Windows for Pentester: Certutil

In this article, we are going to describe the utility of Certutil tool and how vital it is in Windows Penetration Testing.

TL; DR

Certutil is a preinstalled tool on Windows OS that can be used to download malicious files and evade Antivirus. It is one of the Living Off Land (LOL) Binaries.

Disclaimer

The main objective of publishing the series of “Windows for Pentester” is to introduce the circumstances and any kind of hurdles that can be faced by any Pentester while solving CTF challenges or OSCP labs which are based on Windows Operating System. Here, we do not criticize any kind of misconfiguration that a network or system administrator does for providing higher permissions on any kind of programs/binaries/files & etc.”

Table of Content

  • Introduction
    • What is certutil?
    • What is Living off Land?
    • Working with certutil?
    • What is Alternative Data Stream (ADS)?
  • Configurations used in Practical
  • Working with certutil
    • Encoding
    • Decoding
    • Hashing
    • Downloading
    • Reading Error Code
  • Penetration Testing using certutil
    • Compromising using Malicious Executable
    • Compromising with Encoded Malicious DLL
    • Compromising with Malicious Executable inside ADS
  • Mitigation
  • Conclusion

Introduction

What is Certutil?

Certutil is a CLI program that can be used to dump and display certificate authority (CA), configuration information, configures Certificate Services, backup and restore CA components, and verify certificates, key pairs, and certificate chains. It is installed as a part of Certificate Services.

What is Living off Land?

In simple words, it is an attack that works on the idea of using system tools as backdoors. File-less attack is another example of LOL attack. Attackers who use this tactic works with trusted, in most cases, preinstalled system tools to carry out their attack. Attackers use these tactics to hide their malicious activity in plain sight among the other general activity inside the network or system. As these kinds of attacks operate without triggering any alerts, it is almost impossible for investigators to determine who is behind the said malicious activity even if they discover it.

What is Alternative Data Stream (ADS)?

The NTFS file system consists of the ADS feature. This is an inconspicuous feature that was included, to provide compatibility with files in the Macintosh file system. ADS enable files to incorporate more than one stream of data. In any instance, each file consists of at least one data stream. This default data stream in Windows is recognized as :$DATA.

Windows Explorer can’t see what ADSs are in a file (or a way to erase them without actually discarding the original file) but they can be created and accessed with ease. Because they are challenging to detect, thus often used by hackers to hide files on machines that they’ve compromised. Executables in ADSs can be executed from the command line but without showing up in Windows Explorer.

Some of the CTF Challenges over HackTheBox where certutil can be used are:

Access, Arctic, BigHead, Conceal, Ethereal, Fighter, Giddy, Hackback, Jerry, Rabbit.

Configurations used in Practical

Attacker:

  • OS: Kali Linux 2019.4
  • IP:168.1.10

Target:

  • OS: Windows 10 (Build 18363)
  • IP: 168.1.11

Working with certutil

Practical #1: Encoding

Certutil contains an encode parameter. It could help to encode file content into Base64. This is a Windows equivalent to the base64 command in Linux.

When working with an executable file, we came across a scenario. In it, the uploading of the executable file was not smooth. We can use certutil to encode the executable file. Then transfer the encoded data, then decode it on the recipient machine.

In the following practical, we first created a text file named “file.txt” and wrote the “This is a plain text” line in it. We did this with Add-Content cmdlet in PowerShell. We can see that it worked when we checked the file using type command. To convert, we will use certutil with encode parameter. We will provide the text file and the file that it should write the encoded data.

Certutil adds two segments “BEGIN CERTIFICATE” and “END CERTIFICATE”. The converted contents of the file are between these two segments. We can check the encoded text using the type command.

We can use the parameter -encodehex to convert data into Hex encoded files.

Practical #2: Decoding

Certutil can decode the data encoded in Base64. Let’s show you a quick method from which you can decode the data. We will be using the file that we encoded in the previous practical. We will use certutil with -decode parameter. Then provide the encoded file and the file it should write the decoded data. We can check the decoded text using the type command.

We can use the parameter -decodehex to decode the Hex encoded files.

Practical #3: Hashing

Hashing means taking data and giving out an output string of a fixed length. Using the cryptography hashing algorithms — e.g., MD5, SHA-1, SHA-256, you can verify if two files are identical or not. The checksum is a hash value used for performing data integrity checks. It’s a kind of signature for a file. By comparing checksum, we can identify duplicate files.

Time to generate some hashes. We will use the file.txt we created earlier. First, we will generate the MD5 hash using certutil parameter -hashfile. With the parameter, file path and algorithm we can hash the file.

NOTE: While working with Systems like Windows 7, keep in mind that the hash algorithms are case-sensitive. Be sure to type, for example, “MD5”, not “md5”.

Practical #4: Downloading

In scenarios, where wget, BITSAdmin or any other convention method is blocked. Certutil can be used to download files from the internet. We will be downloading 7zip.exe from the 7zip server as shown in the image.

-URLCache Display or delete URL cache entries
-split Split embedded ASN.1 element & Save to files
-f Force Overwrite

Practical #5: Reading Error Code

Suppose you got a system error code without any message. You don’t have any source to look up the meaning of the error. This is a common scenario. Certutil can help to look up the message text for system error codes.

NOTE: Certutil can perform many more functions related to CA Certificates but we will be focusing on Penetration Testing for now.

Penetration Testing using certutil

Practical #6: Compromising using Malicious Executable

During our initial assessment, we saw that the certutil was actively downloading files from the internet without any kind of verification or assessment. This is an instance that is part of the MITRE | ATT&CK Remote File Copy Tactic.

Certutil can be used to copy a file from one system to another to stage some attacking tools or other files throughout an attack. Files can also be transferred from an outer attacker-controlled system through a Command and Control Channel to bring tools or scripts into the target network to support Lateral Movement.

In the previous practical, we downloaded a file from a remote server. Let’s see how we can compromise a Windows System using a Malicious Executable.

We started our attack with Exploit Development. We used the msfvenom tool to generate a Payload for a Reverse TCP Connection to our attacker machine. We provided the msfvenom with the appropriate LHOST and LPORT. The format of the payload was set to an Executable(.exe) File. We named it “shell.exe”. After successful execution, the file was created in our “/root” directory. Now to transfer the newly generated we decided to use the HTTP Server generated by a Python One-liner.

Now that the payload is hosted on the server, before executing the payload on the Target Machine, we need to start a Listener on Attacker Machine to capture the meterpreter session that would be generated after the execution of the payload.

After successfully starting a listener on the Attacker, its time to move to Target Machine. Here, we have a PowerShell Terminal. We need to download the payload to this machine. We will use certutil to fetch it. Certutil will make two connections to the remote web server using two different User-Agents. They will be named “Microsoft-CryptoAPI” and “Certutil URL Agent”.

NOTE: During our assessment, we found that upon execution the above command an Access Denied Error is notified. Using -verifyCTL instead of -URLCache will let you bypass this error.

After the successful transfer of the Payload to Target Machine. We executed the payload as shown in the image.

We went back to our Attacker Machine to see that a meterpreter instance is generated and captured by our listener. We run sysinfo to see the details of the Target System.

We have successfully Compromised the Target Machine using a combination of Certutil and a Malicious Executable.

Practical #7: Compromising with Encoded Malicious DLL

As seen earlier Certutil encodes file content into Base64. This opens up a lot of possibilities. This is an instance that is part of MITRE | ATT&CK Deobfuscate/Decode Files or Information Tactic.

Attackers can use Obfuscated (Difficult to detect/find) Files to conceal evidence of an attack from the analysis. Afterward, they may Deobfuscate (Unhide) those files. This is where certutil comes into the picture. It can decode the data and help bypass Antivirus, IDS/IPS Software. Certutil can also be used to decode a portable executable file that has been hidden inside a certificate file.

Payloads may be compressed, archived, or encrypted to avoid detection. These payloads may be used with Obfuscated Files or Information during Initial Access or later to mitigate detection. Sometimes a user’s action may be required to open it for deobfuscation or decryption as part of User Execution. The user may also be required to input a password to open a password protected compressed/encrypted file that was provided by the attacker. Now onto our Practical.

We started our attack with Exploit Development. We used the msfvenom tool to generate a Payload for a Reverse TCP Connection to our attacker machine. We provided the msfvenom with the appropriate LHOST and LPORT. The format of the payload was set to a Dynamic-Link Library(.dll) File. We named it “dll.txt”. We can name it any other name which is less suspicious. We use the text file so that it doesn’t rise any unnecessary flags. After successful execution, the file was created in our “/root” directory. Now to transfer the newly generated we decided to use the HTTP Server generated by a Python One-liner.

Now that the payload is hosted on the server, before executing the payload on the Target Machine, we need to start a Listener on Attacker Machine to capture the meterpreter session that would be generated after the execution of the payload.

After successfully starting a listener on the Attacker, it times to move to Target Machine. Here, we have a PowerShell Terminal. We need to download the payload to this machine and we need to do this discreetly. We run certutil with a combination of URLCache and encode separated by the pipe (|). Now the file will be downloaded as a text file and gets encoded as another text file which we named “edll.txt” for encoded DLL.

Now to execute the payload to compromise the target, we need to decode it. We use the decode parameter in certutil to decode the payload and saved it as “exploit.dll”. Now to execute this DLL we decide to use regsvr32. It executes DLL directly into memory.

We went back to our Attacker Machine to see that a meterpreter instance is generated and captured by our listener. We run sysinfo to see the details of the Target System. We have successfully Compromised the Target Machine using a combination of Certutil and an Encoded Malicious Executable.

As we talked about evading Antivirus Software. Let’s inspect the files that we generated and used in the attempt to compromise the target. We use VirusTotal for this analysis. We first inspect the “dll.txt”. Upon successful upload and analysis of the dll.txt, we see that it was detected by 54 out of 67 Antivirus Engines. That can’t be good.

So, the inspection of the dll.txt file was not acceptable. Now let’s test the file we encoded using certutil. We uploaded the edll.txt. Upon analysis of the edll.txt, we see that it was detected by 4 out of 56 Antivirus Engines. It is not perfect but it is a huge difference. 

Another flavour of this attack can be as depicted below:

We create a payload in the form of an executable(payload.exe). Then we use certutil to encode it to a specific binary. For example, “payload.enc”. Then post the output of the encoding process on Github, Pastebin or other alternative services. The purpose of this procedure is to separate the encoded payload from the stager to avoid detection. Now use the certutil on the target machine to download the content from the remote server (Github/Pastebin). Finally, decode the malicious payload into an executable extension using Certutil and execute it to compromise the Target.

Practical #8: Compromising with Malicious Executable inside ADS

We started our attack with Exploit Development. We used the msfvenom tool to generate a Payload for a Reverse TCP Connection to our attacker machine. We provided the msfvenom with the appropriate LHOST and LPORT. The format of the payload was set to an Executable(.exe) File. We named it “virus.exe”. After successful execution, the file was created in our “/root” directory. Now to transfer the newly generated we decided to use the HTTP Server generated by a Python One-liner. 

Now that the payload is hosted on the server, before executing the payload on the Target Machine, we need to start a Listener on Attacker Machine to capture the meterpreter session that would be generated after the execution of the payload.

This time we will use a different approach altogether. We are going to use the Alternative Data Stream. Alternative Data Stream (ADS) was created by Microsoft to supporting compatibility with Apple McIntosh’s file system. In the Mac, files have a huge amount of metadata in addition to regular data. To save the exe file into ADS, we need to specify the name of the file in whose ADS we want to save another file, then (:) followed by name and extension of another file. As shown, we saved the virus.exe inside the ADS of harmless.txt file.

Here, it can be observed that there is no file named virus.exe in the directory and the size of harmless.txt is 0 as well as it contains nothing as it was an originally an empty text file.

Now to execute the file that we put in the ADS; we will be using wmic. We will use the create flag followed by the path of the payload as shown in the image. It says that the Execution was successful.

We went back to our Attacker Machine to see that a meterpreter instance is generated and captured by our listener. We run sysinfo to see the details of the Target System.

We have successfully Compromised the Target Machine using a combination of Certutil and a Malicious Executable concealed in Alternative Data Stream.

Mitigation

As tools like certutil can be used by an attacker with physical access to the machine or by malicious code unknowingly downloaded by a user after a phishing or other social engineering attack.

Certutil usage should be monitored, particularly if detected it being used with -decode or -decodeHex options where that would not normally be expected in your network. It is paramount not to depend on tools that simply whitelist built-in or signed code as obviously these will be bypassed by such Living Off the Land (LOL) techniques.

Conclusion

This kind of attack is very much happening in real life. There have been multiple incidents targeted to different office environments as well as banks. It was a fun learning experience working with certutil. We are going to write more articles about other LOLS that we could find. Stay Tuned.

Certutil Operations

Living Off Land binaries

Author: Pavandeep Singh is a Technical Writer, Researcher and Penetration Tester Contact here