Microsoft's June Patch Tuesday brought six security updates - four of which earn the dreaded rating of critical.
The critical fixes are designed to address flaws in Windows' Schannel security package, Internet Explorer, Outlook Express, and a Windows library file flaw.
All four create a means for hackers to inject malicious code onto vulnerable systems. The Schannel flaw is perhaps the worst of the group, since the bug is the target of known exploits.
Schannel implements the Secure Sockets Layer (SSL) and Transport Layer Security (TLS) authentication protocols on Windows boxes. Vulnerabilities in the package mean that unpatched systems visiting maliciously-created web pages on secure servers are liable to become infected.
Redmond's security team also released a fix for a vulnerability in Microsoft Visio that might allow remote code execution. Microsoft describes the flaw as "important", although some security watchers reckon its more severe than that.
The SANS Institute's Internet Storm Centre rates the flaw as "critical", at least on the client side. The sixth update addresses a less noteworthy (moderate) information disclosure flaw in Windows Vista.