TA551 also known as Shathak is an email-based malware distribution campaign that frequently targets English-speaking victims. This campaign has aimed German, Italian and Japanese speakers.
TA551 in the past pushed different families of information-stealing malware like Ursnif and Valak. After mid-July 2020, this campaign has exclusively pushed IcedID malware, another information stealer.
Chain of Events of TA551
TA551 continued to be consistent in its infection process from mid-July to November 2020 (as shown in the flow chart below). The first lure is an email spoofing an email chain. These email chains are retrieved from email clients on previously infected hosts.
The message has an attached ZIP archive and a message informing the user of a password required to open the attachment.
After opening the ZIP archive, the victim finds a Microsoft Word document with macros. If the victim enables macros on a vulnerable Windows computer, the victim’s host retrieves an installer DLL for IcedID malware. This will infect a vulnerable Windows computer.
These waves of malspam every time targeted English-speaking victims until Oct. 27, 2020. After the experts started observing Japanese templates for the Word documents, TA551 consistently targeted Japanese-speaking victims from Oct. 27-Nov. 20, 2020.
After approximately three weeks of Japanese-focused attacks, TA551 switched back to English-speaking victims starting on Nov. 24, 2020. Apart from the targeted group, TA551 continues to push IcedID as its malware payload.
Characteristics of TA551
- TA551 has distributed different families of malware, including Ursnif (Gozi/ISFB), Valak and IcedID.
- TA551 malspam spoofs legitimate email chains based on data retrieved from previously infected Windows hosts. It sends copies of these email chains to recipients of the original email chain.
- The spoofed email contains a short message as the most recent item in the chain. This is a generic statement asking the recipient to open an attached ZIP archive using the supplied password.
- File names for the ZIP archives use the name of the company being spoofed in the email.
- In 2020, experts observed emails with info.zip or request.zip as the attached ZIP archive names.
- These password-protected ZIP attachments contain a Word document with macros to install malware.
- File names for the extracted Word documents follow noticeable patterns.
- URLs generated by the associated Word macros also follow noticeable patterns.
Recent Development
TA551 has changed traffic patterns. For several months before Oct. 2020, URLs generated by Word macros to retrieve installer binaries followed a noticeable pattern. This pattern includes:
- .php?l= in the URL path
- URLs end with .cab
From November 2020, experts noticed slight changes in artifacts generated during IcedID infections, including that outside of the TA551 campaign.
These changes possibly will be an attempt by malware developers to elude detection. At the very least, they might confuse someone conducting forensic analysis on an infected host.
Conclusion
Organizations with sufficient spam filtering, proper system administration and up-to-date Windows hosts have a lesser risk of infection.
Palo Alto Networks Next-Generation Firewall customers are additionally protected from this threat with the Threat Prevention security subscription, which detects the malware. AutoFocus customers can track this activity using the TA551 and IcedID tags.
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