"Tell me about the most interesting security bug you personally found and helped fix. I’m less interested in the exploit mechanics than in how you handled the ambiguity, prioritized the risk, and drove the remediation. If it touched a Meta-like surface such as Facebook, Instagram, WhatsApp, Messenger, Graph API, or an internal service, even better. Walk me through the situation in STAR format."
This question tests ownership, judgment, and influence. In a Security Engineer role at Meta, finding a bug is only part of the job; the harder part is often validating impact, aligning with product and infrastructure teams, making trade-offs under uncertainty, and ensuring the issue is actually fixed without creating unnecessary disruption. Interviewers also want to see whether you can communicate risk clearly to non-security partners and stay constructive if others initially disagree with severity or scope.
A strong answer is specific: name the surface, the class of bug, the stakes, the timeline, and your role. The best responses show how you investigated, how you persuaded others to act, what changed because of your work, and what you learned about prevention—not just the bug itself.