Microsoft has released its monthly set of security patches, fixing problems in Windows, Internet Explorer and in the Secure Sockets Layer (SSL) software used for secure web browsing.

In total, Microsoft released six updates, fixing 15 flaws in its products. Four of these updates are rated critical and administrators should patch them as soon as possible, security experts said Tuesday.

Many of the flaws that Microsoft patched on Tuesday can be exploited via the web. In particular the MS07-033 update, which fixes bugs in Internet Explorer and MS07-031, which addresses a critical flaw in the SSL libraries used by Windows.

The SSL flaw is particularly interesting because it affects users who are visiting supposedly trustworthy websites and because it could also affect non-Microsoft browsers like Firefox and Opera, said Amol Sarwate, manager of Qualys' vulnerability research lab. "It will affect all the browsers that call the libraries that are in the Windows operating system," he said.

Although Microsoft warned that this flaw could be used by attackers to run unauthorised software on the victim's computer, this would be hard to do. "Attempts to exploit this vulnerability would most likely result in the Internet web browser or application exiting," Microsoft wrote in its MS07-031 bulletin. "The system would not be able to connect to websites or resources using SSL…until a restart of the system."

The two other critical updates are MS07-035, which fixes a bug in the core Windows operating system's Win32 application programming interface (API), and MS07-034, which fixes bugs in Outlook Express and Windows Mail.

Sarwate said that, because MS07-035 could be exploited via the web, it should also be patched as soon as possible.

The other updates released fix less-critical flaws in Windows Vista and Microsoft's Visio drawing software.

The Vista update is noteworthy because it marks the first time that Microsoft has patched a flaw that was introduced in the Vista operating system, Sarwate said. Previous Vista patches have repaired "legacy" code that was written for earlier versions of Windows, and then reused in Vista. But the MS07-032 update applies to Vista only.

This flaw, rated "moderate" by Microsoft, could allow an attacker to gain unauthorised access to information such as administrative passwords stored within Windows.