Apple has released Security Update 2005-004 for iSync 1.5.
The update is available for iSync 1.5 on Mac OS X v10.2.8 and Mac OS X v10.3.x.
It exists to protect against a flaw in which: "A buffer overflow in iSync (within iSync helper tool mRouter) could lead to local privilege escalation." Such an instance could "result in the execution of arbitrary commands as root by local system users", Apple explained.
The update repairs this by providing a patched version of mRouter. The software is available through Software Update and for direct download from the link above.