The curious ‘Cdorked’ Apache web server backdoor that alarmed admins a year ago was only one part of a larger 'Windigo' Linux-Unix botnet that has managed to hijack 26,000 Linux servers since 2011, security firm ESET has discovered.

The campaign analysed in some detail by ESET in conjunction with Sweden’s CERT-Bund and physics lab CERN is that of a compact but potentially very powerful botnet whose purposes include sending tens of millions of spam messages per day, stealing SSH credentials, serving drive-by malware and web redirection.

The number compromised Linux servers since it was discovered stood at 26,024, rising at a rate of 38 per day, with prominent victims including the Linux Foundation’s site and a sizable number of hosting firms including cPanel. Although this sounds modest, because these are servers, every PC visiting them could be at risk, ESET said.

The biggest danger was the bot’s penchant for SSH logins, with ESET recording 5,362 being sent through the malware’s exfiltration servers, ranging in length from a ridiculous 3-character length to an apparently secure 50 characters. Many other passwords seemed to be re-used over and over, suggesting admins were accessing several servers using the same key.

What the discovery confirms is the tangled thread of connections between a number of pieces of malware discovered in the last three years, including not just Cdorked (the drive-by module) but with ‘Ebury’ (used for credential theft). More recently, researchers discovered another component, Calfbot, a spam-sending bot written in Perl.

"Windigo has been gathering strength, largely unnoticed by the security community, for over two and a half years, and currently has 10,000 servers under its control," said ESET security researcher Marc-Étienne Léveillé. 

"Over 35 million spam messages are being sent every day to innocent users' accounts, clogging up inboxes and putting computer systems at risk.  Worse still, each day over half a million computers are put at risk of infection, as they visit websites that have been poisoned by web server malware planted by Operation Windigo redirecting to malicious exploit kits and advertisements."

Evidence that researchers might be dealing with one larger system rather than a series if similar but unrelated pieces of malware came when they noticed that they seemed to be present on the same hosts, while two components embedded identical URLs in spam messages. Cdorked and Ebury also shared a suspiciously similar encryption routine.

It almost comes as a relief that Windigo appears to be a criminal enterprise and not another example of state-backed malware but that won’t come as much consolation for the owners of the Linux servers who have had their systems hijacked to infect Windows users.

Remediation? Servers affected by Windigo included those running Apple OS X, OpenBSD, FreeBSD, Microsoft Windows (through Cygwin) and Linux, including Linux on the ARM architecture.

The answer was for admins to check their servers using the console command, $ ssh -G 2>&1 | grep -e illegal -e unknown > /dev/null && echo "System clean" || echo "System infected", ESET said.

Anyone discovering an infection would have to wipe the affected system and re-install the OS.  They should also consider using two-factor authentication in future.

"We realise that wiping your server and starting again from scratch is tough medicine, but if hackers have stolen or cracked your administrator credentials and had remote access to your servers, you cannot take any risks," said Léveillé. 

"Sadly, some of the victims we have been in touch with know that they are infected, but have done nothing to clean up their systems - potentially putting more internet users in the firing line."