Microsoft has released its May set of security patches, fixing critical bugs in Windows, Office and Exchange. The updates mean that the month-old DNS flaw has finally been patched.

Seven groups of patches were released Tuesday, fixing a total of 19 bugs. Microsoft rates all seven of these updates as critical, but security experts said that IT administrators should be particularly concerned with the MS07-026 and MS07-0299 updates, which fix flaws in Exchange and the Windows DNS server.

The Exchange update fixes previously undisclosed flaws in Microsoft's messaging software that could be exploited to seize control of the server, said Paul Zimski, director of product and market strategy at PatchLink. "The biggest impact to organisations is going to be the Exchange update," he said. "It really affects core business."

If hackers were able to develop the right kind of Exchange exploit code, they could install unauthorised software on the server simply by sending it a maliciously crafted email message - something that would be extremely difficult to block, Zimski said.

Unlike the Exchange bugs, the the Windows' DNS server flawhas been known for about a month. Attackers have already developed code that exploits this flaw and security vendors have been seeing some online attacks over the past several weeks.

The DNS flaw "stands as the number one issue," this month, according to Amol Sarwate, manager of Qualys's vulnerability research lab. "A lot of people were waiting very anxiously for that patch."

The problem affects Windows 2000 and Windows Server 2003 systems, which can be tricked into running unauthorised software when an attacker sends them maliciously encoded RPC (Remote Procedure Call) packets to the DNS server. The latest versions of Windows 2000 Professional, XP and Vista are not vulnerable to this attack.

There is no shortage of other worrying bugs this month, however. Microsoft also released updates that fixed vulnerabilities in Excel, Word, Internet Explorer, Office and the CAPICOM cryptography technology used by BizTalk Server.

These updates fix a pair of vulnerabilities in Word and Excel that had been exploited by criminals in online attacks, said Mark Griesi, a security program manager with Microsoft's Security Response Center.

Griesi agreed that the Exchange and DNS server vulnerabilities should be given priority, though many administrators have now taken steps to mitigate the DNS attacks. "If I were an admin, the first thing I'd be doing would be getting the Exchange update deployed, assuming I had the DNS mitigations in place," he said.

Original story by IDG news service