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AusCERT 2013: Cloud-based scanner identifies new malware by its ancestry

Polymorphic malware may be good at evading signature-based scanning engines, but the application of advanced algorithms to terabytes’ worth of malware dumps is enabling one Deakin University PhD student to detect even new strains of malware by assessing their similarity to existing, known malicious code.

The approach is a departure from traditional signature-based antivirus approaches, which are easily defeated by the large volume of malware that modifies its structure or behaviour to avoid detection. Yet by feeding a scanning engine with massive volumes of new malware and collating the results in the cloud-based Simseer service, security researcher Silvio Cesare – who presented his research to AusCERT 2013 today – has been able to identify new malware strains by their heritage.

“Traditional antivirus is good at something, but signatures aren’t very good at detecting entire families of malware because if you change the malware a little bit, it changes the byte-level content of that malware,” Cesare told CSO Australia.

Rather than focusing on scanning at the byte level, Cesare’s technique looks at small ‘structures’ woven through the malware code, which are common to each family of malware. “Even moderate changes to the malware don’t change the structures very much,” he said. “Using structures, you can detect approximate matches of malware, and it’s possible to pick an entire family of malware pretty easily with just one structure.”

Cesare’s analysis translates these structures into a series of control-flow graphs with vectors that can be easily compared to the profile of other malware. To facilitate those comparisons, Cesare has built Simseer to track and categorise the heritage of what has become more than 50,000 strains of malware.

“The visualisation aspect takes a similarity matrix and draws you an evolutionary tree showing the relationships between existing and new code,” he explained. “The servers group samples into clusters, then scans an unknown sample. If it belongs to a known cluster, it’s probably evolved from that cluster.”

To maintain Simseer’s database, Cesare downloads raw malware code from the likes of open malware-sharing network VirusShare and other sources, with between 600MB and 16GB of data fed into his algorithms every night. The service runs on an Amazon EC2 cluster with around a dozen virtual servers; many submitted samples are stored online, and the analysis of new code is load-balanced between the systems.

This data is combed over to scan for similarities with known malware families and to identify new ones; new strains are only catalogued separately if they are less than 98% similar to an existing strain. This threshold has proven effective in furthering Cesare’s goal of grouping malware according to the similarities in their code.

The service is still in the experimental stages and will continue to be refined as well as potentially being commercialised in the future. For now, however, Cesare hopes it will give malware researchers and potential victims an important tool to help in their efforts to catch, analyse and combat new malware.

“I’m still letting people experiment with it to see how it helps them,” he explained. “But the idea is that if you give it a sample, and if it can find you 10 samples that are 90% of similar and eight of them belong to a particular family, it likely belongs to the same family. And, if you know the family that it belongs to, it’s much easier to understand the capability of that new malware.”

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