Which term is used to refer to an attack that exploits a vulnerability before a patch is available?

• A. Day Zero
• B. Day One
• C. Zero Day Attack
• D. Early Access

Answer: (A)

A vulnerability that an attacker exploits before a patch exists is described as a zero-day attack. The key idea is that defenders have zero days to respond with a fix because the vulnerability is unknown and unpatched at the time of the exploit. The term “zero-day” emphasizes both the lack of a patch and the surprise nature of the attack, which is why it’s the precise label for this scenario.

Sometimes people might say “day zero” informally, but the accepted security terminology is zero-day attack (or zero-day exploit). The other terms don’t fit: Day One would imply after a patch or disclosure, and Early Access refers to software release timing, not to exploiting unknown vulnerabilities.