CVE-2026-98303 in Linux
Summary
by MITRE • 10/06/2026
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:
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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.
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Analysis
by VulDB Data Team • 10/06/2026
The Linux kernel contains a vulnerability in its IPv4 ICMP routing logic specifically within the icmp_route_lookup function that allows unreachable network routes to be processed incorrectly, leading to kernel warnings and potential instability. This issue arises during the handling of incoming Internet Control Message Protocol packets when the system attempts to determine the appropriate route for generating an error response or processing specific ICMP types. The core flaw lies in how the routing subsystem handles cases where a forward output route cannot be established. In such scenarios, the code enters a reverse path lookup mechanism and invokes ip_route_input() using the source address of the original packet as the destination to find a valid input route.
The technical deficiency stems from an incomplete validation check within icmp_route_lookup(). The existing logic only rejects routes marked with the RTN_LOCAL type but fails to filter out routes designated as RTN_UNREACHABLE. When ip_route_input() processes packets destined for unreachable addresses, it successfully returns an input route object rather than raising a hard error. However, this returned route has its destination output function pointer set to ip_rt_bug(), which is intended to trigger a warning when invoked unexpectedly. Because the validation logic does not explicitly exclude RTN_UNREACHABLE routes, these invalid paths are accepted and subsequently used for packet processing operations that attempt to invoke the buggy output handler.
This flaw results in the triggering of a WARN_ON_ONCE() macro within ip_rt_bug(), as evidenced by syzkaller fuzzing tests which successfully reproduce the condition. The call trace indicates that the warning is generated during the execution of ip_push_pending_frames, which is called from __icmp_send, ultimately originating from packet reception routines like ip_rcv and tun_get_user. While this does not typically lead to a full kernel panic or remote code execution in standard configurations, it causes significant noise in system logs through repeated warnings and can indicate deeper issues with network stack state consistency under specific routing conditions. The warning is specifically printed for RTN_LOCAL routes due to race condition concerns, but the omission of checks for unreachable addresses represents an oversight that allows malformed routing states to propagate further into the networking subsystem.
From a classification perspective, this vulnerability aligns with CWE-20 Improper Input Validation and CWE-754: Improper Check for Unusual or Exceptional Conditions in Linux kernel network stack components. It also relates to ATT&CK technique T1498 Network Denial of Service if the repeated warnings cause resource exhaustion or log flooding, although primarily it is a stability issue rather than an availability attack vector per se. The root cause is essentially a logic error where the application fails to validate that the returned route object represents a usable path for transmission before attempting to use its output function pointer.
Mitigation strategies involve applying kernel patches that update icmp_route_lookup() to explicitly reject RTN_UNREACHABLE routes in addition to existing checks for local and blackhole types. System administrators should ensure their Linux kernels are updated with the latest security fixes addressing this specific ICMP routing logic flaw. For environments where immediate patching is not possible, monitoring system logs for ip_rt_bug warnings can serve as an indicator of compromise or misconfiguration attempts targeting this code path. Additionally, implementing strict egress filtering and validating source addresses at network boundaries may reduce exposure to packets that trigger the reverse path lookup failure conditions associated with unreachable destinations.