CVE-2026-19577: Out-of-bounds read in IPv6 route forwarding when the nexthop neighbor has no link-layer address

netrouteipv6packet() in subsys/net/ip/routeipv6.c resolved the nexthop's link-layer address with netnbrgetlladdr(nbr->idx) without first checking whether the neighbor cache entry actually had a linked link-layer address. An unresolved neighbor carries idx == NETNBRLLADDRUNKNOWN (0xff), and netnbrgetlladdr() in subsys/net/ip/nbr.c performs no runtime bounds check beyond a NETASSERT, returning &netneighborlladdr[255] — roughly 2.5 KB past the end of an array whose default size is CONFIGNETIPV6MAXNEIGHBORS (8). Because the returned pointer is never NULL, the following lladdr == NULL guard does not catch it.

The function is reached from ipv6routepacket() in subsys/net/ip/ipv6.c for every received unicast IPv6 packet whose destination is not a local address on the receiving interface; CONFIGNETIPV6ROUTE is enabled by default whenever the IPv6 neighbor cache is, so no router or forwarding configuration is needed. netrouteipv6getinfo() returns the packet's destination itself as the nexthop when a neighbor cache entry for it exists, and the cache lookup does not skip INCOMPLETE entries. An unauthenticated attacker on the same link can therefore force the unresolved state — for example by eliciting traffic to a spoofed, non-existent neighbor address so that netipv6sendns() creates an INCOMPLETE entry, or by sending a Router Advertisement with no source link-layer address option, which creates a persistently unresolved router neighbor — and then send a packet addressed to that neighbor.

The result is an out-of-bounds read at a fixed index past the neighbor link-layer address array. On builds with CONFIGASSERT enabled the assertion fires and the device panics, giving a repeatable remote denial of service. With assertions disabled, the stale out-of-bounds struct netlinkaddr drives a memcmp() over an attacker-uninfluenced length and, when its len byte passes the NETLINKADDRMAXLENGTH check, up to 8 bytes of unrelated static RAM are copied into the outgoing frame's destination link-layer address and transmitted on the link, disclosing them to any listener. There is no out-of-bounds write and the offset is not attacker-controlled, which bounds the impact.