Wednesday, October 22, 2008

Risk Factors for AV Scanning

repair If you work in a large company then you probably are familiar with the all-too-regular process of your desktop AV software performing a scheduled full disk scan. This may happen a few times a month, and during the scan (which may last a few hours) you typically experience quite sluggish performance. You may also get the sinking feeling that most files are being needlessly scanned (again). Kaspersky, a security software vendor that includes AV, gets that feeling as well. Treating all files on your desktop as equally likely to contain malware is wasteful, but without any criteria to discern less-likely from more-likely malware candidates, the current regime remains. Kaspersky observes that users are only willing to wait a few seconds at the outside for AV to perform its checks, and typically AV scans are limited to what can be done in these windows of user-defined expectations.

Kapersky has decided to make AV scanning more efficient not by making it faster but by doing less, as determined by risk-based criteria. And they were recently issued a US patent for this approach. If you have been looking for a simple example to highlight the difference between traditional security and risk-based security then your search is over.

Be warned that the patent is repetitive and not very clearly written, in keeping with the style of such documents. Patents are lawyers' solution to the legal optimisation problem of being both vague (to support the broadest claims) and specific (giving details in an embodiment that demonstrates the invention). Also towards the end, beginning with the paragraph describing FIG. 2, you can read the convoluted legalese required to define a "computer" and a "network".

Trading Risk against Speed

The purpose of the patent is "balancing relatively quick (but less thorough) anti-malware checks with more thorough, but also more time-consuming, anti-malware checks". The basic approach is to employ different scanning strategies for known files that have been previously scanned, and new files whose status is unknown to the AV software. Known files will receive a quick signature-based scan, while unknown files may be subject to more detailed scans.

When unknown executable files are first launched a risk assessment is performed to determine the appropriate level of scanning. The risk assessment evaluates a collection of risk factors that produces a metric which determines whether to rate the file as having a high, medium or low risk of containing malware. This rating in turn determines the thoroughness of the scan to be performed on the file. Options for a more detailed anti-malware scan can include heuristics analysis, emulating file execution (in an isolated environment, and steeping through particular instructions) or the statistical analysis of instruction patterns. Beyond local checks, the AV software may also consult online scanning services for additional information. Employing these more sophisticated scanning methods increases the rate of detection at a cost of additional processing time.

Risk Factors

The patent provides some example risk factors for the purpose of satisfying the embodiment requirement of the invention. While the risk factors are intended only as examples, they are interesting nonetheless.

Online Status

The patent mentions that it can take between 15 minutes to 2 hours to update local AV databases when new malware appears. It is suggested to contact an AV server directly to obtain the latest information available. If the executable is sitting on a blacklist then it is likely to be malware (that's why its on the list), and if its on a whitelist then the likelihood of malware being present is low.

File Origin

If the origin of the file is a storage medium such as CD or DVD then it it less likely to have malware than if the software was distributed over the internet. Email attachments are always suspicious. For downloaded files, the URL source of the download should be considered to determine if the origin is a suspicious web site or P2P network.

File Compression

Malware is now commonly compressed (packed) to thwart signature-based methods of virus detection. Packed files should be treated as being more suspicious than unpacked (plain) files (see here for a general discussion on how malware author use packing to hide their payloads).

File Location

The current location and/or path to the file can also be considered, since some executable files install themselves in a particular directory, especially those directories that are infrequently used. For example, the Temporary Internet Files folder is a higher risk than the My Documents folder.

File Size

Relatively small executable files executed are more suspicious than a large executable files since propagating malware does not want to draw attention to itself by transferring large files. Sending a large number of emails with a relatively small attachment is much more practical. Kaspersky states that files sent out in this manner are on the order of 50-100 kilobytes (which, if packed, reduces to something on the order of 20-50 kilobytes).

Installer File

Malware often propagates by sending out small installer files that when executed triggers a process of downloading a much larger malware payload from a web server or a file server on the Internet.

Digital Signature

Files that are digitally signed are less likely to contain malware than unsigned files.

Surprisingly the patent also mentions the possibility of alerting the user with a popup that gives them the option to skip or minimize the scan of the unknown file. The patent states that "as yet a further option, the user can manually choose to run some of the anti-virus scans in the background after the new software has been launched, but not necessarily the entire spectrum of available technologies, which obviously increases the risk that a virus can infect the computer" (italics added).

Risk supporting Business

The idea of Kaspersky is to vary malware scanning sophistication based on well-defined risk-factors. Presumably they have a sufficiently large data set on these risk factors to facilitate a transformation into hard (numeric) decision criteria. The patent does not describe how the various risk factors will be combined to produce a risk decision, but much tuning will be required.

Note that the purpose of the patent is not to reduce the risk of being infected by malware but to provide (security) risk support for the decision to improve user response times by reducing the overall effort for malware scanning. And this is clearly the task of the IT Security Risk function - balancing business and security requirements using risk analysis.

But to be clear, we don't get something for nothing, and the likelihood of being infected by malware will actually increase simply because less scanning will be done and the risk factors will not correlate perfectly with the presence of malware. And this is the next important responsibility of the IT Security Risk function - to make business aware of the residual risk in following the Kaspersky approach and getting acceptance for this risk. Hopefully Kaspersky will provide some data to help here.

I posed the question on LinkedIn whether people would support deploying this type of AV, and the resounding answer was no.

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olj said...

Interesting article. Others are also going into this space calling this trustbased evaluation. It might be that they are not allowed to call it risk-based due to the patent.
So an attacker has first to manipulate the trust database, before he will send his malware out.

Dr. Luke O'Connor said...


I wasn't aware of any of these variants. Please post a few links if you have them. Thanks

regards luke