SLoad (TH-163) is the protagonist of increasing and persistent attack waves against the Italian panorama since Q3 2018 and then in 2019 (e.g N020419, N040619, N010819), but also against the UK and Canada as reported by Proofpoint. Ten months ago, we wrote about the complex infection chain the sLoad malware threat was using during its attack campaigns, and today we are looking at the evolution of the threat by dissecting one of its latest attacks.
During our CSDC monitoring operation, we recently noticed some changes in the infamous attack waves related to sLoad, which is known for adopting a complex infection chain using to spread additional malware. For this reason Cybaze-Yoroi ZLAB dissected one latest ones.
According to CERT-PA investigations, the malware has recently been delivered using legit certified emails (PEC). These recent attack waves were targeting Italians Organizations and consultants affiliated
This time the zip does not hide powershell code, such the appended one recovered in the past waves. The archive contains two files: a corrupted PDF file and a VBScript. The first one is designed to deceive the unaware user and force him to open the runnable script.
In the following tables are shown some basic information about samples contained in the zip archive.
|Brief Description||Sload visual basic script loader|
Table 1: Information about SLoad .vbs dropper
|Brief Description||Sload corrupted pdf file|
Table 2: Information about SLoad .pdf file
Opening the vbs dropper is possible to see an obfuscated script containing several junk instructions like unused variables and commented codes. After a deobfuscation phase is possible to see the inner logic. The purpose of this script is launch start a powershell script retrieved from the attacker infrastructures and, in the meantime, decoy the victim.
Code snippet 1: Deobfuscated vbs dropper
The malware downloads a fake jpg using the using “bitsadmin.exe” tool from “hxxps://dreamacinc[.com/UCP9dATGyt6mJ/srdzHcN4bWUum[.jpg”. The usage of native tools allow the script to operate under the radar avoiding several AVs controls. The fake jpg actually contains a powershell script.
Code snippet 2: Downloaded powershell code
The first action the script does is to set a scheduled task to grant persistence on the infected machine. Then, after selection a random active process on infected machine (“System” in this specific infection) and concatenation it with the “%AppData%\Roaming” path, it stores four different files in his installation folder.
All of them are embedded in the script; furthermore, two of them (“domain.ini” and “main.ini”) are encrypted using the “ConvertFrom-SecureString” native function. Then, the script runs the “UoqOTQrc.tmp” file, having the only purpose to execute the “UoqOTQrc.ps1” file contained in the same folder.
Code snippet 3: content of “UoqOTQrc.tmp” file.
Code snippet 4: content of “UoqOTQrc.ps1” file.
In the same way, the “UoqOTQrc” script decrypts the “mini.ini” file using the “ConvertFrom-SecureString” function and the ecnryption key contained in “$sv8eJJhgWV3xAN7Uu” variable, a sequential integer array.
The decrypted “main.ini” script tries to ping a URL generated selecting three ascii char-codes in ranges [65-90] and [67-122]. Then, it decrypts “domain.ini” using the key in the “$main_key” variable. In the end, it saves the results in the “btc.log” file. Continuing the analysis of “main.ini” is possible to spot that the script also grabs system information to check-in the newly infected host.
At this point, another malicious file is downloaded. The malware retrieves it from “hxxps://<C2_URL>/doc/x2401.jpg”. Once again, this is not a real jpg, but rather another obfuscated powershell layer.
Code snippet 5: Obfuscated content of “x2401.jpg” file.
Code snippet 6: Deobfuscated content of “x2401.jpg” file.
Like previous script, this one perform the same operations and create other four file in “%AppData%\Roaming\<active_process>” path. This time the files are:
The first executed file is “<random_name>.tmp”. It is not obfuscated and its only purpose is the execution of “<random_name>.ps1”. The content of “<random_name>.ps1” file is the following. The latest script decrypt the content of “config.ini” file. The following figure shown both encrypted and decrypted “config.ini” file.
This script performs the same operation described in “main.ini” file but use different URLs stored in the “web.ini” file. Also this time, the file is decrypted using an integer array from 1 to 16 as key and contained in “$mainKey” variable.
Finally, it tries to download the final payload with the following piece of script. However, at the time of analysis, all the C2 URLs seems to be down, so we are not able to detect the final payload family.
Code snippet 7: script to download the final payload
To better understand the evolution of sLoad infection chain, we compared attack attempts observed since 2018 and the latest ones. In both cases, the infection vector is a carefully themed malicious email, weaponized with zip archive containing two files. In the first case the starting point is a “.lnk” file and in the second one the chain starts with a “.vbs” script.
The sLoad attack chain observed months ago was characterized by some pieces of powershell code appended to the tail of the zip archive. Probably, this technique become more detectable during the time, so it could have been deprecated in latest infections attempts. For both malware variants, the archive contains a legit image (or pdf) used to deceive the unaware user. Moreover, in the first analyzed variant, the core of the infection is mainly based on powershell scripts and LOLbins. However, the latest stages uses a mix of Powershell and Visual Basic Scripts.
The agent body is still quite similar in the core structure, however the bot now supports new commands such as “Exec” and “Eval”, the latter is able to download further code through the Bitsadmin utility instead of directly rely on “Net.WebClient” primitive. Also, the “ScreenCapture” function have been removed from the new version of the code, in favor to the enhancement of the agent persistence through scheduled task.
sLoad is keeping evolving their TTPs and represents a vivid threat for the Italian cyber-panorama. Also, many times, especially during the last months, its activities in the country involved the abuse of certified mailboxes (PEC) targeting associated professionals and consultants, along with private companies. Additionally, the quality of the latest phishing emails is high: the group adopted templates and naming conventions actually in use by Italian Revenue Agency (“Agenzia delle Entrate”).
The plentiful usage of LOLbins, Powershell scripts and SSL encrypted channels, makes detection of this threat difficult for automated systems, and frequently requires analysis abilities or high quality threat intelligence sources to detect and tackle
Experts published a post on the Yoroi blog:
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