Cybercrooks have been leveraging malicious macros hidden inside XML files to distribute the Dridex financial malware, researchers have warned.
It’s not uncommon for cybercriminals to use specially crafted Microsoft Office files that contain macros for malware distribution. However, attackers usually rely on Microsoft Word and Excel documents, not the XML (Extensible Markup Language) format.
Last week, Trustwave spotted a spam run in which attackers sent out “remittance advice” emails that appeared to come from various companies. The messages instructed recipients to open the remittance advice attached to them.
According to researchers, the attachment is a Word document saved as an XML file named something like “Rem_0443NF.xml.” The file is opened with Microsoft Word if the application is installed on the device, and if macros are enabled, the malicious Visual Basic for Applications (VBA) code is executed.
“Hidden in this seemingly harmless XML file is a malicious macro document file which is compressed, encoded in base64, and stored in XML format,” explained Trustwave’s Rodel Mendrez.
The obfuscated macro downloads the Dridex Trojan from a remote server. The threat is designed to steal sensitive information, such as banking details, from infected systems.
This campaign has been active since October 2014. In January, Trustwave reported that the cybercriminals had leveraged Word and Excel documents to target banking customers in the United Kingdom.
Researchers at Sophos also reported seeing “a surge in brand new VBA malware packaged in this old and unusual format [XML].” The security firm believes it’s likely that cybercriminals have revived the format because it’s not usually associated with attacks.
“Perhaps, also, malware authors hope that the rarity of XML-type files means that some security products are unable to deconstruct it properly,” Graham Chantry, senior threat researcher with SophosLabs UK, explained in a blog post on new developments in Microsoft Office malware.
Chantry has pointed out that the attackers could have embedded the malware as scrambled data directly in the VBA code, which would have allowed the threat to function even when offline. However, the use of a downloader gives the malicious actors extra flexibility, allowing them to change the downloaded malware at any time. This also enables them to adapt the threat based on the victim’s location, and download clean files that serve as a decoy.
Didier Stevens, a handler at the SANS Institute’s Internet Storm Center, has provided some recommendations on how organizations can filter out emails that have potentially malicious XML files attached to them.
“XML declaration identifies the XML file as a Word document, and attribute w:macrosPresent=”yes” (of element w:wordDocument) indicates the presence of VBA macros,” Stevens said.