A bug in the Macintosh operating system has been discovered by the US Computer Emergency Readiness Team that allows code to exploit the DMG.
US-CERT has discovered that the proof-of-concept exploit is available publically, and targets a flaw in the way that Mac OS X handles disk image structures, or DMG files. The bug could cause memory corruption that results in a denial of service or remote code execution. However, there are no cases of the flaw being exploited in the wild yet.
At the moment, there's no fix available, and Apple has not commented on when the flaw will be patched.
The security company recommends that Mac users disable the command to open "safe" files after downloading with Safari, and that they not access DMG files from untrusted sources.
Security problems are not often issues with Apple Macs, because fewer people use them and therefore they're less interesting to hackers.