Necurs Rootkit Spreading Quickly, Microsoft Warns

Necurs, a nearly 2-year-old rootkit, has been spreading quickly recently and was found on 83,427 unique machines during the month of November, according to researchers at Microsoft.

Necurs is distributed through drive-by download and may occur anytime a user visits an infected website, says Tim Liu, a researcher at Microsoft's Malware Protection Center, in a blog.

As a rootkit, Necurs is particularly dangerous, Microsoft says. Once it infects a machine, it can hide itself at the root level, download additional malware and even stop security applications from functioning.

"In addition, Necurs contains backdoor functionality, allowing remote access and control of the infected computer," Liu's blog says. "Necurs also monitors and filters network activity and has been observed to send spam and install rogue security software. Nefariousness aplenty."

Srinivas Kumar, CTO at malware startup Taasera Inc., agrees. "Rootkits attack the system at the lowest level of the stack, rendering any readings by [antivirus] or endpoint agent software 'unreliable,'" he says. "Once a rootkit or bootkit has infected the system, nothing on that system can be trusted, including [a security vendor's] endpoint agent. At this point, the OS has been hijacked."

Necurs is especially crafty in the way it infects and operates on its victims, Microsoft says. For example, it provides a "command option system" that uses an obfuscated method for determining which commands are valid.

"Necurs uses what appear to be randomly numbered keys to identify if a command needs to be functional," Liu's blog says. "Given the lack of readability of the keys, it follows that the Necurs author[s] maintains some kind of interface to facilitate both the identification and enabling of commands.

"The author has a full Necurs command list, and the attacker has the option to enable some of them or not," Liu continues. "Necurs doesn't want its commands to be easily recognized by the antivirus researcher, so the command keys appear to be random numbers, garbage code or obfuscated code."

Necurs also incorporates a troubleshooting/bug report module that uses an exception-handling mechanism, Microsoft says. When an error occurs, the troubleshooting module can be used by the attacker to locate the error efficiently and improve the malware code to make it more stable.

Like other current malware, Necurs uses MD5 and SHA1 to encrypt/verify its network traffic data in transmission, Microsoft says. It also contains strong anti-security features.

"The [Necurs]driver has a very clear goal: protecting every Necurs component from being removed," the blog says. "In order to bypass PatchGuard on 64-bit operating systems, a test-signing method is enabled."

Necurs blocks a long list of antivirus products, the blog says. "The method used for blocking is simple but efficient: modify the entry point of the executable image in memory and return an unsuccessful status."

Because Necurs blocks AV products, traditional signature-based security defenses won't work against it," Taasera's Kumar observes. "The only way to detect a rootkit is through network behavior analysis," he says.

But Necurs also renders other security software's endpoint agents unreliable, Kumar notes. "The only slim chance is that our agent may flag an event before the rootkit takes charge of the OS," he says. "This is not easy, but possible. The problem is that one cannot even detect the 'egg, because the filesystem is under rootkit control."

A tool that examines unencrypted traffic before and after the infection might be able to mitigate it, Kumar says.

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