Attackers exploit vulnerabilities in two WordPress plugins

The plugins, JetPack and Twenty Fifteen, have a cross-site scripting flaw

A vulnerability within two widely used WordPress plugins is already being exploited by hackers, putting millions of WordPress sites at risk, according to a computer security firm.

The plugins are JetPack, a customization and performance tool, and Twenty Fifteen, used for infinite scrolling, wrote David Dede, a malware researcher with Sucuri. WordPress installs Twenty Fifteen by default, which increases the number of vulnerable sites.

Both plugins use a package called genericons, which contains vector icons embedded in a font. In the package, there is an insecure file called "example.html" which makes the package vulnerable, Dede wrote.

The vulnerability in genericons is hard to detect, Dede wrote. It's an XSS (cross-site scripting) flaw in which the malicious payload runs as a result of modifying a browser's DOM (Document Object Model), which is a programming API that defines how HTML and XML documents are accessed and displayed, according to the W3C.

The payload that is delivered is executed directly in the browser and doesn't go to the server, Dede wrote. That means Web application firewalls can't see it and stop it.

Dede wrote that Sucuri found a way to virtually patch the exploit, but that DOM-based XSS flaws "are very tricky to block."

For a successful attack, a victim would have to be tricked into clicking on a malicious link.

Some hosting companies and services, such as GoDaddy, DreamHost and ClickHost, have either done a virtual patch or taken other steps to protect users, Dede wrote.

WordPress is used widely across the Web for publishing, so vulnerabilities in it are particularly troublesome. By the company's own estimation, it runs 23 percent of the sites on the Internet, including major publishers such as Time and CNN.

Last month, WordPress patched two critical and similar cross-site scripting vulnerabilities. The flaws allow malicious JavaScript entered into a comments field to run.

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