Check your BITS, because deleting malware might not be enough

Rogue Windows Background Intelligent Transfer Service jobs can linger after malware clean-up and re-infect systems

Attackers are abusing the Windows Background Intelligent Transfer Service (BITS) to re-infect computers with malware after they've been already cleaned by antivirus products.

The technique was observed in the wild last month by researchers from SecureWorks while responding to a malware incident for a customer. The antivirus software installed on a compromised computer detected and removed a malware program, but the computer was still showing signs of malicious activity at the network level.

Upon further investigation, the researchers found two rogue jobs registered in BITS, a Windows service that's used by the OS and other apps to download updates or transfer files. The two malicious jobs periodically downloaded and attempted to reinstall the deleted malware.

Even though it's not very common, attackers have abused BITS to download malware since as far back as 2007. An advantage of using this approach is that BITS is a trusted service and is not blocked by the computer's firewall.

However, the new Trojan program discovered by SecureWorks -- part of the DNSChanger malware family -- also abuses a little-known BITS feature to execute the downloaded file. This removes the need for malware to already exist on the system.

After completing the transfer, the rogue job executes a command as a BITS "notification" action. The command creates and launches a batch script called x.bat, which completes the BITS job, checks if the file has been saved and loads it in the computer's memory as a DLL.

Through this technique, the attackers created "self-contained, download-and-execute BITS tasks that persisted even after the original malware was eliminated," the SecureWorks researchers said Monday in a blog post.

Another problem is that while the Windows event log showed information about the previous BITS malicious transfers, the logged information about the pending tasks was limited. The researchers had to use other tools to parse the BITS jobs database to see the full details.

BITS jobs expire after 90 days but can potentially be renewed. In the case investigated by SecureWorks, the computer had been infected on March 4 and was cleaned by antivirus software 10 days later. The BITS job remained until it was discovered in May.

Companies should consider enumerating active BITS tasks on computers that continue to generate network or host security alerts after malware remediation, the researchers said. One way to do this is to execute the bitsadmin client from a cmd.exe session with elevated privileges by typing: bitsadmin /list /allusers /verbose.

Join the newsletter!

Or
Error: Please check your email address.
Rocket to Success - Your 10 Tips for Smarter ERP System Selection
Keep up with the latest tech news, reviews and previews by subscribing to the Good Gear Guide newsletter.

Lucian Constantin

IDG News Service
Show Comments

Essentials

Mobile

Sansai 6-Outlet Power Board + 4-Port USB Charging Station

Learn more >

Exec

Budget

Back To Business Guide

Click for more ›

Brand Post

Bitdefender 2018

Roam freely in the digital world. Critically acclaimed performance and security at your fingertips.

Most Popular Reviews

Latest Articles

Resources

PCW Evaluation Team

Louise Coady

Brother MFC-L9570CDW Multifunction Printer

The printer was convenient, produced clear and vibrant images and was very easy to use

Edwina Hargreaves

WD My Cloud Home

I would recommend this device for families and small businesses who want one safe place to store all their important digital content and a way to easily share it with friends, family, business partners, or customers.

Walid Mikhael

Brother QL-820NWB Professional Label Printer

It’s easy to set up, it’s compact and quiet when printing and to top if off, the print quality is excellent. This is hands down the best printer I’ve used for printing labels.

Ben Ramsden

Sharp PN-40TC1 Huddle Board

Brainstorming, innovation, problem solving, and negotiation have all become much more productive and valuable if people can easily collaborate in real time with minimal friction.

Sarah Ieroianni

Brother QL-820NWB Professional Label Printer

The print quality also does not disappoint, it’s clear, bold, doesn’t smudge and the text is perfectly sized.

Ratchada Dunn

Sharp PN-40TC1 Huddle Board

The Huddle Board’s built in program; Sharp Touch Viewing software allows us to easily manipulate and edit our documents (jpegs and PDFs) all at the same time on the dashboard.

Featured Content

Product Launch Showcase

Latest Jobs

Don’t have an account? Sign up here

Don't have an account? Sign up now

Forgot password?