Imagine a stranger quietly using your computer to make money, while you pay the electric bill. That's the whole game here. A hidden program turns your machine into a money-printer for someone you'll never meet, and you foot the cost in slow performance, a screaming fan, and a fatter power bill.
The tech name for this is crypto malware, and the act of pulling it off is called cryptojacking (secretly using someone else's computer to mine). To get why anyone bothers, you need two quick pieces of background first.
Money, and the puzzle behind it
Cryptocurrency is digital money like Bitcoin, with no bank in charge. Instead, computers all over the world keep a shared record of every payment, called a blockchain.
That ledger doesn't update itself. Mining is when a computer solves hard math puzzles to confirm new payments, and whoever solves one first earns fresh coins. It's completely legal; it's how networks like Bitcoin keep running. The catch? Doing it for real profit takes serious hardware running nonstop, and the electricity costs are brutal. So here's the obvious dark idea: why pay for your own machines and power when you can steal someone else's?
What the malware actually does
Once crypto malware sneaks onto your system, it quietly installs mining software that runs in the background. It's built to hide. Often it throttles itself, using just enough of your processor to earn coins without making the slowdown obvious enough to investigate.
You still pay for it, though. Higher electricity bills. A computer that lags and overheats. Hardware worn out faster from running flat-out for weeks. One infected laptop barely earns the attacker anything. But spread across thousands of hijacked machines, the combined output adds up to real money. Volume is the whole business model.
For companies it's worse. A whole rack of servers or a floor of employee PCs can burn cash for ages before anyone notices something's off.
The browser trick, and CoinHive
Some cryptojacking skips the malware entirely. Attackers hide mining code right inside a web page. Visit the site and your browser starts mining for as long as the tab is open; close it and the mining stops. Sneaky, because nothing gets installed on your machine. The only clues are a sluggish browser and a fan that suddenly won't shut up.
The famous case is CoinHive. Launched in 2017, it handed website owners a small chunk of code that borrowed visitors' processing power to mine Monero, a privacy-focused coin. The pitch wasn't crazy: skip the ads, earn money from visitors' spare computing instead. Some sites were upfront about it. Honestly, that part was fine.
Then it went sideways. Most sites used CoinHive in secret, never telling anyone. Worse, criminals injected the code into hacked websites, turning thousands of normal pages into mining traps without the owners knowing. Security tools started flagging CoinHive as malicious no matter how it was used. The service shut down in March 2019, blaming a crashed crypto market and a Monero update that slashed mining earnings in half. Good riddance, frankly.
Avast End Coinhive; End Cryptojacking
Why it won't die
You'd think falling coin prices would kill this off. Nope. The economics still work because the attacker pays nothing; every hijacked device chips in a little, and a botnet of thousands grinds out steady income at low risk. Cryptojacking isn't a quiet leftover, either. SonicWall counted over a billion cryptojacking hits in 2023, a record high.
It's also brutally hard to spot. No ransom note, no stolen files, no broken-down door. The malware just sits there eating your resources, and plenty of victims never find out at all. That silence is exactly why it keeps paying.
How to protect yourself
- Watch performance. Random CPU or GPU spikes when your machine should be idle are a red flag.
- Run solid security software and keep everything patched; this malware loves outdated systems with known holes.
- Use a script-blocking browser extension like No Coin or minerBlock to stop in-browser mining.
- Check Task Manager or Activity Monitor for strange processes hogging your processor.
- Go easy on sketchy downloads. Pirated software and dodgy email attachments are how a lot of this stuff spreads.
