Updated A server-based botnet that preys on insecure websites is flooding the net with attacks that attempt to guess the login credentials for secure shells protecting Linux boxes, routers, and other network devices.

According to multiple security blogs, the bot compromises websites running outdated versions of phpMyAdmin. By exploiting a vulnerability patched in April, the bot installs a file called dd_ssh, which trawls the net for devices protected by the SSH protocol.

“This bot then conducts brute force SSH attacks on random IP addresses specified by the bot herder,” a user blogged here. Indeed, DShield, an exploit-monitoring service maintained by the SANS Institute, shows a six-fold increase in the number of sources participating in SSH scanning from July 24 to August 10, and close to a three-fold jump in the number of targets.

For reasons that remain unclear, the number of sources over the past two days has plummeted, even as the number of targets has dropped only moderately.

Using a distributed network of compromised web servers to perform the brute-force attacks makes a lot of sense from the hacker's point of view. Not only does it save bandwidth and computing costs, but it also allows the attack to remain concealed from potential victims, since each bot makes only a few login attempts before moving on to the next SSH device. The attacks attempt to guess the passwords used to administer websites by trying huge numbers of possible combinations.

In addition to posing a threat to unpatched websites and SSH-protected devices, the attacks are also creating headaches for large numbers of non-vulnerable sites. As Reg readers have pointed out in comments to this article, the flood of requests for admin.php, setup.php and other PHP-related files can have the effect of a denial-of-service attack. The queries often hit sites running Microsoft's IIS and other platforms that have nothing to do with PHP.

One person commenting on this SANS Diary post reports having success in warding off the attacks with DenyHosts, an open source script that pools IP blacklists from more than 70,000 users. A better countermeasure still is to configure SSH devices to use a cryptographic key, something that is orders of magnitude harder to brute-force than a simple password.

It's tempting to say that it's a given, as well, that admins should update phpMyAdmin and other webserver software to the latest secure versions, but the success that miscreants are showing in installing dd_ssh suggests otherwise. The aforementioned links to Malwarecity.com and SANS provide other useful suggestions for detecting and repelling these attacks. ®

This article was updated to add details about the effect on non-vulnerable sites.