According to the security release notes accompanying Apple's latest Safari versions, the new software address a WebKit vulnerability that could allow for termination of arbitrary code after a user visits a malicious website.
Apple said it patched issues with memory corruption in WebKit by employing "improved memory handling," but offered no further detail on the matter. Looking at the list of Common Vulnerabilities and Exposures IDs (CVE-IDs), Apple discovered five issues fixed with today's release, while credit for the other two was given to Google's Chrome Security Team and an anonymous researcher.
A Safari update in May addressed a nearly identical WebKit problem that could cause Macs to unexpectedly crash.
Safari 6.1.6 and Safari 7.0.6 are free downloads that can be accessed via Software Update. The new software is available for OS X 10.9.4 Mavericks, OS X 10.7.5 Mountain Lion and OS X 10.8.5 Lion.