New point-of-sale malware Multigrain steals card data over DNS

The malware was designed for stealth operation inside restricted PoS environments

Security researchers have found a new memory-scraping malware program that steals payment card data from point-of-sale (PoS) terminals and sends it back to attackers using the Domain Name System (DNS).

Dubbed Multigrain, the threat is part of a family of malware programs known as NewPosThings, with which it shares some code. However, this variant was designed to target specific environments.

That's because unlike other PoS malware programs that look for card data in the memory of many processes, Multigrain targets a single process called multi.exe that's associated with a popular back-end card authorization and PoS server. If this process is not running on the compromised machine, the infection routine exists and the malware deletes itself.

"This shows that while developing or building their malware, the attackers had a very specific knowledge of the target environment and knew this process would be running," security researchers from FireEye said in a blog post.

FireEye did not name the PoS software that Multigrain targeted. However, threats like this show the need for companies to monitor the DNS traffic that originates from their own networks for suspicious behavior.

Multigrain was designed with stealth in mind. It is digitally signed, it installs itself as a service called Windows Module Extension and more importantly, it sends data back to attackers via DNS queries.

Stolen payment card data is first encrypted with a 1024-bit RSA key and then it's passed through a Base32 encoding process. The resulting encoded data is used in a DNS query for log.[encoded_data].evildomain.com, where "evildomain" is a domain name controlled by the attackers. This query will appear in the authoritative DNS server for the domain, which is also controlled by the attackers.

This technique, while not specific to Multigrain, allows attackers to pass data out of restricted environments where other Internet communication protocols are blocked.

"Sensitive environments that process card data will often monitor, restrict, or entirely block the HTTP or FTP traffic often used for exfiltration in other environments," the FireEye researchers said. "While these common internet protocols may be disabled within a restrictive card processing environment, DNS is still necessary to resolve hostnames within the corporate environment and is unlikely to be blocked."

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