SKID: A New Javascript Backdoor


Analyzing a Customer EndPoint, we identified a new threat that expolits a recent exploit S.O. Micorosft Windows technique, discovered on April 2016 and called “Applocker Bypass“.

We called this threat “SKID“, because inside the code there are a lot of keywords with this name.


Indicators of Compromise to detect compromised EndPoint are advertised.

The new threat, identified as “SKID” uses a recent exploit S.O. Windows technique called “Applocker Bypass“; thanks to this technique it is possible to execute VB or JS scripts using the system process regsvr32.exe.

This exploit technique, discovered on April 2016, is detailed documented on this links:

We tracked this process that was exploited to execute a javascript located on the victim user’s share.

SKID” uses the following command line to been activated:

regsvr32.exe /s /n /u /i:”C:\Users\[username]\AppData\Roaming\9DB0F2C13D.txt” scrobj.dll


The analyzed file has the following details:
File name: 9DB0F2C13D.txt
File path: C:\Users\[username]\AppData\Roaming\
File size: 14646 Bytes
MD5: 28DD8E548FAE06C9114D1593150F3860

From the first lines on the header file it is possibile to notice that it’s a Jascript script, but this code appears hidden.

Revealing the code, we obtained these evidences:

  • There is a function that check the Internet connection, making an http request to host
  • The Backdoor executes http GET request to host
  • It reads the registry key HKEY_CURRENT_USER\Software\Microsoft\Notepad\<username> in which we can find references to the installed Backdoor
  • It verifies if the threat is alredy installed, checking through WMI if the regsvr32  process is in execution
  • The script downloads a real Backdoor javascript from the following address hxxps:// nella cartella dell’utentde C: \Users \<username> AppData <filename>.txt to execute it on the system, with the “Applocker Bypass” exploiting technique with the following command:  C: \windows \system32 \regsvr32.exe” /s/n/u/i: scrobj.dll

This first file appears to be a real downloader, created ad-hoc to exploit the exploiting technique described.




The second file analyzed has the following details, and it’s a real Backdoor developed in javascript and now is at the 2.0 version:
File name: changelog.txt
Downloaded from: hxxps://
File size: 43686 Bytes
MD5: 09BCF50D498C67942A4F70FDD72D2913
As the first file, also for this, the header is very similar.

The javascript file is hidden again, and the code has been revealed to analyze malware characteristics:

The Backdoor is able to obtain the following information:
Il file javascript risulta offuscato, ed è stato de-offuscato il codice in modo da analizzare le caratteristiche del malware:

  • Operating System and Architecture (32 o 64 bit)
  • It recognizes if on the system is installed or in execution one of the following antivirus:
  1. Windows Defender
  2. McAfee
  3. Avast
  4. Avira
  5. AVG
  6. TrendMicro
  7. Panda
  8. F-Secure
  9. Kaspersky
  10. Symantec
  11. Sophos
  12. Bitdefender
  13. ESET
  14. Comodo
  15. MalwareBytes
  16. Norton
  17. ClamAV
  18. TrusteerRapport
  19. DeepFreeze
  20. 360 Total Security
  21. Seqrite EndPoint Security
  22. Quick Heal
  23. Fortinet
  24. Bitdefender Endpoint Security
  25. ByteFence
  26. G-Data
  27. Webroot
  • Computer serial number
  • Local computer IP
  • Backdoor installed version (during the analysis it was v2.0)

With this Backdoor the attacker can execute the following tasks:

  • Binary files download and execution (.exe and .dll)
  • Sciprt .js or .vbs download using the exploiting “AppLocker Bypass” technique
  • Threat removal from victim PC
  • Remote command execution on victim PC


The CnC identified is the following:

The domain has been registred on November 2016, the 15 and, also the SSL host certificate is valid from the 15th November 2016 to the 16th November 2017.
From these evidences it’s possible to speculate that the attacker had implemented and distributed the Backdoor’s firsts versions on the end of 2016.


The helpful IoC to investigate and verify if the threat can be identified on further EndPoint are the following:

– MD5

CnC hostname:
CnC URL: hxxps://
Comportamento anomalo: connessioni in uscita verso (Google IP) su porta 80, protocollo HTTP