In the Linux kernel, the following vulnerability has been resolved: netfilter: nfreject: don't leak dst refcount for loopback packets recent patches to add a WARN() when replacing skb dst entry found an old bug: WARNING: include/linux/skbuff.h:1165 skbdstcheckunset include/linux/skbuff.h:1164 [inline] WARNING: include/linux/skbuff.h:1165 skbdstset include/linux/skbuff.h:1210 [inline] WARNING: include/linux/skbuff.h:1165 nfrejectfillskbdst+0x2a4/0x330 net/ipv4/netfilter/nfrejectipv4.c:234 [..] Call Trace: nfsendunreach+0x17b/0x6e0 net/ipv4/netfilter/nfrejectipv4.c:325 nftrejectineteval+0x4bc/0x690 net/netfilter/nftrejectinet.c:27 exprcallopseval net/netfilter/nftablescore.c:237 [inline] .. This is because blamed commit forgot about loopback packets. Such packets already have a dstentry attached, even at PREROUTING stage. Instead of checking hook just check if the skb already has a route attached to it.