Two vulnerabilities have been reported in Microsoft Excel, which can be exploited by malicious people to compromise a user's system.

1) An error may cause an invalid object to be referenced when opening an Excel document and can be exploited via a specially crafted Excel spreadsheet file.

NOTE: According to Microsoft, the vulnerability is currently being actively exploited.

2) An error when processing an unspecified 16-bit value extracted from an Excel document can be exploited to corrupt memory.

Successful exploitation may allow execution of arbitrary code.

Solution
Apply patches.
Further details available to Secunia VIM customers

Provided and/or discovered by
1) Reported as a 0-day.
2) Haifei Li, Fortinet FortiGuard Global Security Research Team

Changelog
Further details available to Secunia VIM customers

Original Advisory
MS09-009 (KB968557, KB959964, KB959988, KB959995, KB959997, KB959993, KB960000, KB960003):
http://www.microsoft.com/technet/security/Bulletin/MS09-009.mspx

Microsoft:
http://www.microsoft.com/technet/security/advisory/968272.mspx

FortiGuard:
http://www.fortiguardcenter.com/advisory/FGA-2009-16.html

Alternate/detailed remediation
Further details available in Customer Area

Deep Links
Links available to Secunia VIM customers