The vulnerability has been exploited over a year and was reported to vendor on 2009-03-26.
According to Symantec the first exploitation of the vulnerability was discovered on 2008-07-25.
Advisory: SB2009060902 - Multiple vulnerabilities in Microsoft Excel
Vulnerable component: Microsoft Excel
CVSSv3 score: CVSS:3.1/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 - Memory corruption
The vulnerability allows a remote attacker to execute arbitrary code on the target system.
The weakness exists due to buffer overflow when parsing the Excel spreadsheet file format. A remote attacker can create a specially crafted Excel file containing a malformed record pointer, trick the victim into opening it, trigger memory corruption and execute arbitrary code on the target system with privileges of the current user.
Successful exploitation of the vulnerability results in arbitrary code execution on the vulnerable system.
Note: according to reports this vulnerability was being actively exploited before Microsoft issued security patch.