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Nearly 11,000 websites in recent months have been infected with a backdoor that redirects visitors to sites that rack up fraudulent views of ads provided by Google Adsense, researchers said.
All 10,890 infected sites,
found by security firm Sucuri
, run the WordPress content management system and have an obfuscated PHP script that has been injected into legitimate files powering the websites. Such files include “index.php,” “wp-signup.php,” “wp-activate.php,” “wp-cron.php,” and many more. Some infected sites also inject obfuscated code into wp-blog-header.php and other files. The additional injected code works as a backdoor that’s designed to ensure the malware will survive disinfection attempts by loading itself in files that run whenever the targeted server is restarted.
“These backdoors download additional shells and a
Leaf PHP mailer script
from a remote domain filestack[.]live and place them in files with random names in wp-includes, wp-admin and wp-content directories,” Sucuri researcher Ben Martin wrote. “Since the additional malware injection is lodged within the wp-blog-header.php file it will execute whenever the website is loaded and reinfect the website. This ensures that the environment remains infected until all traces of the malware are dealt with.”