This attack could pose a serious security threat. You should take immediate action to stop any damage or prevent further damage from happening.
This signature detection a vulnerability in Microsoft Excel which may result in remote code execution.
Microsoft Excel is a spreadsheet application that is part of the Microsoft Office suite.
Excel is prone to a remote code-execution vulnerability when parsing malformed 'FORMAT' records in Excel files. Stack-based buffer overflow in Microsoft Office Excel allows remote attackers to execute arbitrary code via an Excel file with a crafted OBJ (0x5D) record.
Attackers may exploit this issue by enticing victims into opening a malicious Excel file.
Successful exploits may allow attackers to execute arbitrary code with the privileges of the user running the application.
The vendor has released an advisory and updates. Please see the references for details.