Chapter 8. Remote OS Detection

It is sometimes very difficult to determine remotely whether an available service is susceptible or patched for a certain vulnerability. Even obtaining the application version number doesn't always help, since OS distributors often back-port security fixes without changing the version number. The surest way to verify that a vulnerability is real is to exploit it, but that risks crashing the service and can lead to wasted hours or even days of frustrating exploitation efforts if the service turns out to be patched.

OS detection can help reduce these false positives. For example, the Rwho daemon on unpatched Sun Solaris 7 through 9 may be remotely exploitable (Sun alert #57659). Remotely determining vulnerability is difficult, but you can rule it out by finding that a target system is running Solaris 10.

Taking this from the perspective of a systems administrator rather than a pen-tester, imagine you run a large Sun shop when alert #57659 comes out. Scan your whole network with OS detection to find machines which need patching before the bad guys do.

Even after you discover a vulnerability in a target system, OS detection can be helpful in exploiting it. Buffer overflows, format-string exploits, and many other vulnerabilities often require custom-tailored shellcode with offsets and assembly payloads generated to match the target OS and hardware architecture. In some cases, you only get one try because the service crashes if you get the shellcode wrong. Use OS detection first or you may end up sending Linux shellcode to a FreeBSD server.

While it isn't as exciting as busting root through a specially crafted format string exploit, there are many administrative reasons to keep track of what is running on your network. Before you renew that IRIX support contract for another year, scan to see if anyone still uses such machines. An inventory can also be useful for IT budgeting and ensuring that all company equipment is accounted for.

With the ubiquity of mobile devices and cheap commodity networking equipment, companies are increasingly finding that employees are extending their networks in undesirable ways. They may install a $20 wireless access point (WAP) in their cubicle without realizing (or caring) that they just opened up the protected corporate network to potential attackers in the parking lot or nearby buildings. WAPs can be so dangerous that Nmap has a special category for detecting them. Users may also cause sysadmins grief by connecting insecure and/or worm-infected laptops to the corporate network. Regular scanning can detect unauthorized devices for investigation and containment.

Another possible use is social engineering. Lets say that you are scanning a target company and Nmap reports a “Datavoice TxPORT PRISM 3000 T1 CSU/DSU 6.22/2.06”. You could call up the target pretending to be Datavoice support and discuss some issues with their PRISM 3000. Tell them you are about to announce a big security hole, but are first providing the patch to valued customers. Some naive administrators might assume that only an authorized engineer from Datavoice would know so much about their CSU/DSU. Of course the patch you send them is a Trojan horse that gives you remote access to sniff and traipse through their network. Be sure to read the rest of this chapter for detection accuracy and verification advice before trying this. If you guess the target system wrong and they call the police, that will be an embarrassing story to tell your cellmates.