server: add reverse port forwarding restrictions

Although reverse port forwarding (sharing client ports with the server)
should not generally leak any resources from the server to the client,
the facility may nevertheless be abused if the client is able to open a
server port which is otherwise meant for some other purpose on the
server. (This might happen, for instance, if a service on the server has
crashed or becomes somehow disabled, thus freeing the port which would
otherwise be occupied by the service.)

To mitigate such potential abuse, disable reverse port forwarding by
default and introduce server option --reverse to enable it explicitly.
Additionally, subject reverse port forwarding remotes to server-side
--authfile restrictions (for instance, "^R:0.0.0.0:7000$").

README.md

@@ -104,7 +104,9 @@ $ chisel server --help
     when <user> connects, their <pass> will be verified and then
     each of the remote addresses will be compared against the list
     of address regular expressions for a match. Addresses will
-    always come in the form "<host/ip>:<port>".
+    always come in the form "<remote-host>:<remote-port>" for normal remotes
+    and "R:<local-interface>:<local-port>" for reverse port forwarding
+    remotes. This file will be automatically reloaded on change.
 
     --auth, An optional string representing a single user with full
     access, in the form of <user:pass>. This is equivalent to creating an
@@ -117,6 +119,9 @@ $ chisel server --help
     --socks5, Allows client to access the internal SOCKS5 proxy. See
     chisel client --help for more information.
 
+    --reverse, Allows client to specify reverse port forwarding remotes
+    in addition to normal remotes.
+
     --pid Generate pid file in current directory
 
     -v, Enable verbose logging

example/users.json

@@ -7,6 +7,7 @@
 	],
 	"ping:pong": [
 		"^0.0.0.0:[45]000$",
-		"^example.com:80$"
+		"^example.com:80$",
+		"^R:0.0.0.0:7000$"
 	]
-}
+}

main.go

@@ -107,8 +107,9 @@ var serverHelp = `
     when <user> connects, their <pass> will be verified and then
     each of the remote addresses will be compared against the list
     of address regular expressions for a match. Addresses will
-    always come in the form "<host/ip>:<port>". This file will be
-    automatically reloaded on change.
+    always come in the form "<remote-host>:<remote-port>" for normal remotes
+    and "R:<local-interface>:<local-port>" for reverse port forwarding
+    remotes. This file will be automatically reloaded on change.
 
     --auth, An optional string representing a single user with full
     access, in the form of <user:pass>. This is equivalent to creating an
@@ -120,6 +121,9 @@ var serverHelp = `
 
     --socks5, Allows client to access the internal SOCKS5 proxy. See
     chisel client --help for more information.
+
+    --reverse, Allows client to specify reverse port forwarding remotes
+    in addition to normal remotes.
 ` + commonHelp
 
 func server(args []string) {
@@ -134,6 +138,7 @@ func server(args []string) {
 	auth := flags.String("auth", "", "")
 	proxy := flags.String("proxy", "", "")
 	socks5 := flags.Bool("socks5", false, "")
+	reverse := flags.Bool("reverse", false, "")
 	pid := flags.Bool("pid", false, "")
 	verbose := flags.Bool("v", false, "")
 
@@ -167,6 +172,7 @@ func server(args []string) {
 		Auth:     *auth,
 		Proxy:    *proxy,
 		Socks5:   *socks5,
+		Reverse:  *reverse,
 	})
 	if err != nil {
 		log.Fatal(err)

server/handler.go

@@ -95,14 +95,23 @@ func (s *Server) handleWebsocket(w http.ResponseWriter, req *http.Request) {
 		clog.Infof("Client version (%s) differs from server version (%s)",
 			v, chshare.BuildVersion)
 	}
+	for _, r := range c.Remotes {
+		if r.Reverse && !s.reverseOk {
+			clog.Debugf("Denied reverse port forwarding request, please enable --reverse")
+			failed(s.Errorf("Reverse port forwaring not enabled on server"))
+			return
+		}
+	}
 	//if user is provided, ensure they have
 	//access to the desired remotes
 	if user != nil {
 		for _, r := range c.Remotes {
+			var addr string
 			if r.Reverse {
-				continue
+				addr = "R:" + r.LocalHost + ":" + r.LocalPort
+			} else {
+				addr = r.RemoteHost + ":" + r.RemotePort
 			}
-			addr := r.RemoteHost + ":" + r.RemotePort
 			if !user.HasAccess(addr) {
 				failed(s.Errorf("access to '%s' denied", addr))
 				return

server/server.go

@@ -25,6 +25,7 @@ type Config struct {
 	Auth     string
 	Proxy    string
 	Socks5   bool
+	Reverse  bool
 }
 
 // Server respresent a chisel service
@@ -39,6 +40,7 @@ type Server struct {
 	socksServer  *socks5.Server
 	sshConfig    *ssh.ServerConfig
 	users        *chshare.UserIndex
+	reverseOk    bool
 }
 
 var upgrader = websocket.Upgrader{
@@ -53,6 +55,7 @@ func NewServer(config *Config) (*Server, error) {
 		httpServer: chshare.NewHTTPServer(),
 		Logger:     chshare.NewLogger("server"),
 		sessions:   chshare.Users{},
+		reverseOk:  config.Reverse,
 	}
 	s.Info = true
 	s.users = chshare.NewUserIndex(s.Logger)