Attackers Hide Communication With Linux Backdoor

Ducking detection once a malware infection is successful is a key part of life for an attacker.

According to Symantec, a group of sophisticated attackers have developed a sneaky way to do that via a Linux backdoor designed to hide communications.

The backdoor is detected as Linux.Fokirtor, and has thus far only been seen by the company in a single organization – an Internet hosting provider compromised in May of this year. During the attack, the hackers gained access to internal administrative systems and appear to have been targeting customer information, Symantec noted.

“The attackers understood the target environment was generally well protected,” according to the Symantec Security Response Team blog. “In particular, the attackers needed a means to avoid suspicious network traffic or installed files, which may have triggered a security review.”

To do this, the attackers “devised their own stealthy Linux back door to camouflage itself within the Secure Shell (SSH) and other server processes.”

The backdoor enabled the attackers to remotely execute code commands without opening a network socket or attempting to connect to a command-and-control server. Instead, the backdoor code was injected into the SSH process to monitor network traffic and look for a specific sequence of characters – colon, exclamation mark, semi-colon, period (“:!;.”).

When the characters were spotted, the malware would parse the remaining traffic and extract commands that had been encrypted with Blowfish and Base64 encoded, Symantec explained. From there, the attackers could make normal connection requests through SSH or other protocols and embed the sequence of characters within legitimate traffic to avoid detection. The commands would then be executed.

“To identify the presence of this back door on your network, look for traffic that contains the “:!;.” string (excluding quotes),” according to the Security Response team blog. “Traffic which contains this string will not appear in SSH logs.”

Another way to identify the backdoor is to dump the SSHD process and search for the following strings (where [VALUE] can be various values): key=[VALUE]; dhost=[VALUE]; hbt=3600; sp=[VALUE]; sk=[VALUE] and dip=[VALUE].

“While we discovered this in only a single organization, this threat will likely go under the radar of most organization’s security checks,” the Security Response team told SecurityWeek. “As such, we suspect more organizations may have been attacked, but are unaware of it.”