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Nmap Network Scanning

SOLUTION: Hack Version Detection to Suit Custom Needs, such as Open Proxy Detection

Problem

An important part of securing any network is identifying dangerous hosts. Nmap's service detection system is a flexible, reliable way to do this. It can help identify vulnerable versions of software, find misconfigured servers, and more. But sometimes actually trying to misuse services in ways the stock version scan doesn't dare to is the best way to determine if they are actually vulnerable.

Open proxies are servers that will blindly relay requests from untrusted hosts to servers of their choosing. Running these inside a network can be extremely dangerous for many reasons, as attackers can:

  • Launch attacks that appear to come from your network

  • Steal bandwidth or other network services from you

  • Pretend to be an internal client to further escalate their privileges inside your organization

This provides good motivation for hacking version detection to specifically try to exploit open proxies. We could probably map out which ports are proxies by using Nmap's normal proxy match lines, but the best, and only real way to prove an application is vulnerable is to actually exploit it yourself.

[Note]Note

This solution was contributed by Nmap developer Doug Hoyte.

Solution

The first thing we do is copy the nmap-service-probes file so we can work on a temporary copy:

mkdir ~/proxydetect
cp /usr/local/share/nmap/nmap-service-probes ~/proxydetect

Next we want to temporarily force Nmap to use our temporary file:

export NMAPDIR=$HOME/proxydetect

Now we need to add a probe and match line to the file, so open up your favorite editor and place the following text into your copy of nmap-service-probes. A good place to put it is after all the match lines in the NULL probe, but immediately before the next Probe line (GenericLines).

Probe TCP ProxyProbe q|GET http://insecure.org/ HTTP/1.1\r\nHost: insecure.org\r\n\r\n|
rarity 1
ports 1-65535
totalwaitms 20000
match proxy m|^HTTP/1.[01] 200 OK\r?\n.*TITLE>Insecure.O|s p/Open HTTP Proxy!!/

Now Nmap will actually try to request an HTTP download from insecure.org by treating any scanned ports as proxies. We will start to see the following in scans of networks containing open proxies:

PORT   STATE SERVICE VERSION
80/tcp open  proxy   Open HTTP Proxy!!

Discussion

The placement of our probe, the low rarity value, and extensive ports range help ensure that our custom probe is tried very soon into the service scan so that other probes like GetRequest don't simply identify this as a proxy before we've had a chance to use our active probe.

We also used a totalwaitms directive to make Nmap wait longer for this probe to time out. This can be necessary because not only are we dealing with the latency and unreliability of the connection between us and the proxy, but also the latency and unreliability of the connection between the proxy and the server containing the page we requested (insecure.org).

Keep in mind that many other protocols can be proxied in addition to HTTP. Version detection will identify proxies for many of them including FTP, POP3, IMAP, and SMTP. SOCKS proxies have special match lines that determine information on the authentication options the proxy has configured. As we did in this solution, often we can use version detection to tell whether such proxies are open or not by using custom probes files. However, more complicated tests are probably best done with NSE scripts.

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