Hackers can abuse Microsoft Office executables to download malware

The list of LOLBAS files – legitimate binaries and scripts present in Windows that can be abused for malicious purposes, will soon include the main executables for Microsoft’s Outlook email client and Access database management system.

The main executable for the Microsoft Publisher application has already been confirmed that it can download payloads from a remote server. 

LOLBAS stands for Living-off-the-Land Binaries and Scripts and are typically described as signed files that are either native to the Windows operating system or downloaded from Microsoft.

They are legitimate tools that hackers can abuse during post-exploitation activity to download and/or run payloads without triggering defensive mechanisms.

According to recent research, even executables that are not signed by Microsoft serve purposes that are useful in attacks, such as reconnaissance.

Microsoft Office binaries

The LOLBAS project currently lists over 150 Windows-related binaries, libraries, and scripts that can help attackers execute or download malicious files or bypass lists of approved programs.

Nir Chako, a security researcher at Pentera, a company that provides an automated security validation solution, recently set off to discover new LOLBAS files by looking at the executables in the Microsoft Office suite.

He tested all of them manually and found three – MsoHtmEd.exe, MSPub.exe, and ProtocolHandler.exe – that could be used as downloaders for third-party files, thus fitting the LOLBAS criteria.

The researchers shared with BleepingComputer a video that shows MsoHtmEd reaching the test HTTP server with a GET request, indicating an attempt to download a test file.

Later in his research, Chako discovered that MsoHtmEd could also be used to execute files.

Animated by this initial success and already knowing the algorithm to find the appropriate files manually, the researcher developed a script to automate the verification process and cover a larger pool of executables faster.

In a blog post today, he explains the refinements added to the script that enabled listing the binaries in Windows and testing them for download capabilities beyond the intended design.

In total, the Pentera researcher discovered 11 new files with download and execute functionalities that meet the principles of the LOLBAS project.

Standing out are MSPub.exe, Outlook.exe and MSAccess.exe, which an attacker or a penetration tester could use to download third-party files, the researcher says.

While MSPub has been confirmed that it can download arbitrary payloads from a remote server, the other two are yet to be added to the LOLBAS list. They have not been included because of a technical error, Chako told BleepingComputer.

New LOLBAS sources

Apart from Microsoft binaries, Chako also found files from other developers that meet the LOLBAS criteria, one example being the popular PyCharm suite for Python development.

The PyCharm installation folder contains elevator.exe (signed and verified by JetBrains), which can execute arbitrary files with elevated privileges.

Another file in the PyCharm directory is WinProcessListHelper.exe, which Chako says can serve reconnaissance purposes by enumerating all the processes running on the system.

Another example of a LOLBAS reconnaissance tool he provided BleepingComputer is mkpasswd.exe, part of the Git installation folder, which can offer the entire list of users and their security identifiers (SIDs).

Chako’s journey started with two weeks to formulate a correct approach to discover new LOLBAS files, which resulted in finding three.

After understanding the concept, he spent another week creating the tools to automate the discovery. The effort paid off, as the scripts enabled him to go through “the entire pool of Microsoft binaries” in about five hours.

The reward is even larger, though. Chako told us that the tools he developed can also run on other platforms (e.g. Linux or custom cloud virtual machines), either in their current state or with minor modifications, to explore new LOLBAS territory.

However, knowing about the LOLBAS threats can help defenders define adequate methodologies and mechanisms to prevent or mitigate cyberattacks.

Pentera published a paper with full details on how researchers, red-teamers, and defenders can find new LOLBAS files.