Copyright © 2012 Red Hat, Inc. All rights reserved. Moderate 2026-10-06T00:00:00 kernel: ipv4: icmp: reject RTN_UNREACHABLE input routes in icmp_route_lookup 5.5 CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H CWE-617

In the Linux kernel, the following vulnerability has been resolved: ipv4: icmp: reject RTN_UNREACHABLE input routes in icmp_route_lookup When the forward output route cannot be used in icmp_route_lookup(), it enters the "reverse path" and calls ip_route_input() on fl4_dec.daddr, the original packet's source address. ip_route_input() only returns an error for truly invalid packets. For unreachable addresses it will succeed and return an input route whose dst.output is set to ip_rt_bug(). The existing check only rejects RTN_LOCAL routes, so the RTN_UNREACHABLE route types can still be returned and later used for output, syzkaller triggering a WARN_ON_ONCE() in ip_rt_bug() as bellow: ------------[ cut here ]------------ WARNING: net/ipv4/route.c:1273 at ip_rt_bug+0x14/0x20 RIP: 0010:ip_rt_bug+0x14/0x20 Call Trace: ip_push_pending_frames+0xfa/0x100 __icmp_send+0x905/0xf10 ip_options_compile+0xc0/0xd0 ip_rcv_finish_core+0x321/0xae0 ip_rcv+0x1de/0x260 __netif_receive_skb_one_core+0x11a/0x130 netif_receive_skb+0x7b/0x260 tun_get_user+0x11bf/0x1c10 ------------[ cut here ]------------ Reject input route that is RTN_UNREACHABLE to fix it. The net warning is only printed for RTN_LOCAL, as RTN_UNREACHABLE is not the result of a race condition.

A flaw was found in the Linux kernel's IPv4 networking subsystem. When looking up routes to send Internet Control Message Protocol (ICMP) error messages, the kernel fails to properly reject unreachable input routes. A remote attacker can exploit this flaw by transmitting network packets that trigger ICMP responses to unreachable destinations, causing the system to invoke an invalid route handler and trigger a kernel warning, potentially leading to a Denial of Service (DoS).

A flaw in the Linux kernel's IPv4 ICMP error-message routing is rated Moderate by Red Hat Product Security and affects the Red Hat Enterprise Linux 7, 8, 9, and 10 kernels (including kernel-rt), Red Hat In-Vehicle OS 2, and the RHEL CoreOS layers of OpenShift Container Platform 4 and 5. When the kernel generates an ICMP error response and the forward output route cannot be used, icmp_route_lookup() falls back to a reverse-path lookup that calls ip_route_input() on the original packet's source address. For unreachable source addresses, this lookup can succeed and return an RTN_UNREACHABLE input route whose output callback is ip_rt_bug(); because the existing check only rejected RTN_LOCAL routes, the invalid route can be used for output, triggering a WARN_ON_ONCE() in ip_rt_bug() and dropping the outgoing ICMP response. An attacker who can transmit crafted IPv4 packets that cause the kernel to generate ICMP error replies toward unreachable destinations can trigger this condition and cause a denial of service: under default configurations the effect is a kernel warning and a dropped packet, while systems configured with panic_on_warn can panic. Red Hat Enterprise Linux 6 is out of support (OOSS) and will not receive a fix. For all affected streams the fix is deferred to a future update; the residual risk is limited to availability, with no impact on confidentiality or integrity. The Red Hat severity (Moderate, CVSS 3.1 score 5.5, local attack vector) reflects that the publicly demonstrated reproducer requires local privileges to set up the triggering configuration, although a remotely transmitted trigger remains plausible. Mitigation for this issue is either not available or the currently available options do not meet the Red Hat Product Security criteria comprising ease of use and deployment, applicability to widespread installation base, or stability. Red Hat Enterprise Linux 10 Fix deferred kernel Red Hat Enterprise Linux 6 Out of support scope kernel Red Hat Enterprise Linux 7 Fix deferred kernel Red Hat Enterprise Linux 7 Fix deferred kernel-rt Red Hat Enterprise Linux 8 Fix deferred kernel Red Hat Enterprise Linux 8 Fix deferred kernel-rt Red Hat Enterprise Linux 9 Fix deferred kernel Red Hat Enterprise Linux 9 Fix deferred kernel-rt Red Hat OpenShift Container Platform 4 Fix deferred openshift/ose-rhel-coreos-8 Red Hat OpenShift Container Platform 4 Fix deferred openshift/ose-rhel-coreos-9 https://www.cve.org/CVERecord?id=CVE-2026-98303 https://nvd.nist.gov/vuln/detail/CVE-2026-98303 https://git.kernel.org/pub/scm/linux/security/vulns.git/plain/cve/published/2026/CVE-2026-98303.mbox