CWE-412: Unrestricted Externally Accessible Lock

What is Unrestricted Externally Accessible Lock?

• Overview: An Unrestricted Externally Accessible Lock vulnerability occurs when an application uses a lock (such as a mutex) to control access to a resource, but the lock can be manipulated by unauthorized external actors, leading to potential misuse or disruption of the application.

• Exploitation Methods:

• Attackers can hold the lock indefinitely, preventing legitimate processes from accessing the resource.
• Exploit patterns include creating or holding a lock in a way that the application cannot release it, causing denial of service.

• Security Impact:

• Direct consequences include denial of service, where legitimate users or processes are blocked from accessing necessary resources.
• Potential cascading effects include system downtime or degraded performance of dependent services.
• Business impact may involve loss of productivity, customer dissatisfaction, and financial losses due to service interruptions.

• Prevention Guidelines:

• Specific code-level fixes include implementing proper access controls and ensuring only authorized entities can create or hold locks.
• Security best practices involve validating the source and context of lock requests and using secure design patterns to manage locks.
• Recommended tools and frameworks include using established libraries that provide secure lock management and reviewing code for potential lock misuse.

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Technical Details

Likelihood of Exploit: Not specified

Affected Languages: Not Language-Specific

Affected Technologies: Not specified

Vulnerable Code Example

import os
import threading

class LockManager:
    def __init__(self, lock_file_path):
        # The lock file path is set based on external input, which is dangerous
        self.lock_file_path = lock_file_path  # External input used directly here

    def acquire_lock(self):
        # Create a lock file to signal that a resource is locked
        with open(self.lock_file_path, 'w') as lock_file:
            lock_file.write('locked')  # Lock file created at user-defined path

    def release_lock(self):
        # Remove the lock file to release the resource
        if os.path.exists(self.lock_file_path):
            os.remove(self.lock_file_path)

# Usage
lock_manager = LockManager('/tmp/lockfile.lock')
lock_manager.acquire_lock()

Explanation of Vulnerability

• Unrestricted Externally Accessible Lock: The LockManager class allows for the creation of a lock file at a path specified by external input. This is a security risk because an attacker could specify a path to a critical system file, potentially causing denial of service or other unintended effects when the lock file is created or deleted. This vulnerability arises from a lack of validation and restriction on the file path input.

How to fix Unrestricted Externally Accessible Lock?

To fix this vulnerability, we need to ensure that the lock file path cannot be influenced by external actors in an unrestricted manner. The recommended approach is to:

1. Restrict the lock file path: Set a predefined directory for lock files and ensure that any input is sanitized and validated to prevent path traversal or other malicious inputs.
2. Use unique identifiers: Instead of allowing arbitrary file paths, use unique identifiers to generate lock filenames within a controlled directory.
3. Implement file permission controls: Ensure that the lock files are created with appropriate permissions to prevent unauthorized access or modification.

Fixed Code Example

import os
import threading

class LockManager:
    LOCK_DIR = '/var/locks/'  # Predefined and secure directory for lock files

    def __init__(self, resource_id):
        # Generate a lock file path using a unique identifier within a restricted directory
        self.lock_file_path = os.path.join(self.LOCK_DIR, f'{resource_id}.lock')  # Safe path construction

    def acquire_lock(self):
        # Create a lock file with restricted permissions
        with open(self.lock_file_path, 'w') as lock_file:
            os.chmod(self.lock_file_path, 0o600)  # Set file permissions to read/write for owner only
            lock_file.write('locked')  # Securely create lock file

    def release_lock(self):
        # Remove the lock file to release the resource
        if os.path.exists(self.lock_file_path):
            os.remove(self.lock_file_path)

# Usage
lock_manager = LockManager('unique_resource_id')
lock_manager.acquire_lock()

Explanation of Fix

• Restricted Lock Directory: The lock file path is now constructed using a predefined directory (LOCK_DIR), which is controlled and not influenced by external inputs. This prevents path traversal attacks.
• Unique Identifier: The lock filename is generated using a unique resource identifier, preventing arbitrary path injection and ensuring that lock files are only created within the designated directory.
• File Permissions: The lock file is created with permissions set to 0o600, ensuring only the owner can read or write the file, thus enhancing security by preventing unauthorized access to the lock file. This minimizes the risk of tampering by other users.