A security company informs its customers of a new vulnerability that affects web applications.

The best answer is A. Zero-day.

A zero-day vulnerability is a newly discovered vulnerability for which no patch or official fix is yet available. Because defenders have had zero days to fully remediate it, attackers may be able to exploit it before a vendor releases a patch.

The question specifically states that the vulnerability is new and does not have an available patch, which is the defining clue.

Why the other options are incorrect:

B. XSSCross-site scripting is a specific web application attack type, not a description of whether a patch exists.

C. SQLiSQL injection is also a specific attack type, not the broader vulnerability status being asked about.

D. Buffer overflowBuffer overflow is a type of coding flaw, but again it does not specifically describe the condition of having no patch available.

From a Security+ perspective, when a vulnerability is newly identified and lacks a vendor fix, it is best described as a zero-day.