CWE-413: Improper Resource Locking

What is Improper Resource Locking?

• Overview: Improper Resource Locking (CWE-413) occurs when a software application fails to correctly lock a resource that requires exclusive access, allowing unintended modifications.

• Exploitation Methods:

• Attackers can exploit this vulnerability by accessing and modifying the resource concurrently while it is being used by the application.
• Common attack patterns include race conditions and time-of-check-to-time-of-use (TOCTOU) attacks where an attacker manipulates the resource between the check and use phases.

• Security Impact:

• Direct consequences include data corruption, unexpected behavior of the application, and possible system crashes.
• Potential cascading effects might involve the compromise of other system components dependent on the affected resource.
• Business impact includes data loss, reduced system reliability, and potential reputational damage.

• Prevention Guidelines:

• Specific code-level fixes involve implementing proper concurrency controls, such as mutexes or semaphores, to ensure exclusive access to resources.
• Security best practices include conducting thorough code reviews and testing for race conditions.
• Recommended tools and frameworks are those offering built-in concurrency controls, such as threading libraries that provide safe resource management features.

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Technical Details

Likelihood of Exploit: Not specified

Affected Languages: Not Language-Specific

Affected Technologies: Not specified

Vulnerable Code Example

Python Example

import threading

class FileWriter:
    def __init__(self, filename):
        self.filename = filename

    def write_to_file(self, data):
        # Vulnerable code: No proper locking mechanism
        with open(self.filename, 'a') as f:
            f.write(data + '\n')

def thread_function(writer, data):
    writer.write_to_file(data)

# Create a FileWriter instance
writer = FileWriter('example.txt')

# Create multiple threads that write to the same file
threads = []
for i in range(10):
    thread = threading.Thread(target=thread_function, args=(writer, f'Thread {i}'))
    threads.append(thread)
    thread.start()

# Ensure all threads have completed
for thread in threads:
    thread.join()

Explanation:

In this example, the write_to_file method of the FileWriter class does not use any locking mechanism. This can lead to a race condition when multiple threads attempt to write to the file simultaneously. The lack of synchronization can result in data corruption or loss, as concurrent writes can interfere with each other.

How to fix Improper Resource Locking?

To fix the issue, we need to ensure that access to the shared resource (the file, in this case) is properly synchronized. We can achieve this by using a threading lock. A lock ensures that only one thread can access the resource at any given time, preventing race conditions.

Specific Fixes:

• Use a threading.Lock() object to control access to the file.
• Acquire the lock before opening the file and writing data.
• Release the lock after the write operation is complete.

Fixed Code Example

import threading

class FileWriter:
    def __init__(self, filename):
        self.filename = filename
        self.lock = threading.Lock()  # Initialize a lock

    def write_to_file(self, data):
        # Acquire the lock before writing
        with self.lock:
            with open(self.filename, 'a') as f:
                f.write(data + '\n')
        # Lock is automatically released when exiting the 'with' block

def thread_function(writer, data):
    writer.write_to_file(data)

# Create a FileWriter instance
writer = FileWriter('example.txt')

# Create multiple threads that write to the same file
threads = []
for i in range(10):
    thread = threading.Thread(target=thread_function, args=(writer, f'Thread {i}'))
    threads.append(thread)
    thread.start()

# Ensure all threads have completed
for thread in threads:
    thread.join()

Explanation:

In the fixed code, a threading.Lock() object is used to ensure that only one thread can write to the file at any given time. By using the with self.lock: statement, we acquire the lock before writing to the file and release it automatically when the block is exited. This ensures that the file operations are safely synchronized, preventing data corruption and race conditions. Additionally, all threads are joined at the end to ensure that the main program waits for all threads to complete, which is a good practice for thread management.