Internet Security

Agent Tesla Updates SMTP Data Exfiltration Technique
Introduction Agent Tesla is a Windows-based keylogger and RAT that commonly uses SMTP or FTP to exfiltrate stolen data. This malware has been around since 2014, and SMTP is its most common method for data exfiltration. Earlier today, I reviewed post-infection traffic from a recent sample of Agent Tesla. This activity revealed a change in Agent Tesla's SMTP data exfiltration technique. Through November 2021 Agent Tesla samples sent their emails to compromised or possibly fraudulent email accounts on mail servers established through hosting providers. Since December 2021, Agent Tesla now uses those compromised email accounts to send stolen data to Gmail addresses. Shown above: Flow chart of recent change in Agent Tesla SMTP data exfiltration. SMTP exfiltration before the change Agent Tesla is typically distributed through email, and the following sample was likely an attachment from malicious spam (malspam) sent on 2021-11-28. SHA256 hash: bdae21952c4e6367fe534a9e5a3b3eb30d045dcb93129c6ce0435c3f0c8d90d3 File size: 523,919 bytes File name: Purchase Order Pending Quantity.zip Earliest Contents Modification: 2021-11-28 19:55:50 UTC SHA256 hash: aa4ea361f1f084b054f9871a9845c89d68cde259070ea286babeadc604d6658c File size: 557,056 bytes File name: Purchase Order Pending Quantity.exe Any.Run analysis from 2021-11-29: link The packet capture (pcap) from Any.Run's analysis shows a typical SMTP data exfiltration path. The infected Windows host sent a message with stolen data to an email address, and that address was on a mail server established through a hosting provider. Shown above: Traffic from the Any.Run analysis filtered in Wireshark. Shown above: TCP stream of SMTP traffic shows stolen data sent to the compromised email account. Example after the change The following Agent Tesla sample was likely an attachment from malspam sent on 2021-12-01. SHA256 hash: 6f85cd9df964afc56bd2aed7af28cbc965ea56e49ce84d4f4e91f4478d378f94 File size: 375,734 bytes File name: unknown Earliest Contents Modification: 2021-12-01 05:02:06 UTC SHA256 hash: ff34c1fd26b699489cb814f93a2801ea4c32cc33faf30f32165b23425b0780c7 File size: 537,397 bytes File name: Partial Shipment.exe Any.Run analysis from 2021-12-01: link The pcap from Any.Run's analysis of this malware sample shows a new data exfiltration path. The infected Windows host sent a message with stolen data to a Gmail address using a compromised email account from a mail server established through a hosting provider. Shown above: Traffic from the Any.Run analysis filtered in Wireshark. Shown above: TCP stream shows stolen data sent to Gmail address using the compromised email account. Final words The basic tactics of Agent Tesla have not changed. However, post-infection traffic from samples since 2021-12-01 indicates Agent Tesla using STMP for data exfiltration now sends to Gmail addresses. Based on the names of these addresses, I believe they are fraudulent Gmail accounts, or they were specifically established to receive data from Agent Tesla. --- Brad Duncan brad [at] malware-traffic-analysis.net	Brad 422 Posts ISC Handler Dec 30th 2021
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