TL;DR: Look for the fan running at full speed with no heavy programs open, unexpected pop-ups, new browser extensions or a changed homepage you didn't set, and processes in Task Manager or Activity Monitor you don't recognize. Run a full scan with your operating system's built-in security tool (Windows Security or macOS's built-in protections) or a reputable third-party scanner, remove unfamiliar browser extensions, check Task Manager/Activity Monitor for high resource use from unknown processes, and change your important passwords from a separate, trusted device afterward.
Look for the fan running at full speed with no heavy programs open, unexpected pop-ups, new browser extensions or a changed homepage you didn't set, and processes in Task Manager or Activity Monitor you don't recognize.
What Usually Causes This
These symptoms are typically caused by malware installed via a malicious download, a compromised browser extension, or in some cases a legitimate-looking email attachment that ran a script in the background.
What to Do About It
Run a full scan with your operating system's built-in security tool (Windows Security or macOS's built-in protections) or a reputable third-party scanner, remove unfamiliar browser extensions, check Task Manager/Activity Monitor for high resource use from unknown processes, and change your important passwords from a separate, trusted device afterward.
If this looks similar to something else you've seen, it's worth reading How to Check If Your Printer Is a Security Risk.
This pattern shows up elsewhere too — see How to Check If Your Smart Speaker Is Always Listening.
Frequently Asked Questions
What are the real warning signs?
Look for the fan running at full speed with no heavy programs open, unexpected pop-ups, new browser extensions or a changed homepage you didn't set, and processes in Task Manager or Activity Monitor you don't recognize.
What usually causes this?
These symptoms are typically caused by malware installed via a malicious download, a compromised browser extension, or in some cases a legitimate-looking email attachment that ran a script in the background.
What should I do about it?
Run a full scan with your operating system's built-in security tool (Windows Security or macOS's built-in protections) or a reputable third-party scanner, remove unfamiliar browser extensions, check Task Manager/Activity Monitor for high resource use from unknown processes, and change your important passwords from a separate, trusted device afterward.
Should I be worried if I only see one sign?
One sign alone (like a slower device) is usually not cause for alarm — most single symptoms have mundane explanations. Multiple signs together, especially unexpected data usage or unfamiliar apps, are worth investigating.