CVE-2026-7656
HighCVSS 8.1Exploitation Probability (EPSS)
Low risk14th percentile — higher than 14% of all known CVEs
Summary
A logic error in IPv6 Neighbor Discovery handlers in Zephyr RTOS causes incorrect operator precedence, bypassing critical checks (Hop Limit == 255, link-local source, multicast sanity). An adjacent or potentially remote attacker can inject forged RA, NS, and NA packets that are accepted.
Risk Assessment
The organization risks IPv6 routing hijacking, neighbor cache poisoning, man-in-the-middle attacks, traffic redirection, and denial of service – from both local and potentially remote attackers.
Recommendation
Immediately update Zephyr RTOS to a version containing the fix (post v4.4.0) or apply the patch from the project repository. As a temporary workaround, consider disabling IPv6 ND or deploying firewalls to block unauthorized ICMPv6 packets.
Original NVD description (English source)
The IPv6 Neighbor Discovery handlers in subsys/net/ip/ipv6_nbr.c (handle_ra_input, handle_ns_input, handle_na_input) used an incorrect boolean expression that combined the RFC 4861 validity checks with the ICMPv6 code check using the wrong operator precedence: the form was '((length/hop/source/target checks) && (icmp_hdr-code != 0))'. Because every legitimate ND message carries ICMPv6 code 0, an attacker setting code == 0 (the normal value) caused the entire predicate to evaluate false, so the packet was never dropped and all of the other checks were silently skipped. The bypassed checks include the mandatory Hop Limit == 255 verification (which proves an ND packet originated on-link and was not forwarded) and, for Router Advertisements, the requirement that the source be a link-local address, as well as multicast-target sanity checks. As a result, an adjacent on-link attacker — and, because the Hop-Limit-255 guard is bypassed, potentially a remote/off-link attacker whose packets would otherwise be rejected — can have forged Router Advertisement, Neighbor Solicitation, and Neighbor Advertisement messages accepted. A forged RA lets the attacker reconfigure the victim's default router, on-link prefixes (SLAAC), MTU, reachable/retransmit timers, and (with CONFIG_NET_IPV6_RA_RDNSS) DNS servers, while forged NS/NA enable neighbor-cache poisoning, enabling man-in-the-middle, traffic redirection, and denial of service. The flaw is an input-validation/authentication weakness rather than a memory-safety issue: the underlying packet-parsing primitives (net_pkt_get_data, net_pkt_read, net_pkt_skip) are independently bounds-safe and the validated 'length' is the true buffer length, so skipping the length check causes no out-of-bounds access. The defect has existed since the logic was introduced in 2018 and shipped in all releases through v4.4.0; it is fixed by splitting the condition so any failing check drops the packet.