Pierluigi Paganini May 16, 2019

The BlackTech cyber-espionage group exploited the ASUS update process for WebStorage application to deliver the Plead backdoor.

The cyber espionage group tracked as BlackTech compromised the ASUS update process for WebStorage application to deliver the Plead backdoor.

The BlackTech group was first observed by ESET on July 2018, when it was abusing code-signing certificates stolen from D-Link for the distribution of the Plead backdoor that has been in the wild since at least 2012.

According to the experts, the cyber espionage group is highly skilled and most of its victims are in the East Asia region, particularly Taiwan.

At the end of April 2019, experts from ESET observed observed multiple attempts to deploy the Plead backdoor. In the attacks observed by the researhcers, the Plead backdoor was created and executed by a legitimate process named AsusWSPanel.exe that is associated with the Windows client for a cloud storage service called ASUS WebStorage. The executable file used in the attack is digitally signed by ASUS Cloud Corporation.

Experts noticed that all observed samples of the Plead backdoor had the file name ‘Asus Webstorage Upate.exe.’ Experts discovered that
during the software update process, the AsusWSPanel.exe module of ASUS WebStorage can create files with such filenames.

Threat actors might have had access to the update mechanism a circumstance that suggest two attack scenarios:

• Hackers hack compromise the supply chain for the ASUS WebStorage cloud service;
• Hackers were in the position to carry out a MITM attack, given that WebStorage binaries are delivered via HTTP during the update process. 

Experts believe that the second scenario is more plausible updates for the
ASUS WebStorage software are not provided through a secure connection and the process lack of validation for the binaries downloaded.

“The ASUS WebStorage software is vulnerable to a man-in-the-middle attack (MitM).” reads the advisory published by ESET. “Namely, the software update is requested and transferred using HTTP; once an update is downloaded and ready to execute, the software doesn’t validate its authenticity before execution. Thus, if the update process is intercepted by attackers, they are able to push a malicious update.”

Experts from ESET noticed that most of the affected organizations have routers made by the same vendor and their admin panels are exposed online. It is likely that attackers compromised the routers to carry out a MitM attack.

During the update mechanism for ASUS WebStorage, the client sent a request to the server to request the update, in turn the server responds in XML format, with a guid and a link included in the response. The software then checks if the installed version is older, based on the information in the guid element, and requests the update binary via the provided link. 

“Therefore, attackers could trigger the update by replacing these two elements using their own data. This is the exact scenario we actually observed in the wild. attackers inserted a new URL, which points to a malicious file at a compromised gov.tw domain,” says ESET. 

The attackers serve a Plead sample that acts as a first-stage downloader that fetches a fav.ico file from a server, whose name mimics the official ASUS WebStorage server. The downloaded file contains a PNG image and data used by the malware, which is located right after PNG data

The second-stage loader writes itself to the Start Menu startup folder to gains persistence. The loader executes shellcode in memory to load the third-stage DLL, the TSCookie.

“We see that supply-chain and man-in-the-middle attacks are used more and more often by various attackers all around the globe.” ESET concludes.  “This is why it’s very important for software developers not only to thoroughly monitor their environment for possible intrusions, but also to implement proper update mechanisms in their products that are resistant to MitM attacks,”

Pierluigi Paganini

(SecurityAffairs – Plead Backdoor, Zero-day, BlackTech group)

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