Google Chrome explain how users can view extension data READ MORE Jan Rubin, malware researcher at Avast, said: "The extensions’ backdoors are well-hidden and the extensions only start to exhibit malicious behaviour days after installation, which made it hard for any security software to discover". Rubin also added: "Our hypothesis is that either the extensions were deliberately created with the malware built-in, or the author waited for the extensions to become popular, and then pushed an update containing the malware. It could also be that the author sold the original extensions to someone else after creating them, and then the buyer introduced the malware afterwards".