localhost", addr.port())] = struct{}{}
 				uniqueOrigins[net.JoinHostPort("::1", addr.port())] = struct{}{}

cmd/commandfuncs.go

@@ -610,7 +610,7 @@ func AdminAPIRequest(adminAddr, method, uri string, headers http.Header, body io
 	}
 	origin := "http://" + parsedAddr.JoinHostPort(0)
 	if parsedAddr.IsUnixNetwork() {
-		origin = "http://unixsocket" // hack so that http.NewRequest() is happy
+		origin = "http://127.0.0.1" // bogus host is a hack so that http.NewRequest() is happy
 	}
 
 	// form the request
@@ -619,20 +619,24 @@ func AdminAPIRequest(adminAddr, method, uri string, headers http.Header, body io
 		return nil, fmt.Errorf("making request: %v", err)
 	}
 	if parsedAddr.IsUnixNetwork() {
-		// When listening on a unix socket, the admin endpoint doesn't
-		// accept any Host header because there is no host:port for
-		// a unix socket's address. The server's host check is fairly
-		// strict for security reasons, so we don't allow just any
-		// Host header. For unix sockets, the Host header must be
-		// empty. Unfortunately, Go makes it impossible to make HTTP
-		// requests with an empty Host header... except with this one
-		// weird trick. (Hopefully they don't fix it. It's already
-		// hard enough to use HTTP over unix sockets.)
+		// We used to conform to RFC 2616 Section 14.26 which requires
+		// an empty host header when there is no host, as is the case
+		// with unix sockets. However, Go required a Host value so we
+		// used a hack of a space character as the host (it would see
+		// the Host was non-empty, then trim the space later). As of
+		// Go 1.20.6 (July 2023), this hack no longer works. See:
+		// https://github.com/golang/go/issues/60374
+		// See also the discussion here:
+		// https://github.com/golang/go/issues/61431
 		//
-		// An equivalent curl command would be something like:
-		// $ curl --unix-socket caddy.sock http:/:$REQUEST_URI
-		req.URL.Host = " "
-		req.Host = ""
+		// After that, we now require a Host value of either 127.0.0.1
+		// or ::1 if one is set. Above I choose to use 127.0.0.1. Even
+		// though the value should be completely irrelevant (it could be
+		// "srldkjfsd"), if for some reason the Host *is* used, at least
+		// we can have some reasonable assurance it will stay on the local
+		// machine and that browsers, if they ever allow access to unix
+		// sockets, can still enforce CORS, ensuring it is still coming
+		// from the local machine.
 	} else {
 		req.Header.Set("Origin", origin)
 	}