Fix SOCKSv5 method selection

So, RFC 1928 doesn't say anything about the METHODS field in the Method
Selection message being ordered in terms of preference or anything, and
the server is free to pick any of the METHODS offered by the client.

Always sending a Method Selection message with NO AUTHENTICATION REQUIRED
and USERNAME/PASSWORD set is broken on two fronts:

 * If the OpenVPN client can't handle the server picking USERNAME/PASSWORD
   due to the credentials being missing, it shouldn't offer it to the
server.

 * If the OpenVPN client has credentials, then it should always attempt to
   authenticate.  This is a security product.  "You can misconfigure it and
   it will work" is not acceptable.  Setting a username/password when the
   SOCKS server doesn't require/support that as an option is the user not
   configuring it correctly, and should be treated as such.

Also verify that the SOCKS server returned the auth that was requested.

URL: https://github.com/OpenVPN/openvpn/pull/14
Fix trac #377, trac #148
Acked-by: Gert Doering <gert@greenie.muc.de>
Acked-by: Arne Schwabe <arne@rfc2549.org>
Message-Id: <20140413130102.GR16637@greenie.muc.de>
URL: http://article.gmane.org/gmane.network.openvpn.devel/8488

Signed-off-by: Gert Doering <gert@greenie.muc.de>

diff --git a/src/openvpn/socks.c b/src/openvpn/socks.c
index 1551da846b66b59320af73acac2cfb64b48b7edc..6e29e7ad0ea43b4f5a01451d4dc092b01e741294 100644 (file)
--- a/src/openvpn/socks.c
+++ b/src/openvpn/socks.c
@@ -189,10 +189,15 @@ socks_handshake (struct socks_proxy_info *p,
   char buf[2];
   int len = 0;
   const int timeout_sec = 5;
+  ssize_t size;
+
+  /* VER = 5, NMETHODS = 1, METHODS = [0 (no auth)] */
+  char method_sel[3] = { 0x05, 0x01, 0x00 };
+  if (p->authfile[0])
+      method_sel[2] = 0x02; /* METHODS = [2 (plain login)] */
 
-  /* VER = 5, NMETHODS = 2, METHODS = [0 (no auth), 2 (plain login)] */
-  const ssize_t size = send (sd, "\x05\x02\x00\x02", 4, MSG_NOSIGNAL);
-  if (size != 4)
+  size = send (sd, method_sel, sizeof (method_sel), MSG_NOSIGNAL);
+  if (size != sizeof (method_sel))
     {
       msg (D_LINK_ERRORS | M_ERRNO, "socks_handshake: TCP port write failed on send()");
       return false;
@@ -252,6 +257,13 @@ socks_handshake (struct socks_proxy_info *p,
       return false;
     }
 
+  /* validate that the auth method returned is the one sent */
+  if (buf[1] != method_sel[2])
+    {
+      msg (D_LINK_ERRORS, "socks_handshake: Socks proxy returned unexpected auth");
+      return false;
+    }
+
   /* select the appropriate authentication method */
   switch (buf[1])
     {