SSHStalker: The Linux Botnet Reviving 1990s IRC Tech to Hijack Thousands of Servers

A newly uncovered Linux botnet named SSHStalker is making waves in cybersecurity circles—not for cutting-edge sophistication, but for its surprisingly retro approach. Instead of relying on modern command-and-control (C2) frameworks, this botnet uses the decades-old Internet Relay Chat (IRC) protocol to orchestrate its attacks. Think of it as a digital throwback: the same chat tech that powered early internet communities is now being weaponized to hijack thousands of Linux servers worldwide.

The Comeback of a 1990s Classic

IRC, short for Internet Relay Chat, was invented in 1988 and dominated online communication throughout the 1990s. It allowed users to chat in real-time, join group channels, and send private messages—all through simple, text-based interfaces. While most modern messaging platforms have moved on, IRC remains beloved in technical communities for its simplicity, low bandwidth needs, and lack of reliance on graphical interfaces.

Now, SSHStalker is breathing new life into this old-school tech—but with malicious intent. According to researchers at Flare, a leading threat intelligence firm, the botnet uses IRC’s classic mechanics: multiple C-based bots, redundant servers, and channels for resilience. The goal? Reliability and scale over stealth.

“What we actually found was a loud, stitched-together botnet kit that mixes old-school IRC control, compiling binaries on hosts, mass SSH compromise, and cron-based persistence. In other words, a scale-first operation that favors reliability over stealth,” Flare explained.

How SSHStalker Works: A Noisy, Worm-Like Attack

SSHStalker’s attack chain is both aggressive and noisy—a far cry from the stealthy tactics often associated with advanced threats. Here’s how it unfolds:

1. Automated SSH Scanning and Brute Force: The botnet starts by scanning the internet for Linux servers with open SSH ports. Using a Go binary disguised as the popular network tool nmap, it attempts to brute-force its way in by guessing weak or default passwords.

2. Worm-Like Propagation: Once a server is compromised, it becomes part of the botnet and is used to scan for more SSH targets. This creates a self-propagating cycle, rapidly expanding the botnet’s reach.

3. Compiling Payloads on the Fly: To evade detection and ensure compatibility, SSHStalker downloads the GCC compiler onto infected hosts. This allows it to compile C-based IRC bots directly on the victim machine.

4. IRC Bot Enrollment: The first payloads are IRC bots with hard-coded C2 servers and channels. These bots enroll the new victim into the botnet’s IRC infrastructure, ready to receive commands.

5. Persistence via Cron Jobs: To maintain control, the malware sets up cron jobs that run every 60 seconds. These jobs act as a watchdog, checking if the main bot process is running and restarting it if terminated.

6. Exploiting Old Vulnerabilities: SSHStalker also leverages 16 CVEs targeting Linux kernels from 2009-2010. These exploits are used to escalate privileges after gaining initial access as a low-privileged user.

What’s the Endgame? Monetization and Idle Threats

While SSHStalker’s infrastructure is active, its current operations are surprisingly passive. Researchers found that the botnet’s bots connect to the C2 and then enter an idle state, suggesting the operators may be testing the system or hoarding access for future use.

However, the botnet is equipped with tools for monetization:

• AWS Key Harvesting: Scanning for and stealing cloud credentials.
• Website Scanning: Identifying vulnerable targets for further exploitation.
• Cryptomining: Including high-performance miners like PhoenixMiner for Ethereum.
• DDoS Capabilities: While not yet observed in action, the botnet has the tools to launch distributed denial-of-service attacks.

Who’s Behind SSHStalker?

Flare has not definitively attributed SSHStalker to a specific threat group. However, they noted similarities with the Outlaw/Maxlas botnet ecosystem and several Romanian indicators. This suggests the botnet may be linked to long-standing cybercriminal operations with a history of cryptomining and exploitation.

How to Protect Your Systems

SSHStalker’s reliance on brute force, old vulnerabilities, and noisy tactics means it’s both aggressive and preventable. Here are Flare’s top recommendations for mitigation:

• Disable SSH Password Authentication: Use SSH keys instead to prevent brute-force attacks.
• Remove Compilers from Production Images: Eliminate tools like GCC that attackers can use to compile payloads.
• Enforce Egress Filtering: Block outbound connections to suspicious IP addresses and ports.
• Restrict Execution from ‘/dev/shm’: Prevent malware from running from shared memory.
• Monitor for Compiler Installation and IRC Connections: Set up alerts for unusual activity, such as compiler downloads or IRC-style outbound traffic.
• Watch for Short-Cycle Cron Jobs: Be wary of cron jobs running every 60 seconds from unusual paths.

The Bigger Picture: Old Tech, New Threats

SSHStalker’s use of IRC highlights a fascinating trend in cybersecurity: the revival of old technologies for modern attacks. While IRC may seem outdated, its simplicity and reliability make it an attractive choice for botnet operators. Combined with aggressive scanning, worm-like propagation, and a focus on scale, SSHStalker represents a new breed of botnet—one that prioritizes volume over stealth.

As organizations continue to rely on Linux servers for critical infrastructure, threats like SSHStalker serve as a stark reminder: even the oldest tools can be repurposed for harm. Staying vigilant, patching vulnerabilities, and adopting best practices are essential to keeping your systems safe in an ever-evolving threat landscape.

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Viral Sentences:

• “SSHStalker is reviving 1990s IRC tech to hijack thousands of Linux servers—old-school meets new-school cybercrime!”
• “This botnet doesn’t hide—it’s loud, aggressive, and built for scale, not stealth.”
• “Think IRC is dead? Think again. Cybercriminals are bringing it back for mass exploitation.”
• “SSHStalker’s worm-like propagation turns compromised servers into attack machines in minutes.”
• “From brute force to cryptomining, this botnet has all the tools for monetization.”
• “Old vulnerabilities, new threats: SSHStalker exploits 15-year-old Linux kernel bugs.”
• “The botnet’s bots are idle now—but they could spring to life at any moment.”
• “Protect your servers: disable SSH passwords, remove compilers, and watch for IRC connections.”
• “SSHStalker proves that even outdated tech can be weaponized in the hands of cybercriminals.”
• “Stay vigilant: the threat landscape is evolving, and old tricks are new again.”

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