Security experts are warning that an escalating series of online attacks designed to break into poorly-secured WordPress blogs is fueling the growth of an unusually powerful botnet currently made up of more than 90,000 Web servers.

Over the past week, analysts from a variety of security and networking firms have tracked an alarming uptick in so-called “brute force” password-guessing attacks against Web sites powered by WordPress, perhaps the most popular content management system in use today (this blog also runs WordPress).

According to Web site security firm Incapsula, those responsible for this crime campaign are scanning the Internet for WordPress installations, and then attempting to log in to the administrative console at these sites using a custom list of approximately 1,000 of the most commonly-used username and password combinations.

Incapsula co-founder Marc Gaffan told KrebsOnSecurity that infected sites will be seeded with a backdoor the lets the attackers control the site remotely (the backdoors persist regardless of whether the legitimate site owner subsequently changes his password). The infected sites then are conscripted into the attacking server botnet, and forced to launch password-guessing attacks against other sites running WordPress.

Gaffan said the traffic being generated by all this activity is wreaking havoc for some Web hosting firms.

“It’s hurting the service providers the most, not just with incoming traffic,” Gaffan said. “But as soon as those servers get hacked, they are now bombarding other servers with attack traffic. We’re talking about Web servers, not home PCs. PCs maybe connected to the Internet with a 10 megabit or 20 megabit line, but the best hosting providers have essentially unlimited Internet bandwidth. We think they’re building an army of zombies, big servers to bombard other targets for a bigger cause down the road.”

Indeed, this was the message driven home Thursday in a blog post from Houston, Texas based HostGator, one of the largest hosting providers in the United States. The company’s data suggests that the botnet of infected WordPress installations now includes more than 90,000 compromised sites.

“As I type these words, there is an on-going and highly-distributed, global attack on WordPress installations across virtually every web host in existence,” wrote HostGator’s Sean Valant. “This attack is well organized and again very, very distributed; we have seen over 90,000 IP addresses involved in this attack.”

That assessment was echoed in a blog post Thursday by CloudFlare, content delivery network based in San Francisco. Cloudflare CEO Matthew Prince said the tactics employed in this attack are similar to those used by criminals to build the so-called itsoknoproblembro/Brobot botnet which, in the Fall of 2012, was responsible for a series of rather large cyber attacks against the largest US financial institutions.

“One of the concerns of an attack like this is that the attacker is using a relatively weak botnet of home PCs in order to build a much larger botnet of beefy servers in preparation for a future attack,” Prince wrote. “These larger machines can cause much more damage in DDoS attacks because the servers have large network connections and are capable of generating significant amounts of traffic.”

HostGator’s Valant urged WordPress administrators to change their passwords to something that meets the security requirements specified on the WordPress website. These requirements are fairly typical of a secure password: upper and lowercase letters, at least eight characters long, and including “special” characters (^%$#&@*). For more on picking strong passwords, see this tutorial. Users can also restrict access to wp-admin so that it is only reachable from specific IP addresses.

Also, WordPress users can take advantage of a third-party plugin from Duo Security, which enables secure logins using one-time codes pushed via text message or an associated mobile app.

Matthew Mullenweg, the founding developer of WordPress, suggests site administrators chose a username that is something other than “admin”. In addition, he urged WordPress.com-hosted blogs to turn on two-factor authentication, and to verify that the site is running the latest version of WordPress. “Do this and you’ll be ahead of 99% of sites out there and probably never have a problem,” Mullenweg wrote.

Daniel Cid, chief technology officer of Sucuri Security, a company that helps site owners prevent and recover from security breaches, said his team isn’t seeing infected sites being used to attack others; according to Cid, most of the password brute-forcing is being conducted by desktop systems under the attackers’ control.

“We saw a big increase in the number of brute force attacks (almost tripled) since previous month’s average,” Cid wrote in an instant message interview. “However, at least from our data, they are not re-using the compromised sites to build a botnet to scan others. I assume that is speculation. On the sites we looked [at] that were hacked, the attackers injected backdoors and malware on them,” including the Blackhole Exploit Kit. Cid also shared a copy of the username/password list that the attackers have been using for the brute-forcing.

“The brute force attacks do not seem to be coming from servers, but from desktops,” Cid said. “However, this is still very early, since they are injecting backdoors (a variation of the Filesman backdoor) they can later use the sites to inject malware or even create a botnet and brute force other sites.”

According to Sucuri, WordPress administrators who have been hacked should strongly consider taking the following steps to evict the intruders and infections:

– Log in to the administrative panel and remove any unfamiliar admin users.

– Change all passwords for all admin users (and make sure all legitimate accounts are protected with strong passwords this time).

– Update the secret keys inside WordPress (otherwise any rogue admin user can remain logged in).

– Reinstall WordPress from scratch or revert to a known, safe backup.

Update, 3:05 p.m. ET: Corrected Gaffan’s title.

Update, 6:29 p.m. ET: Added quotes and tips from Sucuri Security.

Update, Apr. 13, 2013, 12:14 p.m. ET: Added comments from Mullenweg.

Tags: brute-force botnet, CloudFlare, Daniel Cid, Duo Security, HostGator, Incapsula, Marc Gaffan, Matthew Prince, Sean Valant, Sucuri, wordpress, wordpress attack, WordPress botnet