The flaw was most likely introduced in August 2013. The vulnerability was reported to vendor - 2014-02-04.
Private fully functional exploit code existed long before the vendor released security patch. We consider this vulnerability a zero-day.
Vulnerable component: Microsoft Internet Explorer
CVSSv3 score: CVSS:3.0/AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:H/A:H/E:F/RL:O/RC:C
CWE-ID: CWE-119 - Improper Restriction of Operations within the Bounds of a Memory Buffer
The vulnerability allows a remote attacker to execute arbitrary code on the target system.
The weakness exists due to use-after-free when accessing an object in memory. A remote attacker can create a specially crafted Web site, trick the victim into visiting it, trigger memory corruption and execute arbitrary code with privileges of the current user.
Successful exploitation of the vulnerability results in arbitrary code execution on the vulnerable system.
Note: the vulnerability was being actively exploited.