ZyXEL ZyNOS contains a flaw that may allow a remote denial of service. The issue is caused due to an error in the handling of malformed fragmented IP packets. Upon receiving a large number of fragmented packets, the CPU consumes a large amount of resources for the re-assembling and processing of the packets resulting in a loss of availability for the router.
Upgrade to the firmware version 3.40 (GT.5) or higher, as it has been reported to fix this vulnerability. An upgrade is required as there are no known workarounds.