New “ThreadKit” Office Exploit Builder Emerges

A newly discovered Microsoft Office document exploit builder kit has been used for the distribution of a variety of malicious payloads, including banking Trojans and backdoors, Proofpoint reports.

The exploit builder kit was initially discovered in October 2017, but Proofpoint’s researchers have linked it to activity dating back to June 2017. The builder kit shows similarities to Microsoft Word Intruder (MWI), but is a new tool called ThreadKit.

In June 2017, the kit was being advertised in a forum post as being able to create documents with embedded executables and embedded decoy documents, and several campaigns featuring such documents were observed that month. The documents would perform an initial check-in to the command and control (C&C) server, a tactic also used by MWI.

The documents were targeting CVE-2017-0199 and were focused on downloading and executing a HTA file that would then download the decoy and a malicious VB script to extract and run the embedded executable. The payload was Smoke Loader, which in turn downloaded banking malware.

In October, ThreadKit started targeting CVE 2017-8759 as well, but continued to use the initial C&C check-in and the HTA file to execute the embedded executable, Proofpoint says. However, changes were made to the manner in which the exploit documents operate and new exploits were integrated as well.

In November, ThreadKit was quick to incorporate exploits for new Microsoft Office vulnerabilities, and started being advertised as capable of targeting CVE 2017-11882 too. Soon after, campaigns that featured the previously observed check-in already started to emerge.

In February and March 2018, the kit was embedding new exploits, targeting vulnerabilities such as an Adobe Flash zero-day (CVE-2018-4878) and several new Microsoft office vulnerabilities, including CVE-2018-0802 and CVE-2017-8570.

At the same time, the researchers noticed a large spike in email campaigns featuring ThreadKit-generated Office attachments packing these exploits. The exploits appear copied from proofs of concept available on a researcher’s GitHub repo.

As part of these attacks, the attachments would drop the contained packager objects into the temp folder, then the exploits would execute the dropped scriptlet file, thus leading to the execution of the dropped batch files, which in turn run the executable.

Proofpoint found that not all ThreadKit documents contain a valid URL for the statistics check-in (some contain placeholder URLs). Furthermore, not all documents followed the same execution chain, with some scripts modified to perform other actions, a customization that may be provided as a service by the kit author.

“In 2017, several new vulnerabilities entered regular use by threat actors and the first months of 2018 have added to that repertoire. Document exploit builder kits like ThreadKit enable even low-skilled threat actors to take advantage of the latest vulnerabilities to distribute malware. Organizations and individuals can mitigate the risk from ThreadKit and other document exploit-based attacks by ensuring that clients are patched for the latest vulnerabilities in Microsoft office and other applications,” Proofpoint concludes.

Related: Microsoft Patches Zero-Day Vulnerability in Office

Related: Microsoft Manually Patched Office Component: Researchers

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Ionut Arghire is an international correspondent for SecurityWeek.

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