Hunting Kernel Rootkits: A Deep Dive into Linux Kernel Backdoor Analysis

Recently, I tackled an interesting CTF challenge that simulated a real-world kernel rootkit investigation. The scenario involved a compromised system where traditional detection tools had failed, and an attacker had established deep persistence through a kernel-level backdoor. Here's how I uncovered and analyzed this sophisticated threat.

The Challenge Setup

The investigation began with a simple premise: a system was suspected of running a kernel-level backdoor, but the attacker's presence remained hidden from conventional security tools. This mirrors real-world scenarios where advanced persistent threats (APTs) deploy rootkits that operate at the kernel level, making them nearly invisible to userspace detection mechanisms.

Initial Reconnaissance: Module Analysis

The first step in any kernel rootkit investigation probably is examining the loaded kernel modules. Using the lsmod command, I discovered an interesting anomaly among the standard system modules:

Module                  Size  Used by
spatch                 12288  0
# ... other standard modules

The spatch module immediately raised red flags:

• Unknown origin: Not a standard Linux kernel module
• Suspicious naming: Could be attempting to blend in with patch-related tools
• No dependencies: Small size (12KB) with zero dependencies suggested a lightweight backdoor
• + Matching the CTF name 😁

Deep Dive: Module Metadata Analysis

Further investigation using modinfo revealed the smoking gun:

root@tryhackme:~# modinfo spatch
filename:       /lib/modules/6.8.0-1016-aws/kernel/drivers/misc/spatch.ko
description:    Cipher is always root
author:         Cipher
license:        GPL

The module's metadata was practically a confession:

• Description: "Cipher is always root" - explicit reference to privilege escalation
• Author: "Cipher" - clearly an attacker's handle, not a legitimate developer
• Location: Placed in drivers/misc/ to blend in with legitimate hardware drivers

Kernel Symbol Analysis

The next crucial step was examining the kernel symbol table to understand the rootkit's functionality:

cat /proc/kallsyms | grep spatch

This revealed several telling function names:

• proc_fops and proc_entry - Indicates creation of a /proc interface
• execute_command and read_command_output - Command execution capabilities
• proc_write - Handles writing to the proc entry
• cipher_bd_init and cipher_bd_exit - Backdoor initialization and cleanup

Discovery of the Backdoor Interface

Based on the symbol analysis, I searched for the proc entry created by the rootkit:

find /proc -name "*cipher*" -o -name "*spatch*" -o -name "*bd*" 2>/dev/null

This led to the discovery of /proc/cipher_bd - the backdoor's command interface.

Technical Analysis: How the Backdoor Works

The rootkit implements a simple command execution system:

1. Proc Interface: Creates /proc/cipher_bd as a writable file
2. Command Processing: When data is written to the proc entry, the proc_write function processes it
3. Execution: The execute_command function runs the input as a shell command
4. Output Handling: Results are logged to kernel message buffer (dmesg) rather than returned directly

The Communication Protocol

The backdoor uses an interesting communication method:

# Write command to proc entry
echo "whoami" > /proc/cipher_bd

# Read output from kernel logs
dmesg | tail -5

This approach has several advantages for an attacker:

• Stealth: Output doesn't appear in standard command output
• Persistence: Commands execute with kernel privileges
• Evasion: Difficult to detect with standard monitoring tools

Flag Extraction: The Final Piece

The investigation culminated in discovering the rootkit's built-in functionality. Using strings on the module revealed hardcoded commands, including get_flag. When executed:

echo "get_flag" > /proc/cipher_bd
dmesg | tail -5

The backdoor responded with:

[CIPHER BACKDOOR] Here's the secret: 54484d7b73757033725f73<REDACTED>

The hex-encoded flag `54484d7b73757033725f73 decoded to: THM{REDACTED}

Real-World Implications

This CTF challenge demonstrates several real-world attack techniques:

Kernel Module Persistence

• Rootkits often masquerade as legitimate kernel modules
• Placement in standard directories helps avoid detection
• Minimal dependencies reduce the attack surface

Communication Channels

• /proc filesystem abuse is a common technique
• Kernel message buffers provide covert communication
• Commands execute with elevated privileges

Detection Evasion

• Standard monitoring tools may miss kernel-level activity
• Legitimate-looking module names and locations

Detection and Mitigation Strategies

Based on this analysis, here are key detection strategies:

1. Module Verification: Regularly audit loaded kernel modules
2. Symbol Analysis: Monitor kernel symbol tables for suspicious functions
3. Proc Monitoring: Watch for unusual /proc entries
4. Behavioral Analysis: Look for processes that can't be killed or hidden network activity
5. Memory Forensics: Use tools like Volatility for deep memory analysis

Technical Deep Dive: Code Analysis

The rootkit's architecture reveals sophisticated design:

// Simplified structure based on symbol analysis
static struct proc_dir_entry *proc_entry;
static struct file_operations proc_fops = {
    .write = proc_write,
    // other operations
};

static ssize_t proc_write(struct file *file, const char __user *buffer, 
                         size_t count, loff_t *pos) {
    // Command execution logic
    execute_command(user_input);
    return count;
}

Conclusion

This investigation showcased the complexity of modern kernel rootkits and the methodical approach required to detect them. The "Cipher" backdoor demonstrated several advanced techniques:

• Stealth through legitimacy: Mimicking standard kernel modules
• Covert communication: Using kernel message buffers
• Privilege escalation: Operating at kernel level
• Persistence: Surviving standard detection methods

Understanding these techniques is crucial for cybersecurity professionals dealing with advanced persistent threats. The combination of static analysis, dynamic investigation, and deep system knowledge proved essential in uncovering this sophisticated backdoor.

Remember: in the world of kernel security, assumption is the enemy of detection. Always verify, always dig deeper, and never trust that a system is clean until you've thoroughly investigated every anomaly.

# Always remember the fundamentals
echo "Trust but verify" > /proc/security_mindset

This analysis was performed in a controlled CTF environment. Never attempt to analyze suspected malware on production systems without proper isolation and expert guidance.