Security researchers uncovered new delivery and evasion techniques adopted by Agent Tesla remote access trojan (RAT) to get around defense barriers and monitor its victims.

Typically spread through social engineering lures, the Windows spyware not only now targets Microsoft’s Antimalware Scan Interface (AMSI) in an attempt to defeat endpoint protection software, it also employs a multi-stage installation process and makes use of Tor and Telegram messaging API to communicate with a command-and-control (C2) server.

The differences between v2 and v3 of Agent Tesla appear to be focused on improving the success rate of the malware against sandbox defenses and malware scanners, and on providing more C2 options to their attacker customers.

During the height of the pandemic, a variant of the malware was found to spread via COVID-themed spam campaigns to steal Wi-Fi passwords alongside other information such as Outlook email credentials from target systems.

Then in August 2020, the second version of Agent Malware increased the number of applications targeted for credential theft to 55, the results of which were then transmitted to an attacker-controlled server via SMTP or FTP.

Agent Tesla now attempts to modify code in AMSI in a bid to skip scans of malicious payloads fetched by the first-stage downloader, which then grabs obfuscated base64-encoded code from Pastebin (or Hastebin) that acts as the loader for the Agent Tesla malware.

AMSI is an interface standard that allows applications and services to be integrated with any existing antimalware product that’s present on a Windows machine.

To achieve persistence, the malware copies itself to a folder and sets that folder’s attributes to “Hidden” and “System” in order to conceal it from view in Windows Explorer, the researchers explained.

The email accounts used to spread Agent Tesla are often legitimate accounts that have been compromised. Organizations and individuals should, as always, treat email attachments from unknown senders with caution, and verify attachments before opening them.