A critical vulnerability found recently in a popular newsletter plug-in for WordPress is actively being targeted by hackers and was used to compromise an estimated 50,000 sites so far.The security flaw is located in MailPoet Newsletters, previously known as wysija-newsletters, and was fixed in version 2.6.7 of the plug-in released on July 1. If left unpatched, it allows attackers to upload arbitrary PHP files on the Web server and take control of the site.[Critical vulnerability in popular WordPress newsletter plug-in endangers many blogs]MailPoet Newsletters has been downloaded almost 2 million times from the official WordPress plug-in repository to date.Several days ago researchers from Web security firm Sucuri spotted an automated attack that injected a PHP backdoor file into many WordPress sites. A deeper analysis revealed that the attack exploited the MailPoet file upload vulnerability patched at the beginning of the month."The backdoor is very nasty and creates an admin user called 1001001," the Sucuri security researchers said Wednesday in a blog post. "It also injects a backdoor code to all theme\/core files. The biggest issue with this injection is that it often overwrites good files, making very hard to recover without a good backup in place."The Sucuri free website scanner, which people use voluntarily, detects a few thousand sites compromised by this attack every day, according to Daniel Cid, chief technology officer at Sucuri. However, Sucuri estimates that up to 50,000 sites were infected so far, he said Thursday via email.Some sites that didn't have MailPoet installed or were not even using WordPress were also compromised, because of what Cid calls cross-contamination. If one Web hosting account has a WordPress site vulnerable to this attack, the PHP backdoor uploaded through it can infect all sites hosted under that same account."On most shared hosting companies -- GoDaddy, Bluehost, etc. -- one account can not access files from another account, so the cross-contamination would be restricted to sites within the same account," Cid said. However, in other cases, "if the server is not properly configured, which is not uncommon, then [the infection] can spread to all sites and accounts on the same server."The injection script used in the initial attack had a bug that damaged legitimate site files, resulting in obvious errors. That's no longer the case, as attackers fixed their code and the latest variation of the malware no longer breaks websites, Cid said.[Jetpack for WordPress pushes patch for two year-old flaw]In order to protect their WordPress websites from this attack, administrators should update the MailPoet plug-in to the latest version, which at this time is 2.6.9. Version 2.6.8 of the plug-in, released on July 4, addressed an additional security issue.