What you copy is not always what you paste.

"ClickFix": the fake error that asks you to paste

A page shows a fake CAPTCHA or "fix this error" box and tells you to press Win+R (or open a terminal) and paste. The command was silently pre-loaded onto your clipboard by the page, so you run the attacker's installer thinking you are just proving you are human.

Backdoor path: the pasted one-liner fetches and runs an infostealer or RAT (Lumma Stealer, DarkGate, NetSupport, AsyncRAT), handing over your credentials, session, and a foothold.

Seen in the wild: named by Proofpoint, who coined "ClickFix" from a 100,000-message campaign by the broker TA571. It is now its own MITRE ATT&CK technique (T1204.004), ranked by Red Canary as the #2 initial-access vector of 2025, and adopted by nation-state actors: Kimsuky (DPRK), MuddyWater (Iran) and APT28 (Russia). Microsoft, which tracks the technique too, reports campaigns hitting thousands of devices daily.

Clipper malware: your clipboard, quietly rewritten

Malware on the machine watches the clipboard and swaps what you copied for something else (classically a cryptocurrency wallet address replaced with the attacker's look-alike), so the value you paste is not the value you copied.

Backdoor path: you copy a wallet address and paste it to send funds; the money goes to the attacker. The swap happens silently between copy and paste, with no visible change.

Seen in the wild: a long-running malware class: CryptoShuffler (2017, ~$140k stolen) and the Laplas Clipper malware-as-a-service (2022), which mints a look-alike address matching the first and last characters of yours. More reporting.

The paste that presses Enter for you

Modern terminals and shells defend against this by default: on a current emulator with interactive bash (>= 5.1, Dec 2020), zsh or fish, a pasted newline is held on the command line for you to review -- "bracketed paste," on by default. But it is not universal. It fails wherever the reader does not honour it: bash before 5.1, minimal shells (POSIX /bin/sh (dash), busybox ash), raw REPLs and readers that never request it (cat, a language REPL, a serial or recovery console), ssh into an old host, or an app that turns the mode off. There, a snippet ending in a newline still submits the moment it lands -- and it can be bypassed on unpatched terminals by crafted clipboard content (CVE-2021-31701).

The catch: on such a reader, combined with a hidden line, there is no pause to notice anything: the copy, the paste, and the execution are one motion.

Backdoor path: no hidden line is even needed -- on a reader without bracketed paste, a visible command that ends in a newline runs the moment it lands, before you can cancel, and any multi-line paste executes every line in sequence.

Seen in the wild: paste-and-run is the payload step of ClickFix (MITRE ATT&CK T1204.004), an active initial-access campaign detailed above.

The extra line you never saw

Text placed off-screen with CSS, or tucked into a zero-height element, is still part of what you select and copy. The page shows one command; the clipboard gets two. Like invisible ink on a page you photocopy: the copy captures what your eyes skipped.

The catch: if the hidden portion ends in a newline, a reader without bracketed paste submits it the moment you paste and the second command runs before you have read the first. Modern bash/zsh/fish hold it by default (see Auto-run on paste) -- older or minimal shells, REPLs and old SSH hosts do not, so do not rely on it.

Backdoor path: the hidden line is a real command (a curl | sh installer, a key exfiltration); on an unprotected reader its trailing newline runs it the instant you paste, before your eyes reach the harmless line you meant to copy.

Proof of concept: the page-side hidden-copy trick was demonstrated as pastejacking (2016; no CVE -- inherent browser clipboard behavior). Its active in-the-wild social-engineering cousin, ClickFix, is above.

Characters that ride along unseen

Zero-width and control characters copy perfectly but display as nothing. A command that reads pip install requests can carry an invisible break, or a look-alike letter, so the thing your shell runs is not the thing you read: an invisible extra character wedged inside a word.

The catch: the payload survives a glance, a code font, even a careful proofread. It is not on the screen to be seen.

Backdoor path: the look-alike or zero-width character binds the command to a different target -- a typosquatted package name, a different host -- so the shell fetches attacker code while the text still reads as the trusted one.

Proof of concept: the invisible-character class was formalized as Trojan Source (CVE-2021-42574); research disclosure, no documented in-the-wild use of this copy-paste vector.