Dissecting the XOR.DDoS Linux Trojan: Sample, Crontab Abuse, and Defense Steps

Three Engineer Tricks

Many engineers rely on three quick fixes: restarting processes or the whole system, reinstalling the operating system, and replacing aging hardware. These actions can temporarily resolve performance issues but may mask deeper problems such as hidden malware.

Malware Sample Identification

The first captured trojan sample, labeled 101 , is a Linux/XOR.DDoS remote‑control malware. It modifies /etc/crontab to schedule a malicious script /etc/cron.hourly/gcc.sh, which starts all network interfaces and copies /lib/libudev.so to /lib/libudev.so.6 before executing it.

# cat /etc/crontab*
SHELL=/bin/bash
PATH=/sbin:/bin:/usr/sbin:/usr/bin
MAILTO=root
HOME=/
# run-parts
01 * * * * root run-parts /etc/cron.hourly
02 4 * * * root run-parts /etc/cron.daily
22 4 * * 0 root run-parts /etc/cron.weekly
42 4 1 * * root run-parts /etc/cron.monthly
3 * * * * root /etc/cron.hourly/gcc.sh

Program Entry (gcc.sh)

#!/bin/sh
PATH=/bin:/sbin:/usr/bin:/usr/sbin:/usr/local/bin:/usr/local/sbin:/usr/X11R6/bin
for i in `cat /proc/net/dev|grep :|awk -F: {'print $1'}`; do
    ifconfig $i up &
    done
cp /lib/libudev.so /lib/libudev.so.6

Attack Path and Forensics

Log analysis (e.g., last output) reveals a login from a typical broadband IP address, indicating the attacker leveraged a compromised home user to gain initial access. The source IP and timestamps help trace the intrusion back to the originating ISP.

Emergency Removal Procedure

Restore the original crontab file.

Delete the malicious gcc.sh script.

Remove the copied /lib/libudev.so.6 file.

Terminate any malicious processes.

It is crucial to follow this order; merely killing processes is ineffective because the malware can respawn itself.

XOR.DDoS Trojan Overview

Identified in October 2014 by MalwareMustDie, XOR.DDoS targets 32‑bit and 64‑bit Linux servers, desktops, and ARM devices. After a successful SSH brute‑force login, the attacker runs a shell installer that performs environment checks, compiles, extracts, and installs a rootkit to evade antivirus detection. The trojan primarily attacks gaming and educational websites, generating up to 1500 Gbps of malicious traffic.

Source Code Highlights

The malware is highly polymorphic, constantly changing its binary signatures to evade signature‑based detection. It can add or delete services, execute arbitrary programs, hide processes and files, download additional payloads, gather system information, and launch DDoS attacks.

Polymorphic Nature

Polymorphism means the malware mutates its file size, hash, and other characteristics during replication, making static analysis and signature scanning extremely difficult.

Defense Challenges

Defending against such threats is akin to a siege: limited resources, long defensive lines, and slow ROI on security investments. Organizational factors—such as fragmented responsibility, KPI‑driven security mandates, and a lack of dedicated security expertise—further weaken defenses. Attackers now operate as large‑scale “crime‑as‑a‑service” outfits, offering on‑demand DDoS capabilities that far outpace typical defensive tooling.

Collaboration Benefits

Effective mitigation often requires cooperation with third‑party security teams. In this case, rapid sample capture, thorough analysis, and coordinated hardening allowed the client to remain largely unaware of the breach while the threat was neutralized.