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Network Testing 101: If Your Name's Not Down, You're Not Getting In

Looking at the basics of network testing, user enumeration is critical. If we can get usernames, access is only a hop skip and a jump away. Well, perhaps only a decent dictionary brute-force away.

The thing is how do we get these usernames? A few basic network pentesting tricks are listed here. Also, as a lot of user names are predictable combinations (such as a combination of first and last names, and initials) it can be fun to find amusing user names on a network.

Simple User Name Enumeration

Time to start with some of the simple stuff, SNMP (Simple Network Management Protocol). Some interesting MIBs (Management Information Base) that result in user enumeration are:



*nix in general



  • Windows    USERS
  • Windows    SHARES

The MIBs listed above give away usernames. Some are obvious. The ones that are less obvious are RUNNING SOFTWARE PATHS and (in Windows) INSTALLED SOFTWARE these may disclose information in the path names as shown below:

. = STRING: “/usr/bin/login”
. = STRING: “/bin/bash”
. = STRING: “/home/auser/tail”

As we can see, the user name auser is disclosed if the full path of the running binary is used. Remember this works only if the user has used the full path to run the process.

The snmpwalk tool is a good place to start for enumerating SNMP data out of a host. There is also the small matter of the community string you’ll also need, however in many cases you can go with the defaults and get information back. Changing these from the default is often overlooked when SNMP is enabled on servers.

Print My User Name

Web and telnet interfaces on printers are often unauthenticated, unencrypted or use default or weak passwords. It is often possible to connect to these repositories of information leakage and grab document names, share locations, and most importantly user names.

As a lot of printers have no lockout controls, even if admin account passwords have been changed you can often brute force passwords on these safely. If we can gain admin access to the printer, there may be other interesting options available as well. In one case, we came across an option to fax a copy of every document printed to a number of our choice. 

Old Problems Never Die

Username enumeration on a Windows domain can be easy or a pain. On a box that accepts null connections we win. We can get the users and also the password policy, shares etc, and tools like enum and enum4linux still have a valuable place in the tool kit. But in a modern Windows AD domain don’t forget the use of LDAP. If it is possible to use null binds via LDAP, tools like, ldp.exe and nmap (—script ldap-search) are a good starting point to give you that user list. However, if you don’t have null shares or anonymous bind then you may need to make authenticated connections to the domain to get the same data. This means that one bad password on the network is a foothold to accessing the rest of the domain.

Research, Research, Research

In a lot of Exchange environments the user’s email address will contain their username. Robert Smith, for example, is [email protected] - it’s likely that “rsmith” is his login. But do remember that with common names this may not be the case.  A lot of this kind of data can be gathered from company web pages or Intranet sites…. or even bouncing a couple of emails into the organisation can work for this. If the company has an internal anonymously accessible wiki this can be a nice resource as well.

Listen, Did You Smell That?

Sniffing network traffic can also help out with delivering those user names. You may even get those passwords you’re looking for - never discount the amount of clear text protocols that are still in use. Also many companies will use TLS/SSL on their public web sites, but not encrypt internally.

I Never Metadata I Didn’t Like

In Office documents the metadata will contain, amongst other things, the name of the user who created that document. If we know the schema, this can give you the username. Also if the company writes Silverlight or .NET applications, then decompilation can give you pathnames, again with valid usernames.

Now Pay Attention 007

There is a reason to wear headphones with no music playing. If you are sitting onsite amongst IT Staff or developers you may hear the phrase: “What user should I log on as?” around you. If you are lucky they may even shout out passwords. Also there is the old tried and true method of just asking. Some call it social engineering but that’s a topic for another post. 

Sharing Your Toys

So you find a file share. Now there are lots of awesome things you can do: SMB relay attacks, trojan documents, DLL injection (if some one is dumb enough to share the wrong thing). But one of the other things you can do when a domain user visits the share is have a file there that points back to us. This could be an image in the document, a second embedded document in our Excel sheet (that we host), or a malicious shortcut file with an icon on our machine. When they access this, a bit of metasploit SMB sniffing and we can get the username as well as NTHASH and (if they are using it) the LMHASH. 

Guess Who - Are You “bin”?

On smtp, ftp, and ssh there have been ways to brute force out usernames. This is ok, but is really dependant on the list of usernames you start with. In the spirit of recycling, never throw anything away - every time you gather a name, put it in a file. Next time you have a chance to brute-force out names on an SMTP server via RCPT EXPN and VRFY you will have a good starting point.

Go Wide!

So you’ve got your big list of users? Now we take a big dictionary and hit go! - lock out the accounts and get asked to leave OR perhaps there is a better way? Time to chose a common password and wide-band it across all the accounts. A usual rule is to assume they lock the accounts after 3 failed attempts. So we could choose 2 candidate passwords and try those. If you haven’t found anything at all about the password policy before this stage, now would be a good time to do it. When we know what we can risk, we can make the call and do some brute forcing.  For me, Medusa is my brute forcer of choice. A nice feature is that Medusa that will let you look for “Joe Logins” as well as blank passwords. The nice thing about this one is the tool is modular and supports a large list of protocols. So now from a big list of users we send out 2 passwords per user per hour/day/week. Eventually we get a hit. Next we can use this authenticated access to get more user names and start the brute force loop again. Voila!


Exploiting the Pizza Thief

A while back we came across an exploitation scenario with an FTP server that we were assessing that we thought was interesting enough to share - largely because its an issue that has been known about since 1999, but doesn’t seem to be widely exploited - at least publicly.

First its important to understand how FTP works when in passive mode, which is the most common configuration we come across in deployment nowadays. FTP uses two separate TCP connections to the FTP server - a command channel and a data channel. FTP commands are sent over the command channel, which is usually on port 21. The data channel is the connection that is used for a transfers of data, including directory listings or file downloads and uploads, and in passive mode this is another connection from the client to the FTP server on a port opened by the FTP server in order to send or receive the data.

For example, this is how a sample passive FTP session might go, including the commands that will be issued to the FTP server in the background. Note the response to the PASV command, which supplies the IP address and port (in high/low byte order) for the client to connect to:

220 FTP server ready.

Name: user

---> USER user

331 Password required for user.

Password: password

---> PASS password

230 User user logged in.

---> SYST

215 UNIX Type: L8

Remote system type is UNIX.

Using binary mode to transfer files.

ftp> passive

Passive mode on.

ftp> ls

---> PASV

227 Entering Passive Mode (192,168,1,1,195,149).

---> LIST

150 Opening ASCII mode data connection for file list

drwx------ 3 user users 104 Jul 27 01:45 my_files

226 Transfer complete.

ftp> quit

---> QUIT

221 Goodbye.

Visually this will look something like the following:

Interestingly, looking at the RFC for FTP (RFC959) shows that the two connections do not both have to come from the same client, which allows FTP to support some less common usage scenarios such as server to server transfers using a common client. For our purposes it raises the possibility that if we can hit the data port that is allocated on the FTP server at the right time, we will get the file transfer or directory listing that another client has requested, like follows:

This race condition issue, as it turns out, has been known since 1999 as the “Pizza Thief” exploit (CVE-1999-0351), and turns out to be fairly easy to exploit in actual usage scenarios that we’ve come across.

In practice, guessing the port comes down to a combination of two factors - how much load the FTP server is under, and how randomly it allocates the data ports for downloads. If the port allocation is sequential (which is not uncommon) this is fairly trivial, however as enterprise FTP deployments commonly have a small fixed range of ports allowed through a firewall for passive FTP, this can also be practical to exploit in cases where the data port allocation is randomised (even aside from any weaknesses in just how random the allocation actually is). If the server allows anonymous FTP, or you can obtain an account through other means, this just makes the job of predicting the data ports easier for you.

We ended up exploiting this scenario by writing a quick Python script to brute-force connect to the range of ports the server we were looking at was using, and grabbed a number of documents the organisation was sharing with an international business partner. Turns out FTP isn’t so secure after all? ;)