BKDR_YEMROK.A
W32/Staser.AD!tr (Fortinet), Trojan.Win32.Staser.sio (Kaspersky), TrojanDownloader:Win32/Yemrok.A (Microsoft), Win32/ServStart.EN trojan (Eset),
Windows 2000, Windows Server 2003, Windows XP (32-bit, 64-bit), Windows Vista (32-bit, 64-bit), Windows 7 (32-bit, 64-bit)
![](/vinfo/imgFiles/legend.jpg)
Threat Type: Backdoor
Destructiveness: No
Encrypted: No
In the wild: Yes
OVERVIEW
This backdoor arrives on a system as a file dropped by other malware or as a file downloaded unknowingly by users when visiting malicious sites.
It executes commands from a remote malicious user, effectively compromising the affected system.
It retrieves specific information from the affected system.
It deletes the initially executed copy of itself.
TECHNICAL DETAILS
Arrival Details
This backdoor arrives on a system as a file dropped by other malware or as a file downloaded unknowingly by users when visiting malicious sites.
Installation
This backdoor drops the following copies of itself into the affected system and executes them:
- %System%\{random 6 letter}.exe
(Note: %System% is the Windows system folder, which is usually C:\Windows\System32.)
It adds the following mutexes to ensure that only one of its copies runs at any one time:
- Nationalrxs
Autostart Technique
This backdoor registers itself as a system service to ensure its automatic execution at every system startup by adding the following registry entries:
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\
Enum\Root\LEGACY_NATIONALRXS\
0000
Service = "Nationalrxs"
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\
Enum\Root\LEGACY_NATIONALRXS\
0000
DeviceDesc = "Nationallmh Instruments Domain Service"
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\
Services\Nationalrxs
ImagePath = "%System%\{random 6 letter}.exe"
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\
Services\Nationalrxs
DisplayName = "Nationallmh Instruments Domain Service"
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\
Services\Nationalrxs
Description = "Providesqix a domain server for NI security."
It registers as a system service to ensure its automatic execution at every system startup by adding the following registry keys:
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\
Enum\Root\LEGACY_NATIONALRXS
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\
Services\Nationalrxs
Backdoor Routine
This backdoor executes the following commands from a remote malicious user:
- Download other file
It connects to the following URL(s) to send and receive commands from a remote malicious user:
- xl.{BLOCKED}ddos.com
Information Theft
This backdoor retrieves the following information from the affected system:
- Computer Name
- OS Version
- Memory availability
- CPU speed
Other Details
This backdoor deletes the initially executed copy of itself
SOLUTION
Step 1
Before doing any scans, Windows XP, Windows Vista, and Windows 7 users must disable System Restore to allow full scanning of their computers.
Step 2
Identify and terminate files detected as BKDR_YEMROK.A
- Windows Task Manager may not display all running processes. In this case, please use a third-party process viewer, preferably Process Explorer, to terminate the malware/grayware/spyware file. You may download the said tool here.
- If the detected file is displayed in either Windows Task Manager or Process Explorer but you cannot delete it, restart your computer in safe mode. To do this, refer to this link for the complete steps.
- If the detected file is not displayed in either Windows Task Manager or Process Explorer, continue doing the next steps.
Step 3
Delete this registry key
Important: Editing the Windows Registry incorrectly can lead to irreversible system malfunction. Please do this step only if you know how or you can ask assistance from your system administrator. Else, check this Microsoft article first before modifying your computer's registry.
- In HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Enum\Root
- LEGACY_NATIONALRXS
- LEGACY_NATIONALRXS
- In HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services
- Nationalrxs
- Nationalrxs
Step 4
Scan your computer with your Trend Micro product to delete files detected as BKDR_YEMROK.A. If the detected files have already been cleaned, deleted, or quarantined by your Trend Micro product, no further step is required. You may opt to simply delete the quarantined files. Please check this Knowledge Base page for more information.
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