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 analysed by Sucuri researchers, but it keeps changing so it's hard to know for sure, Cid said.
It's hard to say with certainty how the servers with the SSH backdoor had been compromised in the first place, because in most cases the server logs were gone by the time Sucuri had the chance to analyse them, Cid said. However, the infection was often found on servers that had weak root passwords or were running outdated versions of Plesk - a Web-hosting control panel.
On servers that use the RPM Package Manager administrators should run the "rpm -Va" command in order to check the integrity of their software packages, Cid said. "If you see any change to the SSH binaries, it is a red flag," he said.
Simply checking when the files were last modified using the "ls -la" command won't reveal anything suspicious because the attackers change the mtime (last modification time) timestamps of the backdoor files to match those of the original files, Cid said.
If this SSH backdoor is found on a server, it's better to completely reinstall it from scratch because you never know what else might be there, the researcher said.