The Sasser worm was infecting thousands of systems by exploiting a recently announced Microsoft flaw, security experts said Monday.

Researchers at Internet Security Systems estimated 500,000 to one million Sasser infections.

Sasser exploits the Microsoft Local Security Authority Subsystem Service (LSASS) vulnerability, which Microsoft announced and issued a patch for last month.

The worm spreads by scanning randomly chosen IP addresses on unpatched Microsoft systems, according to Symantec. The vendor upgraded a second version of the worm, Sasser.B, to a Category 4 threat, meaning a severe threat.

Sasser affects Windows XP and Windows 2000 systems. A patch for the vulnerability the worm exploits can be downloaded from Microsoft's web site at