Ebury (SSH backdoor) – Malware

Overview

Ebury is an SSH backdoor targeting Linux operating systems. Attackers require root-level access, allowing them to replace SSH binaries (such as ssh, sshd, ssh-add) or modify a shared library used by OpenSSH (libkeyutils).

This payload was used to compromise kernel.org in August 2011 and later affected cPanel Support, leading to the infection of numerous cPanel servers. It is a credential-stealing payload that captures SSH keys, passwords, and potentially other credentials.

Ebury is part of a broader suite of tools detailed in the “Operation Windigo” whitepaper by ESET. A decade ago, ESET researchers raised awareness of Ebury through this whitepaper, which documented a campaign leveraging Linux malware for financial gain. Despite the arrest and conviction of one of the Ebury perpetrators following the publication, the botnet continued to expand.

Targets

Among the victims are many hosting providers and data centers.

How they operate

The initial compromise is performed via credential stuffing attacks, where attackers use stolen credentials to log into servers. Once a server is compromised, the malware exfiltrates a list of inbound and outbound SSH connections from wtmp and the known_hosts file and steals SSH authentication keys, which are then used to attempt logins on other systems.

Alternatively, attackers may exploit known vulnerabilities in the server software to gain further access or elevate their privileges. In the next phase, the malware operators intercept SSH traffic on targeted servers within data centers using Address Resolution Protocol (ARP) spoofing to redirect traffic to a server under their control. When a user logs into a compromised server via SSH, Ebury captures the login credentials.

If the compromised servers host cryptocurrency wallets, Ebury uses the captured credentials to empty the wallets automatically. ESET reports that Ebury targeted at least 200 servers using this method throughout 2023, including Bitcoin and Ethereum nodes. Monetization strategies vary and include stealing credit card information from payment sites, redirecting web traffic to generate revenue from ads and affiliate programs, using compromised servers to send spam, and selling captured credentials.

In late 2023, ESET observed the introduction of new obfuscation techniques and a domain generation algorithm (DGA) system that allows the botnet to evade detection and improve its resilience against blocks. The malware modules spread via the Ebury botnet, based on ESET’s latest observations, include:

HelimodProxy: Proxies raw traffic and relays spam by modifying the mod_dir.so Apache module, allowing the compromised server to run arbitrary commands and support spam campaigns.

HelimodRedirect: Redirects HTTP traffic to attacker-controlled websites by modifying various Apache and nginx modules to redirect a small percentage of web traffic to malicious sites.

HelimodSteal: Exfiltrates sensitive information from HTTP POST requests by adding an input filter that intercepts and steals data submitted via web forms, such as login credentials and payment details.

KernelRedirect: Modifies HTTP traffic at the kernel level to redirect visitors by using a Linux kernel module that hooks into Netfilter, changing the Location header in HTTP responses to redirect users to malicious URLs.

FrizzySteal: Intercepts and exfiltrates HTTP requests by hooking into libcurl, enabling it to capture and steal data from HTTP requests made by the compromised server.

References:

• Ebury is alive but unseen: 400k Linux servers compromised for cryptocurrency theft and financial gain