CVE-2026-98379 in Linux
Summary
by MITRE • 10/09/2026
In the Linux kernel, the following vulnerability has been resolved:
netfilter: ip6t_rpfilter: reject routes without inet6_dev
ip6_route_lookup() can return an error-free route whose rt6i_idev is NULL. Lowering an external nexthop device's MTU below IPV6_MIN_MTU tears down its inet6_dev while fib6_ifdown() leaves routes using nexthop objects in the FIB. An unprivileged user can construct this state with rtnetlink in a private user and network namespace, then trigger a NULL dereference through an IPv6 rpfilter lookup:
Oops: general protection fault, probably for non-canonical address 0xdffffc0000000000 KASAN: null-ptr-deref in range [0x0000000000000000-0x0000000000000007]
RIP: rpfilter_mt (net/ipv6/netfilter/ip6t_rpfilter.c:75) Call Trace: ip6t_do_table (net/ipv6/netfilter/ip6_tables.c:316) nf_hook_slow (net/netfilter/core.c:619) ipv6_rcv (net/ipv6/ip6_input.c:351) __netif_receive_skb_one_core (net/core/dev.c:6216) process_backlog (net/core/dev.c:6680) __napi_poll (net/core/dev.c:7739) net_rx_action (net/core/dev.c:7959) handle_softirqs (kernel/softirq.c:622) do_softirq.part.0 (kernel/softirq.c:523) __local_bh_enable_ip (kernel/softirq.c:450) __dev_queue_xmit (net/core/dev.c:4913) packet_sendmsg (net/packet/af_packet.c:3139) __sys_sendto (net/socket.c:2252) __x64_sys_sendto (net/socket.c:2259) do_syscall_64 (arch/x86/entry/syscall_64.c:94) entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:121) Kernel panic - not syncing: Fatal exception in interrupt
Reject routes without an inet6_dev immediately after lookup. Such routes are not eligible for reverse-path filtering, and the check protects all later rt6i_idev dereferences.
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Analysis
by VulDB Data Team • 10/09/2026
The Linux kernel vulnerability identified within the netfilter subsystem specifically affects the IPv6 reverse path filter implementation in ip6t_rpfilter.c. This flaw arises from a race condition or state inconsistency during route lookup operations where ip6_route_lookup returns a valid routing entry without an associated error, yet the resulting route structure contains a NULL pointer for its inet6_dev field. The root cause of this inconsistent state is triggered when the Maximum Transmission Unit (MTU) of an external nexthop device is lowered below IPV6_MIN_MTU. This action causes fib6_ifdown to tear down the corresponding inet6_dev object, but it fails to remove or update routes that rely on nexthop objects still present in the Forwarding Information Base (FIB). Consequently, subsequent lookups may return these stale route entries where the device pointer is no longer valid, creating a dangerous condition for any code path that assumes the presence of a valid inet6_dev.
An unprivileged user can exploit this vulnerability by constructing the specific network state required to trigger the NULL dereference within a private user and network namespace using rtnetlink system calls. Once the environment is configured such that a route points to a torn-down device, sending packets through the IPv6 reverse path filter mechanism causes the kernel to attempt accessing memory at an invalid address. The technical impact manifests as a general protection fault with KASAN reporting a null pointer dereference in range zero to seven. This leads to a crash of the network processing soft interrupt handler, specifically within rpfilter_mt during packet reception via ipv6_rcv and subsequent NAPI polling stages. In severe cases, this results in a kernel panic that halts system operation, effectively causing a denial of service for all services running on the affected host or virtual machine.
From a security classification perspective, this vulnerability aligns with CWE-476 NULL Pointer Dereference, as the code fails to check for a null pointer before dereferencing it in critical network processing paths. It also relates to CWE-362 Concurrent Execution using Shared Resource with Improper Synchronization Race Condition, given that the state inconsistency arises from asynchronous device teardown and route lookup operations not being properly synchronized or validated against each other's current states. In terms of adversary behavior, this flaw facilitates Denial of Service (T1499) by allowing local users to crash the kernel without requiring elevated privileges initially, although it may serve as a precursor for privilege escalation if combined with other vulnerabilities that allow exploitation of the resulting memory corruption or state confusion.
The mitigation implemented involves modifying ip6t_rpfilter.c to immediately reject any routes returned from lookup operations that lack an associated inet6_dev structure. This check is performed right after the route lookup and before any further processing, ensuring that all subsequent dereferences of rt6i_idev are protected against null pointers. Since reverse path filtering relies on device-specific information contained within the inet6_dev object, routes without this data are inherently ineligible for such filtering anyway. By explicitly rejecting these invalid entries early in the packet processing pipeline, the kernel prevents access to freed or uninitialized memory structures. System administrators should apply the relevant kernel update that includes this patch to restore stability and security integrity of IPv6 network stack operations under conditions involving dynamic MTU changes on nexthop devices.