Microsoft today issued an emergency update to plug a critical security hole present in all versions of its Internet Explorer Web browser, a flaw that hackers have been leveraging to steal data from millions of Windows users. The patch, which Microsoft dubbed MS08-078, fixes a security vulnerability that Microsoft says already has been used to attack more than 2 million Windows users. As Security Fix and other members of the tech community have chronicled, attackers have been busy compromising thousands of Web sites by seeding them with code that installs password-stealing software on computer systems of Web site visitors who use Internet Explorer. Microsoft estimated Monday that one in every 500 Windows users had been exposed to sites that try to exploit the flaw. Additionally, it said the number of victims was increasing at a rate of 50 percent daily. Vulnerability management company nCircle said Microsoft’s decision to issue the