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Help: I Got Hacked. Now What Do I Do?

Published: May 7, 2004

Security Management

Jesper M. Johansson, Ph.D., CISSP, MCSE, MCP+I

Security Program Manager
Microsoft Corporation

See other Security Management columns.

Welcome back. After the very long Patch Management article last month, this months article is much shorter and to the point. Let’s just say you did not install the patches like we discussed last month. Now you got hacked. What to do?

Cleaning a Compromised System

So, you didn’t patch the system and it got hacked. What to do? Well, let’s see:

  • You can’t clean a compromised system by patching it. Patching only removes the vulnerability. Upon getting into your system, the attacker probably ensured that there were several other ways to get back in.

  • You can’t clean a compromised system by removing the back doors. You can never guarantee that you found all the back doors the attacker put in. The fact that you can’t find any more may only mean you don’t know where to look, or that the system is so compromised that what you are seeing is not actually what is there.

  • You can’t clean a compromised system by using some “vulnerability remover.” Let’s say you had a system hit by Blaster. A number of vendors (including Microsoft) published vulnerability removers for Blaster. Can you trust a system that had Blaster after the tool is run? I wouldn’t. If the system was vulnerable to Blaster, it was also vulnerable to a number of other attacks. Can you guarantee that none of those have been run against it? I didn’t think so.

  • You can’t clean a compromised system by using a virus scanner. To tell you the truth, a fully compromised system can’t be trusted. Even virus scanners must at some level rely on the system to not lie to them. If they ask whether a particular file is present, the attacker may simply have a tool in place that lies about it. Note that if you can guarantee that the only thing that compromised the system was a particular virus or worm and you know that this virus has no back doors associated with it, and the vulnerability used by the virus was not available remotely, then a virus scanner can be used to clean the system. For example, the vast majority of e-mail worms rely on a user opening an attachment. In this particular case, it is possible that the only infection on the system is the one that came from the attachment containing the worm. However, if the vulnerability used by the worm was available remotely without user action, then you can’t guarantee that the worm was the only thing that used that vulnerability. It is entirely possible that something else used the same vulnerability. In this case, you can’t just patch the system.

  • You can’t clean a compromised system by reinstalling the operating system over the existing installation. Again, the attacker may very well have tools in place that tell the installer lies. If that happens, the installer may not actually remove the compromised files. In addition, the attacker may also have put back doors in non-operating system components.

  • You can’t trust any data copied from a compromised system. Once an attacker gets into a system, all the data on it may be modified. In the best-case scenario, copying data off a compromised system and putting it on a clean system will give you potentially untrustworthy data. In the worst-case scenario, you may actually have copied a back door hidden in the data.

  • You can’t trust the event logs on a compromised system. Upon gaining full access to a system, it is simple for an attacker to modify the event logs on that system to cover any tracks. If you rely on the event logs to tell you what has been done to your system, you may just be reading what the attacker wants you to read.

  • You may not be able to trust your latest backup. How can you tell when the original attack took place? The event logs cannot be trusted to tell you. Without that knowledge, your latest backup is useless. It may be a backup that includes all the back doors currently on the system.

  • The only way to clean a compromised system is to flatten and rebuild. That’s right. If you have a system that has been completely compromised, the only thing you can do is to flatten the system (reformat the system disk) and rebuild it from scratch (reinstall Windows and your applications). Alternatively, you could of course work on your resume instead, but I don’t want to see you doing that.

This list makes patching look not so bad, yes? We may hate patches, but the alternative is decidedly worse.

The topic for the next article is still up to debate. If you have ideas, comments or feedback of any kind, as always you may click the “Contact Us” link below and tell me.

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