Intrinsic Security

Serpent – The Backdoor that Hides in Plain Sight

This article was written by Darshan Rana.

Overview: 

A new backdoor malware campaign known as ‘Serpent’ is targeting French government agencies and construction firms. To distribute the attack chain, the threat actor uses a macro-based Microsoft Word document file. The attack vector is exploiting a third-party Windows package manager to install Serpent. 

The initial document has a macro showing some of the malicious URL that tries to connect and download the payload. Later, this payload will attempt to connect to a command-and-control C2 server to steal sensitive data. 

 

Behavioural Summary: 

The figure below shows an overall process chart of serpent activity. 

 Figure 1: Process Chart of Serpent Backdoor 

The initial email contains a Microsoft Word document with a malicious macro script. When macros are enabled by the user, the document starts to execute the malicious VBA macro code. 

Figure 2: GDPR Themed Document 

 

Macro content:  

Figure 3 below indicates the malicious VBA macro details of the document file in which the malicious URLs are found: 

“hxxps[://]www[.]fhccu[.]com/images/ship3[.]jpg” 

Figure 3: Macro view of Document 

The above-mentioned URL is used to download a “ship3.jpg” file to the system. The malware is able to detect and extract steganographic embedded data from this file containing base64 encoded PowerShell commands, as shown in Figure 4. 

Figure 4: Downloaded Steganographic Image 

Figure 5: Extract the embedded code from Image File 

The decoded PowerShell script is shown below in Figure 6. The Chocolatey package is downloaded and installed by using this script. The script will also install Python, including the pip package, by using the Chocolatey package. 

hxxps[://]www[.]fhccu[.]com/images/7[.]jpg 

Figure 6: Base64 decoded PowerShell script 

The above-mentioned URL is used to download a “7.jpg” file to the system. Just like the “ship3.jpg” above, it contains a base64 encoded PowerShell script that is embedded by steganography. The Python script, stored within 7.jpg, is saved as “MicrosoftSecurityUpdate.py”. This python script creates a new bat file and executes it. Then executed bat file brings a new python script which has a final serpent payload. Shown in Figure 8. 

The exploit chain wraps up by opening a shortened URL that leads to the Microsoft Office help site.

Figure 7: Downloaded Image from payload 

Figure 8 below represents a portion of the decoded Python script that indicates it is actual “Serpent Backdoor”. 

Figure 8: Extracted and Base64 decoded Python Script 

For command and control (C2), the threat actor deploys a Tor proxy, for example: 

cmd_url_order = ‘hxxp[://]mhocujuh3h6fek7k4efpxo5teyigezqkpixkbvc2mzaaprmusze6icqd[.]onion[.]pet/index[.]html’  

This Serpent backdoor pings this “cmd_url_order” server, located at a onion[.]pet Tor proxy domain, on a regular basis. These pings expect responses for the attacker to perform further command action on infected machine to gain access or steal the sensitive data. 

Figure 9: Extracted and Base64 decoded Python Script 

The malware connects to termbin[.]com, a website associated with a command-line Pastebin application named Termbin, to transmit the results of any specified command. Termbin allows for text to be blindly submitted to a central website and will return a URL to access that data later. The malware will transmit the data and extract this unique URL. 

The malware then sends a request to the “cmd_url_answer” server with the hostname and the TermBin URL included in the header. 

cmd_url_answer = ‘hxxp[://]ggfwk7yj5hus3ujdls5bjza4apkpfw5bjqbq4j6rixlogylr5x67dmid[.]onion[.]pet/index[.]html’ 

The attacker could use this “cmd_url_answer” URL to monitor the bin outputs and see what the compromised host’s response. 

Serpent Attack Chain: 

The Serpent Backdoor cycle shown below, explains how the attack vector works and how it proceeds. 

Figure 10: Serpent Backdoor Attack Chain 

MITRE ATT&CK TIDs

TID  Tactic  Description 
T1566.001  Initial Access  Phishing: Spear phishing Attachment 
T1059.001  Execution  Command and Scripting Interpreter: PowerShell 
T1059.005  Execution 

Command and Scripting Interpreter: Visual Basic 

T1059.006  Execution  Command and Scripting Interpreter: Python 
T1041  Exfiltration 

Exfiltration Over C2 Channel 

T1133  Persistence  External Remote Services 

T1027.003 

Defense Evasion 

Obfuscated Files or Information: Steganography 

 

Table 1: MITRE ATT&CK TIDs 

 

YARA 

rule Serpent_Backdoor 

{   

 

    meta:   

 

           description = “Serpent Backdoor”   

           author = “VMware Threat Research”   

           exemplar_hashes = “8912f7255b8f091e90083e584709cf0c69a9b55e09587f5927c9ac39447d6a19”   

   

  strings:   

 

        $string1 = /www\.fhccu\.com\/images\/[a-z0-9A-Z]+\.jpg/ nocase   

        $string2 = /Microsoft_Office_Word_Update-[0-9]+-[a-zA-Z]+\.bat/ nocase   

        $string3 = “NaHash” wide ascii nocase   

        $string4 = “Une mise a jour de Microsoft Word est necessaire” wide ascii nocase   

        $string5 = /http:\/\/([a-zA-Z]+(\d[a-zA-Z]+)+)\.onion\.pet\/index\.html/ nocase   

    

 condition:   

 

        all of them   

 

} 

 

Indicators of Compromise (IOCs) 

Indicator  Type  Context 
f6d2becc3531e98e7c6331d3e5b269a54a83c1af8f9605d6daea6531a6d72b99  SHA256  Serpent Backdoor  
11c4774cde50030cdd0eb9926debb7d0d6a5323fa5e19cd94dde4d0b2a052348  SHA256  Serpent Backdoor  
8912f7255b8f091e90083e584709cf0c69a9b55e09587f5927c9ac39447d6a19  SHA256  Serpent Backdoor  
f988e252551fe83b5fc3749e1d844c31fad60be0c25e546c80dbb9923e03eaf2  SHA256  Serpent Backdoor  
64d7efad5d25b855cea56d47acc033ad48cf955ec3e16fbe122313eb0b25ba77  SHA256  Serpent Backdoor  
aab32bd7b6e2a2098eb0d7a2e738d5a26280146de229f22fcbd6a7d717cc53a4  SHA256  Serpent Backdoor  
5d1889cc28a2b17f7fa993440a498deeff66042eda42433c265aa1feb831cafb  SHA256  Serpent Backdoor  
8f469afa7040aeefd994109b994981d3844f3672  SHA1  Serpent Backdoor  
bfae2bfe69aa1d38e74968d0d7bf63347729b7b0  SHA1  Serpent Backdoor  
2d6f1ed1236727b36a92dd44cd987c36d6fb7e35  SHA1  Serpent Backdoor  
7061126f43f46b32b9e3b845a27e035b8f04c44b  SHA1  Serpent Backdoor  
0293f35f9d2232dea64b51bea00a4756963c74a3  SHA1  Serpent Backdoor  
ba5b233e352302357dca40b506a50e423413b335  SHA1  Serpent Backdoor  
22b9558d009736a59e41c2bcb80d664fc1cd64c3  SHA1  Serpent Backdoor  
855147e49bd9320984a9bc642623ef73  MD5  Serpent Backdoor  
fe5d7c63cdd96c80f5610a228238edb7  MD5  Serpent Backdoor  
321e04294c04db10d5dbf05051e540e2  MD5  Serpent Backdoor  
2dc1ee3b6dde3b12085cdcb4da5f4e8a  MD5  Serpent Backdoor  
6b2a8a0e3016ab637288cd362f4c7d4e  MD5  Serpent Backdoor  
a8413c1c31055637a657394eafa025ad  MD5  Serpent Backdoor  
f127db6ba149431cb38ca114d07d62d7  MD5  Serpent Backdoor  
hxxps[://]www[.]fhccu[.]com/images/ship3[.]jpg  URL  Serpent Backdoor  
hxxps[://]www[.]fhccu[.]com/images/7[.]jpg  URL  Serpent Backdoor  
hxxp[://]mhocujuh3h6fek7k4efpxo5teyigezqkpixkbvc2mzaaprmusze6icqd[.]onion[.]pet/index[.]html  URL  Serpent Backdoor  
hxxp[://]ggfwk7yj5hus3ujdls5bjza4apkpfw5bjqbq4j6rixlogylr5x67dmid[.]onion[.]pet/index[.]html  URL  Serpent Backdoor  

Table 2: Indicator of Compromise  

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