A rare type of Linux malware that targets VoIP telephony switches to steal metadata from call details has been discovered. ESET, a cybersecurity firm from Slovakia, said the CDRThief malware is designed to target two softswitches produced in China, the Linknat VOS2009 and VOS3000.

CDRThief works by querying internal MySQL databases used by the softswitches to gain an understanding of the VoIP platform architecture. The malware then exfiltrates private data from the switch, including call detail records (CDRs), which contain information about caller and recipient IP addresses, call duration, fee and starting time of the call.

"Based on the described functionality, we can say that the malware’s primary focus is on collecting data from the database," wrote Anton Cherepanov, senior malware researcher at ESET. "The malware can be deployed to any location on the disk under any file name."

"At the time of writing we do not know how the malware is deployed onto compromised devices," he added. "We speculate that attackers might obtain access to the device using a brute-force attack or by exploiting a vulnerability. Such vulnerabilities in VOS2009/VOS3000 have been reported publicly in the past."

ESET said it's difficult to determine the ultimate goal of attackers using CDRThief. They conclude that since the malware is designed to steal sensitive information like call metadata, it is most likely being used for cyberespionage or VoIP fraud.

And since attackers focus on stealing information on VoIP softswitch and gateway activity, the data may be used to perform International Revenue Share Fraud (IRSF). That scheme involves premium phone numbers, which are typically used to support automatic, phone-based purchases. Those numbers are offered by International Premium Rate Number (IPRN) providers, which charge telephone companies a high fee to relay calls on those numbers. Those costs are passed down to customers through monthly invoices or real-time phone crediting systems. Companies renting the premium numbers also get a cut of the profits for driving callers to that number.

Many unscrupulous IPRNs have realized they can drive more traffic, and profits, by enabling spammers and criminal groups to abuse their networks. The result is IRSF schemes, which have become increasingly popular and are difficult to detect. Malware is one of the most common gateways to this type of scheme.