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Vulnerability Squid Affected Those running Squid 2.2.STABLE5 and earlier Description Oezguer Kesim found following. He found a security bug in squid, a web proxy cache, freely available at http://squid.nlanr.net/ Here you find the short Buglog-entry as shown at http://squid.nlanr.net/Versions/v2/2.2/bugs/ Please note that the bug applies whenever a external authenticator is used. After decoding the base64 encoded "user:password" pair given by the client, squid doesn't strip out any '\n' or '\r' found in the resulting string. Given such a string, any external authenticator will receive two lines instead of one, and most probably send two results. Now, any subsequent authentification exchange will has its answer shifted by one. Therefore, a malicious user can gain access to sites he or she should not have access to. 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. 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. Solution Fixed in 2.3 branch. Patch available at: http://squid.nlanr.net/Versions/v2/2.2/bugs/squid-2.2.stable5-newlines_in_auth.patch