CVE-2026-10686 in Zephyr
Summary
by MITRE • 07/31/2026
Zephyr's IPv6 forwarding path re-sent routed unicast packets without ever decrementing the IPv6 hop limit. Both routing branches of ipv6_route_packet() (subsys/net/ip) were affected: the explicit-route path (net_route_packet()) and the on-link cross-interface path (net_route_packet_if()). Each set the packet forwarding flag and called net_send_data() with the hop limit untouched and no expiry check.
Per RFC 8200 the hop-limit decrement is the mechanism that bounds packet lifetime and terminates routing loops; without it, a device acting as an IPv6 router relays looping packets indefinitely. An on-path attacker who can induce or exploit a transient L3 loop turns it into a permanent forwarding storm, causing CPU/bandwidth resource exhaustion (availability DoS) on the forwarder and adjacent links; path-discovery and loop diagnostics that rely on hop-limit expiry are also defeated.
Affected configurations. In every affected release the forwarding path is reached via CONFIG_NET_ROUTE (enabled by default when CONFIG_NET_IPV6_NBR_CACHE is set), together with CONFIG_NET_ROUTING for cross-interface routing. Note that CONFIG_NET_IPV6_FORWARDING and CONFIG_NET_IPV4_FORWARDING — which appear in the fix and in this advisory's evidence notes — were introduced after v4.4.0, when the routing options were split and renamed; they do not exist in any affected release. When auditing a v4.4.1-or-earlier configuration, look for CONFIG_NET_ROUTE and CONFIG_NET_ROUTING.
IPv4 is not affected in any release. The IPv4 forwarding path (net_route_ipv4_packet() in route_ipv4.c) was added after v4.4.0 and has never shipped in a release. Its TTL decrement and IPv4 header-checksum recomputation landed on main as part of the same fix, so the evidence notes below discuss it, but no released version is reachable by way of IPv4.
Affected releases are v1.8.0 through v4.4.1: v1.8.0 introduced net_route_packet() and v2.2.0 added net_route_packet_if(), and neither decremented the hop limit. v4.3.1 carries the explicit-route fix but not the on-link one, so it is affected as well. Fixed on main by 7d8f1afa7345 (explicit-route path) and 589eadc74efa (on-link path).
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Analysis
by VulDB Data Team • 07/31/2026
The vulnerability described in this CVE affects the Zephyr operating system's implementation of IPv6 packet forwarding, specifically within the networking subsystem. This flaw exists in versions 1.8.0 through 4.4.1 and represents a critical design oversight that undermines fundamental IPv6 routing mechanisms. The issue manifests in the ipv6_route_packet() function located in the subsys/net/ip directory, which handles packet routing decisions for IPv6 traffic. Both primary routing branches of this function are impacted: the explicit-route path managed by net_route_packet() and the on-link cross-interface path handled by net_route_packet_if(). These pathways are activated when specific configuration options are enabled, namely CONFIG_NET_ROUTE and CONFIG_NET_ROUTING, which are typically active in default networking setups.
The technical root cause stems from a failure to properly implement RFC 8200 compliance for IPv6 packet handling. The hop limit field within IPv6 headers serves as a critical mechanism that prevents packets from circulating indefinitely through routing loops while also bounding their lifetime on the network. In affected versions, when these routing functions process packets destined for forwarding, they set the packet forwarding flag but neglect to decrement the hop limit value before transmission. This omission occurs regardless of whether the packet follows an explicit route or crosses interface boundaries, meaning that every packet processed through either branch remains with its original hop limit intact. The absence of any expiry check or hop limit decrement creates a persistent state where packets can be endlessly forwarded without ever reaching their intended destination.
The operational impact of this vulnerability is severe and manifests as a denial-of-service condition that affects network availability. An attacker positioned within the network path who can induce or exploit transient Layer 3 routing loops can transform these temporary conditions into permanent forwarding storms. This occurs because the vulnerable system never decrements the hop limit, allowing packets to circulate indefinitely through any routing loop present in the network topology. The result is a significant consumption of CPU resources and bandwidth on both the affected forwarder device and adjacent network links. Network administrators lose the ability to rely on standard diagnostics that depend on hop limit expiry for path discovery and loop detection, as these mechanisms become ineffective. This vulnerability directly violates fundamental security principles outlined in the Common Weakness Enumeration (CWE) category CWE-125, which addresses out-of-bounds read conditions, and also aligns with ATT&CK technique T1498.001 for network denial of service attacks.
The affected configurations are specifically tied to older Zephyr releases where the networking subsystem was still under development and had not yet implemented proper hop limit handling. The configuration options that trigger this vulnerability include CONFIG_NET_ROUTE, which is enabled by default when CONFIG_NET_IPV6_NBR_CACHE is set, and CONFIG_NET_ROUTING for cross-interface routing capabilities. It's important to note that while later versions introduced additional configuration options such as CONFIG_NET_IPV6_FORWARDING and CONFIG_NET_IPV4_FORWARDING, these were not present in any affected release and therefore should not be considered when auditing older system configurations. The IPv4 forwarding path was never implemented in versions prior to v4.4.0 and thus remains unaffected by this particular vulnerability, though the fix for this issue did address IPv4-related components as part of a broader networking enhancement effort.
Mitigation strategies for this vulnerability require immediate action to either upgrade to a patched version of the Zephyr operating system or implement workarounds that prevent the vulnerable code paths from being executed. Organizations should prioritize updating to versions that contain the fixes referenced in commit hashes 7d8f1afa7345 (for explicit-route path) and 589eadc74efa (for on-link path). System administrators should audit their current configurations for the presence of CONFIG_NET_ROUTE and CONFIG_NET_ROUTING options, particularly in environments running Zephyr versions 4.4.1 or earlier. Additionally, network monitoring solutions should be enhanced to detect unusual forwarding patterns that could indicate the presence of routing loops being exploited through this vulnerability. The fix implemented addresses the core issue by ensuring that hop limit values are properly decremented during packet forwarding operations, thereby restoring proper IPv6 routing behavior and preventing the indefinite relaying of packets that would otherwise consume network resources indefinitely.