ostap Propose Change

Ostap is a commodity JScript downloader first seen in campaigns in 2016. It has been observed being delivered in ACE archives and VBA macro-enabled Microsoft Office documents. Recent versions of Ostap query WMI to check for a blacklist of running processes:

AgentSimulator.exe
anti-virus.EXE
BehaviorDumper
BennyDB.exe
ctfmon.exe
fakepos_bin
FrzState2k
gemu-ga.exe (Possible misspelling of Qemu hypervisor’s guest agent, qemu-ga.exe)
ImmunityDebugger.exe
KMS Server Service.exe
ProcessHacker
procexp
Proxifier.exe
python
tcpdump
VBoxService
VBoxTray.exe
VmRemoteGuest
vmtoolsd
VMware2B.exe
VzService.exe
winace
Wireshark

If a blacklisted process is found, the malware terminates.

Ostap has been observed delivering other malware families, including Nymaim, Backswap and TrickBot.

References

Yara Rules

rule js_ostap_w0 {
	meta:
		author = "Alex Holland @cryptogramfan (Bromium Labs)"
		date = "2019-08-29"
		sample_1 = "F3E03E40F00EA10592F20D83E3C5E922A1CE6EA36FC326511C38F45B9C9B6586"
		sample_2 = "38E2B6F06C2375A955BEA0337F087625B4E6E49F6E4246B50ECB567158B3717B"
        malpedia_reference = "https://malpedia.caad.fkie.fraunhofer.de/details/js.ostap"
        malpedia_version = "20190905"
        malpedia_license = "CC BY-SA 4.0"
        malpedia_sharing = "TLP:WHITE"
		
	strings:
		$comment = { 2A 2A 2F 3B } // Matches on **/;
		$array_0 = /\w{5,8}\[\d+\]=\d{1,3};/
		$array_1 = /\w{5,8}\[\d+\]=\d{1,3};/
				
	condition:
		((($comment at 0) and (#array_0 > 100) and (#array_1 > 100)) or
		((#array_0 > 100) and (#array_1 > 100))) and
		(filesize > 500KB and filesize < 1500KB)
}