WordPress pingback used for DDoS attacks

Millions of sites could potentially be used in attack.

Security firm

Incapsula

reports that it has discovered a DDoS attack on a gaming website using thousands of legitimate

WordPress

blogs without the need for them to be compromised.

Today’s most common method for DDoS attacks uses DNS amplification (also known as ‘DNS reflection’): a DNS request is made to an open DNS resolver with the source IP address forged so that it is the IP address of the targeted site – to which the response is thus sent. By making requests that generate a response that can be 100 times larger than the query (hence the name ‘amplification’), the targeted site is quickly flooded with more traffic than it can handle. The real problem here is the existence of open DNS resolves, which the

Open DNS Resolver Project

attempts to eradicate.

The attack discovered by

Incapsula

also uses legitimate third-party servers to perform the attack, but it uses HTTP rather than DNS. The attack makes uses of a feature in the

WordPress

blogging platform called ‘pingback’, which allows the author of one blog to send a ‘ping’ to a post on another blog to notify the latter that it has been referenced. Though rarely used these days, it is still enabled by default.

Using pingback for DDoS is not a new concept: a

WordPress bug

detailing the possibility has been unresolved for more than six years, while late last year a

similar vulnerability

was discovered that could turn third-party blogs into a powerful port-scanning engine.

While the latter vulnerability was subsequently fixed by

WordPress

, DDoS attacks are still possible. Indeed, we had little trouble replicating

Incapsula

‘s findings, using nothing more than basic command-line tools. With millions of sites running

WordPress

, things could easily turn out quite nasty. However, in this case the requests do not appear to be amplified, which means the attackers would have to be able to control a large botnet in order for such attacks to be successful. Nevertheless,

Incapsula

‘s suggestion that site owners remove or rename the

xmlrpc.php

script (which is used for pingbacks) seems a sensible one.

More at

Incapsula

‘s blog

here

.

Posted on 01 May 2013 by

Martijn Grooten