Lesson: Beware of any link to any content not under your direct control because it can be switched out on a moment's notice without your consent.
Stealth fallout: Total loss of control
Hackers have been using stealth methods to hide their maliciousness since the beginning days of malware. Heck, the first IBM-compatible PC virus, Pakistani Brain, from 1986, redirected inquiring eyes to a copy of the unmodified boot sector when viewed by disk editors.
When a hacker modifies your system in a stealthy way, it isn't your system anymore — it belongs to the hackers. The only defenses against stealth attacks are the same defenses recommended for everything (good patching, don't run untrusted executables, and so on), but it helps to know that if you suspect you've been compromised, your initial forensic investigations may be circumvented and fought against by the more innovative malware out there. What you think is a clean system and what really is a clean system may all be controlled by the wily hacker.
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