CWE-432: Dangerous Signal Handler not Disabled During Sensitive Operations

What is Dangerous Signal Handler not Disabled During Sensitive Operations?

• Overview: This vulnerability occurs when a program's signal handler does not prevent other signal handlers from running concurrently, leading to potential corruption of shared resources like global variables.

• Exploitation Methods:

• Attackers can exploit this by sending a signal that triggers another handler while the first is still executing.
• Common attack patterns include sending multiple signals in rapid succession to induce race conditions or inconsistent states.

• Security Impact:

• Direct consequences include corruption of program state, leading to unexpected behavior or crashes.
• Potential cascading effects include data loss, data corruption, or enabling further attacks due to compromised control flow.
• Business impact could involve service downtime, loss of customer trust, or exposure of sensitive information.

• Prevention Guidelines:

• Specific code-level fixes include properly masking signals or using atomic operations to protect shared state.
• Security best practices recommend designing signal handlers to be reentrant or stateless, minimizing shared resources.
• Recommended tools and frameworks include static analysis tools to detect improper signal handling and libraries that abstract safe signal management.

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Technical Details

Likelihood of Exploit: Not specified

Affected Languages: Not Language-Specific

Affected Technologies: Not specified

Vulnerable Code Example

import signal
import time

# Shared state variable
shared_state = {"running": True}

def handler(signum, frame):
    print("Signal handler called with signal:", signum)
    # Dangerous modification of shared state without masking other signal handlers
    shared_state["running"] = False

# Registering the signal handler
signal.signal(signal.SIGINT, handler)

# Sensitive operation that should not be interrupted
while shared_state["running"]:
    print("Running sensitive operation...")
    time.sleep(1)

Explanation:

• The code registers a signal handler for SIGINT using signal.signal().
• The signal handler modifies a shared state variable, shared_state, without ensuring that other signals are masked or blocked during the operation. This can lead to inconsistent state or unexpected behavior if another signal interrupts the handler during its execution.
• The shared state is directly modified without any synchronization mechanism, making it vulnerable to race conditions.

How to fix Dangerous Signal Handler not Disabled During Sensitive Operations?

To fix this vulnerability, implement the following best practices:

1. Block Signals During Handler Execution: Ensure that the signal handler execution is atomic by blocking other signals during its execution. This can be done by using signal masking techniques.
2. Use a Dedicated Lock or Flag: Introduce a lock mechanism or a flag to ensure that signals are handled atomically, preventing other signal handlers from running simultaneously.

By implementing these practices, you prevent concurrent modifications to shared state and ensure that sensitive operations are correctly protected from unwanted interruptions.

Fixed Code Example

import signal
import time
import threading

# Shared state variable
shared_state = {"running": True}
lock = threading.Lock()

def handler(signum, frame):
    print("Signal handler called with signal:", signum)
    # Use a lock to ensure atomic modification of the shared state
    with lock:
        shared_state["running"] = False

# Registering the signal handler
signal.signal(signal.SIGINT, handler)

# Sensitive operation that should not be interrupted
while True:
    with lock:
        if not shared_state["running"]:
            break
    print("Running sensitive operation...")
    time.sleep(1)

Explanation:

• A threading.Lock() is introduced to ensure that the modification of shared_state is atomic, preventing race conditions.
• The signal handler now acquires the lock before modifying the shared state, ensuring that it cannot be interrupted by other signals.
• The sensitive operation checks the shared state within a with lock: block to ensure atomicity and consistent state checking.
• This approach ensures that the shared state is protected from concurrent modifications during sensitive operations, mitigating the security vulnerability effectively.