Mean time to exploit is the average time between vulnerability disclosure and the first observable attacker use of that flaw. In some cases, the clock is effectively negative: attackers begin using the vulnerability before a patch is widely available, or even before defenders finish their normal change process. This metric matters because it measures attacker speed, not just patching speed.
In cyber security, a short mean time to exploit means exposure becomes dangerous quickly, especially on internet-facing systems such as firewalls, VPN gateways, and other edge devices. Defenders use this idea to prioritize urgent fixes, monitor exploit intelligence, and verify that remediation actually stuck after deployment. It also shows why patch tickets alone are not enough: a vulnerability can be “fixed” on paper but still exploitable if the change failed, drifted, or was rolled back.