Today Apple released Security Update 2007-006 which addresses:
An HTTP injection issue exists in XMLHttpRequest when serializing headers into an HTTP request. By enticing a user to visit a maliciously crafted web page, an attacker could conduct cross-site scripting attacks. This update addresses the issue by performing additional validation of header parameters.
An invalid type conversion when rendering frame sets could lead to memory corruption. Visiting a maliciously crafted web page may lead to an unexpected application termination or arbitrary code execution. Credit to Rhys Kidd of Westnet for reporting this issue.
The update can be downloaded and installed via Software Update preferences, or from Apple Downloads.