Adding to a growing trend of phishing attacks wherein Windows and Office functionalities are abused to compromise victim systems, Cofense Intelligence™ has analyzed a recent campaign that uses the URL file type to deliver subsequent malware payloads. This file type is similar to a Windows LNK shortcut file (both file types share the same global object identifier within Windows) and can be used as a shortcut to online locations or network file shares. These files may abuse built-in functionality in Windows to enhance the ability of an attacker to deliver malware to endpoints.
By abusing these built-in functionalities, threat actors can complicate detection and mitigation in these scenarios, because the software is behaving exactly as it was designed to. The proliferation of abuse techniques indicates that threat actors may be increasingly prioritizing the use of such methodologies due to detection difficulties.
The emails analyzed by Cofense Intelligence include a nondescript phishing campaign that informs recipients of an attached bill, receipt, or invoice. The analysis performed for Threat ID 10993 focused on emails that deliver attached URL shortcut files with their target resource identified using the “file://” scheme. Windows environments use this scheme to denote a file resource that is on the hard drive or hosted on a network file share.
However, the target for these Uniform Resource Identifiers (URIs) can also be a remote resource. When a URL shortcut file is written to disk, Windows will attempt to validate the target denoted by the “file://” scheme. If validated, the remote resource can be downloaded to the local machine. The use of this file format and URI scheme may indicate that threat actors seek to abuse the resource resolution functionality associated with these shortcut files to deliver malware onto victims’ machines at the time the URL file is extracted from a Zip archive.
Figure 1 – URL shortcut files can reference remote file shares to deliver malware
Figure 2 – Downloading a payload over SMB is a less-common method for malware delivery
This technique showcases yet another method in which commonplace Windows features are abused by threat actors, adding to the expanding set of delivery applications crafted to distribute malware.
The nature of these files reveals the risk involved with applications that obtain files simply by issuing connection requests without user interaction. Incident responders and network defenders must devise a response plan to address this scenario, especially if enterprises and organizations operate on a Windows environment. This campaign also demonstrates that as threat actors develop new attack methodologies, more emails are likely to reach user inboxes. Therefore, it is crucial that those users can identify and report such campaigns, because they are the final line of defense at that point.
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