HTTP Tunneling and ACE

Sandeep Singh — Tue, 29 Aug 2017 12:15:15 GMT

Introduction

HTTP tunneling is a method in which non-legitimate data is sent using HTTP protocol. This is acheived using a client-server HTTP tunneling application, and is usually done to bypass the security provided by common firewalls, as firewalls usually allow HTTP traffic. A properly formatted HTTP packet is used which passes the standard HTTP checks and L4 firewalls. HTTP tunneling works with a host running the server side applciation. The outside user runs the client side applciation, which is used to send HTTP encapsulated communication to the server application. The server then interprets this information and opens the connection to the desired host, and this bypasses the security.

Real Servers and Server Farms

To provide services to clients, you configure real servers (the actual physical servers) on the ACE. Real servers provide client services such as HTTP or XML content, hosting websites, FTP file uploads or downloads, redirection for web pages that have moved to another location, and so on. The ACE also allows you to configure backup servers in case a server is taken out of service for any reason.

Servers are organized into related groups called server farms. Servers within server farms often contain identical content (referred to as mirrored content) so that if one server becomes inoperative, another server can take its place immediately. Also, mirrored content allows several servers to share the load of increased demand.

Blocking HTTP Tunnels

Most standard firewalls cant block HTTP tunnels which are cleaverly disguised. Although firewalls do have HTTP inspection; it only checks for proper HTTP packet and header, which is there in HTTP tunnels. Several HTTP tunneling methods exist, like HTTP CONNECT, POST, GET, PUT, DELETE tunneling. To block this traffic requires a deeper packet inspection using custom regex. Custom regex is not possible in case of firewalls like PIX and FWSM

pixfirewall(config-cmap)# match request header ?

mpf-class-map mode commands/options:
accept               Accept field
accept-charset       Accept-Charset field
accept-encoding      Accept-Encoding field
accept-language      Accept-Language field
allow                Allow field
authorization        Authorization field
…
pixfirewall(config-cmap)# match request header X-?
ERROR: % Unrecognized command

ACE/PIX/ASA come with a built-in “port-misuse” directive in HTTP inspection engine. But it doesn’t really help here. The engine looks for these fixed regexes:
FIRETHRO: "[/\\\\]cgi[-]bin[/\\\\]proxy"
HTTP_PORT: "[ \t]+photo[.]exectech[-]va[.]com"
UAGENT_GNUTELLA: "[Gg][Nn][Uu][Tt][Ee][Ll][Ll][Aa]"
YAHOO: "YMSG.*"
KAZAA: "[Xx][-][Kk][Aa][Zz][Aa][Aa].*“

Configure ACE to block HTTP Tunneling

Follow these steps to block HTTP tunneling using ACE:

1) Define a class-map that matches port 80:
switch/lab(config)# class-map match-any http
switch/lab(config-cmap)# match port tcp eq www

2) Build header regexes: if we see both headers, match!
switch/lab(config)# class-map type http inspect match-all headers
switch/lab(config-cmap-http-insp)# match header X-Counter header-value .*
switch/lab(config-cmap-http-insp)# match header X-Session header-value .*

3) Instruct ACE to drop packets that match the regex
switch/lab(config)# policy-map type inspect http all-match htpolicy
switch/lab(config-pmap-ins-http)# class headers
switch/lab(config-pmap-ins-http-c)# reset

4) Apply the HTTP policy to port-80 traffic:
switch/lab(config)# policy-map multi-match nofirepass
switch/lab(config-pmap)# class http
switch/lab(config-pmap-c)# inspect http policy htpolicy url-logging

5) Map the policy to the client-side interface:
switch/lab(config)# int vlan 20
switch/lab(config-if)# service-policy input nofirepass

SSL Tunneling

Following is a sample configuration for SSL tunneling

parameter-map type http CASE_PARAM

case-insensitive

persistence-rebalance

set header-maxparse-length 65535

set content-maxparse-length 65535

class-map match-all CLEAR_TEXT_VIP

2 match virtual-address 172.20.120.19 tcp eq www

policy-map multi-match JORGE-MULTIMATCH

class CLEAR_TEXT_VIP

loadbalance vip inservice

loadbalance policy POLICY_TO_ENCRYPT_TRAFFIC

loadbalance vip icmp-reply active

appl-parameter http advanced-options CASE_PARAM

policy-map type loadbalance first-match POLICY_TO_ENCRYPT_TRAFFIC

class class-default

serverfarm ENCRYPTED-SERVERFARM

ssl-proxy client SSL-PROXY-JORGE

ssl-proxy service SSL-PROXY-JORGE

key TAC-key

cert TAC-cert

serverfarm host ENCRYPTED-SERVERFARM

rserver JORGE-SERVER 443

inservice

Verify

Check for the following ACE logs to verify if blocking is working fine

%ACE-6-302022: Built TCP connection 0x76 for vlan20:10.20.10.100/52431 (10.20.10.100/52431) to vlan100:10.10.10.101/80 (10.10.10.101/80)
%ACE-5-304001: User:10.20.10.100 Accessed URL 10.10.10.101:/cgi-bin/firepass-1.1.2a/fpserver/fpserver.cgi
%ACE-6-302023: Teardown TCP connection 0x76 for vlan20:10.20.10.100/52431 to vlan100:10.10.10.101/80 duration 0:00:00 bytes 508 Policy Close