A Google researcher at Google Project Zero discovered a number of flaws in a well known peer-to-peer file sharing application.
The vulnerabilities “allow an attacker to infect a victim with malware or collect data on the users’ past downloads.”
I won’t repeat the content but you can read a technical explanation here or follow this link for a less geeky version of the whole story.
No mitigation is known so you should stop using the web version and the desktop app until a fix is released.