As Emotet’s apparent hiatus continues, new malspam campaigns have risen to take its place (including a strange Emotet-Dridex hybrid eSentire reported in June ). Ostap has generally stayed out of the mainstream spotlight in the three years since its introduction  but appears each year with new modifications. Ostap’s most recent variant adds to its 2018 blacklist of processes the malware can detect and react to including VirtualBox and Hybrid Analysis. Interestingly, the authors also included the names of two security researchers in their blacklist: Hong Lee and Peter Wilson. The authors also removed Windows XP from the list, indicating they may no longer expect sandboxes to be running an XP environment.
A critical difference from Emotet is Ostap’s destructive propagation feature. Upon failure to download the payload, Ostap will search out documents (such as *.xls, *.doc, *.pdf, *.txt, *.rtf, and *.odt) on all attached drives (including mapped network drives) and replace them with copies of itself. The functionality was first observed in 2017, as well as in current samples examined by eSentire’s Advanced Threat Analytics team. In this way, Ostap is able to propagate across network shares and removable media.
The Payload: Trickbot
The Trickbot payload delivered by Ostap has been observed harvesting credentials from common applications such as Chrome, Firefox, Internet Explorer, Filezilla, Windows Remote Desktop Protocol, and VNC. The malware can also infect PoS devices with a separate module .
Another Evolving Threat
Recent Campaign Traces
Jul 3 – Ostap source code is pasted on pastebin 
July 22 – A public hybrid analysis sample points to Trickbot at 185.159.82[.]15 but fails to run, invoking a manufactured “document error”. 
July 26 – Twitter User Jammy (@jcandt) on twitter observed Ostap serving Trickbot via 185.159.82[.]15/hollyhole951/c644.php
July 30 – Twitter User Kirk Sayre (@bigmacjpg) observed trickbot being served from 185.130.104[.]236/deerhunter/inputok.php
July 31 – eSentire observes Ostap delivering trickbot and begins deobfuscating the malware. Last public deobfuscation was 2018 . C2 source was the same reported by Kirk Sayre (July 30).
Aug 5 - During the writing of this article, TrendMicro published their own analysis on the malware delivered by the C2 reported by Jammy (July 26), but did not identify the JS dropper as Ostap .
Most static indicators are outlined well by TrendMicro . However, we include one that could be useful for network monitoring as well as propose a behavioral detection.
HTTP header response: RedSparrow
Human detection of malicious documents: Malicious spam often arrives disguised as invoices, tax documents, and shipping orders. Financial departments tend to be highly targeted. Consider implementing protocols around email handling for the financial department that includes phishing awareness training and safe scrutiny of attached documents.