A buffer overrun flaw in Macromedia Flash can be used to inject malicious code into target systems.
Potential attackers could try to persuade victims to download maliciously altered versions of Macromedia Flash movies (SWF) but there's still a risk there as crackers are hardly adverse to disguising Trojan code as prOn download or Warez. Unusually, the vulnerability affects users of multiple operating systems (Windows, Unix, Mac) who use Macromedia's application.
However there is a fix, available as a link from Macromedia's minimalist description of the problem here.
A fuller description can be obtained from an eEye advisory. ®