squid.exploit.txt

`Holla,  
  
since some people asked how to exploit the squid bug, I send a description  
along.  
  
Assumptions:  
~~~~~~~~~~~~  
  
1.) You use plain squid-2.2-STABLE5 or below. Also, external authentification  
is active using a some external authentication program, which basically  
follows the implementation guidelines given on the squid-webpages.  
  
2.) Your ACL's for external authentication apply often enough so that external  
authentificatoin actually happens maybe every 20 seconds to 20 minutes.  
This also depends on your password-cache settings.  
  
3.) In general, users enter correct user:password pairs.  
  
4.) No other user has sent a user:passwd pair with a newline at the end to  
the proxy until now (so we can acctually describe the effect when it  
occurs the first time).  
  
  
The exploit:  
~~~~~~~~~~~~  
  
1.) Create a base64-encoded "user:passwd\n" string, f.e.:  
# echo "foo:bar" | mimencode  
# Zm9vOmJhcgo=  
  
Note that  
# echo -n "foo:bar" | mimencode  
(notice the -n option!) will strip the trailing newline and can't be used.  
  
The newline at the end is essential for the exploit, since most external  
authenticators will read _two_ lines from the proxy and sent _two_ results  
back to the proxy, shifting all subsquent responses to authentication  
request by one.  
  
2.) telnet to your proxy and sent a valid but not authorized request (lines  
marked with a * are your input lines):  
  
# telnet proxy 8080  
Trying 123.123.123.123  
Connected to proxy.home.net  
Escape character is '^]'  
* GET http://some.domain.net HTTP/1.0  
* Proxy-Authorization: Basic Zm9vOmJhcgo=  
*  
  
Please notice the last extra newline needed for the Protocol (it has  
nothing to do with the exploit, though).  
  
An ACL must match the given domain (here, some.domain.net), which uses the  
external authentication program.  
  
3.) You will see the response for you user:passwd pair and due to assumption  
4.), this answer is accurate.  
  
Now, wait. Once a different user sents his user:password pair -- which in  
turn is correct in general as stated in assumption 3.) -- he will get the  
authentication response of _your_ empty line and most probably will be a  
HTTP/1.0 407 Proxy Authentication Required answer, but then, the user will  
try again and... get the _correct_ answer of his or her _first_ try.  
  
Now, the second answer (which most probably will be OK) is pending!  
  
4.) Try to connect again with another fake user:password (without extra  
newline), most likely using your favorite browser. Now you should profit  
from the pending OK in step 3 and get the page you want.  
  
Thats it. Please notice, that when caching is active, you can surf as long  
the name:password pair is available in the cache -- which can be quite long.  
  
cheers,  
Oezguer Kesim  
[email protected]  
  
`