Exploiting Cisco Routers: Part 1
by Mark Wolfgang
This two-part article will focus on identifying and exploiting vulnerabilities and poor configurations in Cisco routers. We will then discuss the analysis of the router configuration file and will attempt to leverage this access into other systems. Additionally, we will cover the possibilities of what one may do once access to the device has been achieved. We chose to focus this article on Cisco routers due to their overwhelming market share.
Don't Forget the Router...
Pen testers may often go after the more glamorous or fun systems to hack, such as the vulnerable Solaris 8 system, or the Microsoft 2000 server vulnerable to the slew of recent RPC DCOM holes, leaving the core network infrastructure devices alone. I would caution the pen tester to think twice about overlooking these critical systems, for these are the proverbial keys to the kingdom. "So what that the telnet service is accessible to everyone on the Internet", or "I need SNMP open in order to manage my devices", the client may say. These two services alone (though not necessarily vulnerable because of bugs in the code) will usually give a pen tester (or attacker) more than enough opportunities to compromise the device.
As important as these network devices are to the overall security, reliability, and availability of the network, it is pertinent that the pen tester takes a good, hard look at them before blessing them as being secure.
Full control of the routing device can often lead to full control of the network - either by using credentials learned from the router on other network devices and systems, or by acting more deviously and diverting some traffic through a third-party system on its way to the intended destination. I once held a power company's network in my hands, so to speak. I had brute-forced the SNMP write community string, enabled TFTP, and sent the config file to my TFTP server. I then installed the required management software (it was an older Bay ARN router), and could have very easily reconfigured to router to do what I pleased. I could have removed Access Control Lists (ACLs), used the system to telnet or ssh to internal network systems, or even completely shut the network down. To make matters worse, I had seized control of this critical network device from the comfort of my office in Northern Virginia.
Identifying a Router
Routers can be configured to look just like any other system on the network - they can run a web server, an SSH daemon, chargen, and they can even appear to be running multiple X servers. For this reason, they can may often be mistaken as Unix systems.
Probably the easiest and most accurate way of identifying a host on the network as a router is by using Nmap - the venerable port scanner with very accurate OS fingerprinting. A port scan of a typical Cisco router might look like this:
If a login service such as telnet or SSH is accessible, one can simply use a standard telnet client and connect to the appropriate port. A basic Cisco router might look like the following:
The "User Access Verification" line is a trademark Cisco telnet banner. Of course one can't rely on banners alone, since system administrators sometimes modify them for deceptive reasons.
SING is a tool that one can use to assemble custom ICMP packets, to include ICMP netmask requests (ICMP type 17). In my experience, routers are typically the only devices that reply to this type of ICMP packet. Sending this type (and others) of ICMP packet may also help to positively identify a system as a router.
Using traceroute to find the organization's border router is also quite easy. Once the target site's Internet accessible systems have been identified, one can traceroute to those systems to learn the pathway to them. Typically the last hop before an Internet accessible system is the organization's border router.
Other methods exist, but typically a Telnet client or NetCat is all one needs to connect and verify the device is a router.
Vulnerability scanners typically do a great job in identifying known vulnerabilities, but can often miss significant configuration errors. Nessus 2.0.6 for example, has a list of about 44 community strings to brute-force the SNMP daemon, which maybe enough to catch the usage of common default community strings such as public, and private, but of course can't take into account site-specific strings that might be in use. As with most penetration tests, vulnerability scanners can be a good start, but are simply inadequate in matching the human element that goes into a penetration test.
For its market penetration and popularity, Cisco's IOS has relatively few high-risk vulnerabilities leading to the direct compromise of its devices. Rather, I find that poor system administration leads to the compromise of more routers than software bugs. Correspondingly, a vulnerability scanner may report that everything is fine, when there are actually a couple different avenues a pen tester could take in assessing the router, such as brute-forcing available services.
Exploiting Vulnerabilities in Cisco IOS
A vulnerability that affects most Cisco routers (when conditions are right) is the HTTP Configuration Arbitrary Administrative Access Vulnerability. This particular vulnerability should be found by all vulnerability scanners, and is trivial to exploit. It often yields full remote administrative control of the affected router. The pen tester's tool of choice is simply a web browser.
Firstly, a pen tester will fire up his web browser and point it to the vulnerable router. It may look like the following image:
Figure 1: Cisco Router HTTP Basic Authentication Prompt
After clicking the "Cancel" button, the pen tester enters the following URL into the address bar. http://10.0.1.252/level/99/exec/show/config and is presented with the startup configuration of the device.
Figure 2: Cisco Router Config Displayed
Clearly visible in the web browser is the configuration of the target router. In it, we see exactly how the router is configured, other interfaces, the Access Control Lists (ACLS) if any, the SNMP community strings, and the easily decrypted passwords! Of the three different methods in IOS of storing the password, the network administrator has chosen Vigenere - an easily reversed encryption scheme. In this case, I fire up the freely available tool GetPass from Boson and instantly reverse the hash into plain text.
Figure 3: Decrypting a Vigenere Password
Several other decryption tools exist, and most of them are free. There are web pages with CGI scripts that do the decryption, several Unix programs, and even software for your Palm Pilot. Once the password is cracked, the pen tester logs in via telnet and gains complete administrative control on the router.
As mentioned, there are three methods IOS can use to represent a password in a router config file. They are:
It should be obvious that the most secure option is the one-way hash MD5, which by its very nature, cannot be reversed. It's implemented using the command, "enable secret 0 password".
Now you may think that a vulnerability such as this one found almost two years ago should be eradicated by now, but you may be surprised. I mostly find it on test routers on a target's internal network, but that's not to say you won't find a router configured like this that's accessible from the Internet.
Exploiting Configuration Errors
The preceding section demonstrates the exploitation of a bug in Cisco's IOS that yields full remote administrative control. Due to the nature of IOS, and the few bugs like this it has, I'm going to say that more often than not, the pen tester is going to have resort to mundane stuff like brute-forcing. This section explores two different avenues for brute-forcing.
Brute-Forcing Services: SNMP is Always Fun
A basic port scan may reveal that UDP port 161 (SNMP) is open and accessible. A vulnerability scan may have produced no results and the pen tester may think that all is secure. However, if the SNMP community strings are dictionary words, the pen tester is in luck.
There are a few different tools for brute-forcing SNMP, and it is usually a fairly safe and quiet way of attacking a border router. If you're fortunate enough to have a license to the Solarwinds suite of network management tools, you'll be please to learn that quite a few tools are included that the pen tester can use to assess Cisco routers. However, for SNMP, I like to use the free SNMP audit scanner ADMsnmp, written by the group ADM. ADMsnmp is a console-based application that will mow through a wordlist pretty quickly, letting you know of any community strings it guesses. When determining the wordlist to use, I typically surf the target website and attempt to learn as much about the target as possible. I then take a bunch of acronyms, names, and departments, and try those words. If I have no luck using those words, I'll use a bigger dictionary wordfile until I'm convinced I've been thorough. Linux ships with a wordfile (/usr/share/dict/words) that contains 45427 words. Wordfiles of all shapes and sizes are available in abundance. Some are in different languages, other's have common themes like Greek Gods, or Star Trek, and others contain words that you may be completely foreign to you -- like sports-related talk for me :-)
Below is a screenshot of ADMsnmp guessing a community string. Note the differences that appear on the screen telling the user the community string guessed and the associated level of privileges.
Figure 4: ADMsnp Guessing a Read/Write Community String
The "send setrequest" string in the above image lets the user know that he has gained Read/Write privileges on the device. The first thing I usually do after gaining this level of access is to briefly walk to MIB (Management Information Base) to learn more about the device.
Once I learn the device is a router, and running Cisco's IOS, I will then have the router send its config file to one of my systems using TFTP. I do this with the following command:
I'll then check my tftpboot directory, and sure enough, the router's config file is now on my system. The Solarwinds suite has a GUI Windows-based tool that does the same thing, but I prefer to work in Linux, so I'll use the UCD-SNMP utilities.
The Solarwinds MIB browser can also be very helpful if SNMP is they only mechanism for accessing the device. It contains the vendor's standard MIBs for an astounding number of different operating systems and devices. Perhaps one can even enable Telnet via the device's MIB. Of course, what one can do via SNMP depends on the vendor and how they decide to utilize SNMP.
Cisco has chosen to invest quite a lot in SNMP, giving network managers (and less scrupulous individuals) access to a lot of useful information such as the device's routing tables, interfaces, IP addresses and more. There are also several configuration items one can set using the Cisco generic MIB.
The below image depicts a generic MIB on a Cisco device. Using Solarwinds you can browse through the MIB, and double click on the items you want to set or retrieve and the application does the work.
Figure 5: Solarwinds MIB Browser on a Cisco Device
Brute-Forcing Login Services
Brute-forcing login services such as Telnet and SSH can be somewhat harder and often noisier, but can also yield positive results for the pen tester. One of the first things to do before conducting this type of attack on the router is to determine whether or not the router is using some type of extended authentication like Tacacs or Radius. Though IOS doesn't have any means of natively locking out users after X number of login attempts, lockouts can be enabled when authentication is passed off to another system via Tacacs or Radius. The easiest way to tell if authentication is being passed to another system is to simply connect to the router using a standard Telnet client.
If the device prompts for a username, you can almost be sure that it is using some form of extended authentication. In this case, Tacacs is implemented and brute-forcing is going to be more difficult, because the pen tester will have to guess two variables, both the username and password instead of just the password. If finger is running, usernames can be gathered, but the chances of locking out a legitimate user account will likely stop most pen testers from conducting this sort of an attack. Of course, if you have permission to assess the site via "whatever means necessary" you might still want to conduct some brute-forcing up to the lockout threshold.
If it is determined that extended authorization is not in use, then the pen tester can brute-force the Telnet daemon in much the same way he did with SNMP. Brutus is a Windows-based brute-forcing tool that does a number of different protocols, and can be customized for new protocols on the fly. THC's hydra is also an excellent, Unix-based tool that is very capable of brute-forcing a number of different services. Other tools can be written in Expect, Perl, or other languages the pen tester is familiar with.
Concluding Part One
In Part One of this series we've looked at few different ways of gaining access into a Cisco router. These methods are by no means exhaustive, but they do take advantage of common vulnerabilities and configuration errors that you're likely to find while conducting penetration testing. So, by this stage in the pen test, we've gained access to the device. Now what? Come back for Part Two in a few weeks and we'll discuss where you can go from here.
This article originally appeared on SecurityFocus.com -- reproduction in whole or in part is not allowed without expressed written consent.