January 24, 2013, 1:57 PM — A group of hackers that are infecting Web servers with rogue Apache modules are also backdooring their Secure Shell (SSH) services in order to steal login credentials from administrators and users.
The hackers are replacing all of the SSH binary files on the compromised servers with backdoored versions that are designed to send the hostname, username and password for incoming and outgoing SSH connections to attacker-controlled servers, security researchers from Web security firm Sucuri said Wednesday in a blog post.
"I saw some SSHD [SSH daemon] backdoors in the past in very small scale or part of public rootkits, but not like this one," Daniel Cid, Sucuri's chief technology officer, said Thursday via email. "They do not only modify the ssh daemon, but every ssh binary (ssh, ssh-agent, sshd) and their main goal is to steal passwords."
This allows the attackers to regain control of a compromised server if the passwords get changed or to compromise additional servers if users access them from the compromised server via SSH.
In many cases investigated by Sucuri, the server administrator had removed the rogue Apache module and changed his password, but the infection re-appeared a few days later, Cid said.
"It's a clever trick to get new credentials once you have root access," Denis Sinegubko, the creator of the Unkmask Parasites website security scanner, said Wednesday via email. This might also suggest that the attackers gained initial access to those servers by guessing, brute forcing or stealing valid credentials and not by exploiting some security vulnerability, he said.
Sinegubko documented a wave of incidents that involved attackers obtaining administrator (root) access to Web servers and installing rogue Apache modules during August and September 2012. The purpose of the modules was to inject rogue iframes into legitimate websites hosted on those servers.
This website infection method continued to be used during the following months and the attacks were linked to a cybercriminal toolkit called DarkLeech that was being sold on hacker forums.
It's not clear whether the SSH backdoor is actually a new feature of DarkLeech. The backdoor was not part of an early version of the toolkit analyzed by Sucuri researchers, but it keeps changing so it's hard to know for sure, Cid said.