In at least three separate campaigns, online attackers have used a feature of Microsoft Office documents—known as the Dynamic Data Exchange (DDE) protocol—to download and execute malware, internet security firm Zscaler stated in a Nov. 15 analysis of the attacks.
The DDE fields allow a document, such as an Excel spreadsheet, to automatically update its data from external sources. However, the feature can also be used to embed a link to malicious code, causing the Office applications to download and execute malware. Microsoft acknowledged the issue on Nov. 8 but does not consider it a vulnerability because the program alerts the user to the activity.
Yet, over the past few weeks, Zscaler has noted at least three separate attack campaigns using the protocol to attempt to compromise victims’ systems.
“[A]ttackers are using the framework for malicious purposes, such as stealing a user’s sensitive information, uploading and executing malware on a user’s machine, altering the user’s data, and so on,” Mohd Sadique, head researcher with Zscaler’s ThreatLabZ team, stated in the analysis. “In recent cases, the DDE protocol led to ransomware, including Locky, which encrypts the victim’s data and demands ransom for the decryption.”
The attacks are the latest attempt to compromise systems using automated features of Microsoft’s Office applications. In the past, attackers have used automated scripts, such as macros, to attempt to compromise systems.
Microsoft has added warnings to give users more information about malicious actions. In the latest case, a victim would get at least one warning, and most likely two, whenever a document tried to open external data, but some users may not heed the warnings. The alert that all users should see states: “This document contains links that may refer to other files. Do you want to update this document with the data from the linked files?”
The three attack campaigns seen by Zscaler included one that downloaded a PowerShell script to exploit the system, while another attack downloaded Locky ransomware, encrypting data on the infected system. The final attack appears to be linked to Russia cyber operations and downloads spyware to the compromised system.
In Microsoft’s Nov. 8 advisory on the issue, the company urged users to take care when opening attachments from untrusted sources and detailed the scenarios under which the DDE fields could be used to deliver malware.
“The attacker would have to convince the user to disable Protected Mode and click through one or more additional prompts,” the advisory stated. “As email attachments are a primary method an attacker could use to spread malware, Microsoft strongly recommends that customers exercise caution when opening suspicious file attachments.”
The company outlined ways, primarily updating registry keys, to block Office applications from heeding the DDE fields and automatically update a document to expose the system to malware.
In addition, users of the Windows 10 Fall Creators Update can use new Windows Defender functionality, known as Exploit Guard, to prevent—among other dangerous activity—Office apps from launching child processes.
“Emerging exploits like DDEDownloader use the Dynamic Data Exchange (DDE) popup in Office documents to run a PowerShell downloader; however, in doing so, they launch a child process that the corresponding child process rule blocks,” Microsoft stated in the advisory.
Microsoft continues to research the issue and will post more information as it becomes available.