The leading computer security experts Symantec says it has discovered one of the most sophisticated pieces of malicious software on Windows PCs across the globe, that appears to be used for spying and data collection.

The bug, named ‘Regin’, the tool is a Trojan that has infected several organizations’ and individuals’ computer systems between 2008 and 2011, and again since 2013. Symantec’s researchers say Regin has been developed with “considerable effort into making it highly inconspicuous,” making it hard to detect, track and understand its activities.

Experts say computers in Russia, Saudi Arabia and Ireland have been hit most. Once installed on a computer, it can do things like capture screenshots, steal passwords or recover deleted files.

The team also says that “Regin remain undiscovered and additional functionality and versions may exist,” and that they’ll continue to post updates on future discoveries as they happen. You can read more about Regin on Symantec’s blog.