Security researchers are sounding the alarm on a newly discovered vulnerability in the widely used web server management software cPanel and WebHost Manager (WHM). 

The bug allows hackers to hijack and take full control of the servers running the affected software, which is thought to be used by tens of millions of website owners around the world.

Many commercial web hosting companies have patched their customers’ systems already. But the cPanel maker urged customers to ensure that their systems are patched as the bug affects all supported versions of the software.

cPanel and WHM are two software suites used for managing web servers that host websites, manage emails, and handle important configurations and databases needed to maintain an internet domain. The two suites have deep-access to the servers that they manage, allowing a malicious hacker potentially unrestricted access to data managed by the affected software.

The bug, officially tracked as CVE-2026-41940, allows malicious hackers to remotely bypass its login screen to gain full access to the software’s administration panel. 

Given the ubiquity of the cPanel and WHM software across the web hosting industry, hackers could compromise potentially large numbers of websites that haven’t patched the bug.

Canada’s national cybersecurity agency said in an advisory that the bug could be exploited to compromise websites on shared hosting servers, such as large web hosting companies.

The agency said that “exploitation is highly probable” and that immediate action from cPanel customers, or their web hosts, is necessary to prevent malicious access.

Web hosting giant Namecheap, which uses cPanel to allow its customers to manage their web servers, said the company blocked access to customers’ cPanel panels after learning of the flaw to prevent exploitation, and to give it time to patch its customers’ systems. 

HostGator also said it patched its systems and is considering the bug a “critical authentication-bypass exploit.”

One web hosting company says it found evidence that hackers have been abusing the vulnerability for months before the attempts were discovered.

KnownHost CEO Daniel Pearson said in a post on Reddit that his company has seen attempts to exploit the vulnerability as far back as February 23. The company said it also briefly began blocking access to customer systems before applying patches.

According to Pearson, around 30 servers at KnownHost showed signs of unauthorized attempted access out of thousands of computers on its network. Pearson likened the efforts to attempts, and has not seen signs of active compromise. cPanel also said it rolled out a security fix for WP Squared, a similar tool for managing WordPress websites.