Microsoft issued four critical updates Tuesday that quashed 12 bugs in Office, the company's business suite, including a flaw in Excel that has been exploited by attackers for more than two months.

Tuesday's tally was a dramatic decrease from February's, when Microsoft unveiled 11 security bulletins and plugged 17 holes. Of the dozen vulnerabilities disclosed today, however, 11 were ranked "critical," Microsoft's highest rating in its four-step threat-scoring system. That was more than double the number of critical bugs crushed last month. The twelfth vulnerability of today was pegged as "important," the second-highest rating.

There's no question that MS08-014, the bulletin that fixes seven flaws in Microsoft Excel, is the one to work first, said Andrew Storms, director of security operations at nCircle Network Security Inc. "[MS08-]014 is definitely the most important of today's bulletins. It covers so many vulnerabilities and at least one was already known and was being exploited."

The sheer number of bugs quashed by the Excel update and the number of researchers Microsoft credited led Storms to some speculation. "The number of acknowledgments tells me that the exploit was more widely used, and the fix for [it and the others] more detailed than we'd been led to believe. That's probably why it took them the two months to come up with a fix."

Microsoft tipped its hat to eight different security researchers from VeriSign, iDefense Labs, Fortinet, TippingPoint, Websense and other vendors in the description of the Excel patches. Two, Mike Scott of SAIC and Matt Richard of VeriSign, were credited with notifying Microsoft of the currently-exploited bug.

That vulnerability, said Microsoft, was in the code that handled macros within spreadsheets. Exploits have circulated for two months minimum, with a spike spotted just yesterday by several security organizations, including Symantec and US-CERT.

In mid-January, Microsoft issued a security advisory that noted targeted attacks had been discovered exploiting a then-unknown Excel bug. At the time, Microsoft offered several workarounds, including one that recommended Office 2003 users run suspect Excel worksheets through MOICE (Microsoft Office Isolated Conversion Environment), a free conversion tool released last year that converts Office 2003 formatted files into the more secure Office 2007 formats.

Yesterday, both the SANS Institute's Internet Storm Center (ISC) and Homeland Security's US-CERT warned of new attacks using the Excel bug; earlier today, Symantec weighed in, too.

"The incidents...have been limited to a very specific targeted attack and were not widespread," said ISC analyst Maarten Van Horenbeeck Monday in a note posted on the group's site. "We [counted] approximately 21 reports of attacks using only 8 different files, from within the same two communities, so far."

But Storms thought the recent uptick was probably just a coincidence. "It may have been [attackers] using the exploit one last time before Microsoft patched it, but then again, the vulnerability has been out there for months." The increase might have been caused by other hackers joining the fray, he said. "Once an exploit it being used, it only takes time before someone captures and retools it," Storms noted. The bump in discovered attacks could have come from those second-tier hackers.