Methods and systems for selectively blocking, allowing and/or reformatting IPv6 headers by traversing devices are provided. According to one embodiment, a traversing device receives an Internet Protocol (IP) version 6 (IPv6) packet or packet fragment. One or more security checks are applied to extension headers of the IPv6 packet or packet fragment. If a security check of the one or more security checks is determined to be violated, then one or more appropriate countermeasures are identified and performed.