A vulnerability in older versions of the Windows operating system very bad.
The National Security Agency has chimed in alongside Microsoft to encourage users to download an update containing a fix.
The vulnerability is called "BlueKeep" and could lead to malware similar to the "WannaCry" ransomware from 2017.
That bug prevented users from accessing their data unless they paid a ransom.
This new bug could also lead to denial-of-service attacks, which have the potential to shut down crucial systems.