Excel users have been urged to immediately apply a patch released earlier this month that tackles attack code that exploits a bug in Microsoft's spreadsheet application.
The exploit, which was posted to the milw0rm.com site, is the first made public for any of the seven vulnerabilities that were patched by Microsoft several days earlier in the security update tagged as MS08-014. That bulletin fixed multiple flaws in Excel 2000, 2002, 2003 and 2007 on Windows, and Excel 2004 and Excel 2008 on the Mac.
"The vulnerability that this exploit is designed to leverage was originally exploited in the wild on January 15, 2008," said Symantec security analyst Aaron Adams in an alert to customers of the company's DeepSight threat notification service. "We believe it leverages CVE-2008-0081 ... [and] involves the manipulation of an uninitialized stack variable by specially crafting an Excel file such that stack data will be pre-populated with user-supplied data and therefore able to influence the value of the uninitialized variable."
Microsoft labelled CVE-2008-0081 'critical' on Excel 2000, and 'important' on Excel 2002 and 2003.
Microsoft first acknowledged the Excel bug more than two months ago, when it confirmed that hackers were attacking Windows machines via Excel. At the time, the company's security team characterised the attacks as "targeted and not widespread".
Once the attack code was publicly posted on Friday, Adams advised users to apply MS08-014 immediately. "This should be considered a high priority in light of the availability of exploit code," he said. "Additionally, users should be advised to carry out extreme caution when handling Excel files received online. If possible, Excel files should be filtered at the e-mail gateway until the updates can be applied."
The MS08-014 update was the same one that Microsoft had to re-release last week after it discovered one of the Excel fixes had produced a regression error that generated wrong results in some calculations.