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CVE-2026-72421
ipv4: fib: Don't ignore error route in local/main tables.
Published 2026-08-15 (2 months ago) by the Linux kernel CNA. Subsystem include/net. Debian urgency: not yet assigned.
What to do, per Debian release
Debian's own status for CVE-2026-72421 in every release this site tracks, and what each one means in practice. Every status below is the one the Debian Security Team publishes for this CVE. source
| Release | Status | Version | What this means |
|---|---|---|---|
| Debian 12bookworm | fixed | 6.1.180-1 | Update the kernel and reboot. Fixed in 6.1.180-1. A running kernel keeps the old code until the machine restarts. |
| Debian 13trixie | fixed | 6.12.100-1 | Update the kernel and reboot. Fixed in 6.12.100-1. A running kernel keeps the old code until the machine restarts. |
| Debian 14forky | fixed | 7.1.5-1 | Update the kernel and reboot. Fixed in 7.1.5-1. A running kernel keeps the old code until the machine restarts. |
| Debian unstablesid | fixed | 7.1.5-1 | Update the kernel and reboot. Fixed in 7.1.5-1. A running kernel keeps the old code until the machine restarts. |
The vulnerability
What the bug is
When CONFIG_IP_MULTIPLE_TABLES is enabled but no rule is added, fib_lookup() performs route lookup directly on two tables. Since the first lookup does not properly bail out, the result of an error route in the merged local/main table could be overwritten by another route in the default table: # unshare -n # ip link set lo up # ip route add 192.168.0.0/24 dev lo table 253 # ip route add unreachable 192.168.0.0/24 # ip route get 192.168.0.1 192.168.0.1 dev lo table default uid 0 cache <local> Once a random rule is added, the error route is respected: # ip rule add table 0 # ip rule del table 0 # ip route get 192.168.0.1 RTNETLINK answers: No route to host Let's fix the inconsistent behaviour.
Where it lives
The fix touches these files. If the subsystem is one you do not use, the practical exposure is lower, though the package is still the vulnerable one.
include/net/ip_fib.h
- What this is
- NETWORKING [IPv4/IPv6]MAINTAINERS section matched on include/net/ip* · netdev@vger.kernel.org · Maintained
Show whether Debian builds this, per release
Not established. This site could not find the build rule for the affected file in the kernel trees Debian ships, so it has no answer about whether Debian compiles this code. Read that as unknown, never as absent.
- bookworm, trixie, forky, sid?
- ?
- not established - no Makefile entry for the affected file
Where a release above reads "not established", no build rule for the affected file was found in that release's kernel tree. That is a gap in this lookup, not a finding that the code is absent.
From the configuration Debian publishes for every kernel flavour it builds. It covers only the architectures Debian ships a linux-config package for, and nothing about a kernel you built yourself. Debian's own status above stays authoritative about the source package.MAINTAINERS
Signals
- Published
- 2026-08-15 (2 months ago)source
- CVSS
- 10.0 critical
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:Nsource recompute it - Attack vector
- network (AV:N)source
- EPSS
- 0.57%, higher than 46.1% of all CVEssource
- CISA KEV
- not listedthe KEV catalogue
- Debian urgency
- not yet assignedsource
- Subsystem
- include/net
Upstream fixes
The versions the CNA records as introducing and fixing this, and the commit for each. source
| Introduced | Fixed in | Commit |
|---|---|---|
| 3.6 | 5.10.261 | a29e95fbc51e |
| 3.6 | 5.15.212 | 5ae18d87a456 |
| 3.6 | 6.1.178 | 9127589aabde |
| 3.6 | 6.6.145 | 49eaf1403201 |
| 3.6 | 6.12.97 | fd25996f57a9 |
| 3.6 | 6.18.40 | 828fad4fd418 |
| 3.6 | 7.1.5 | a668fa160247 |
| 3.6 | 7.2 | b72f0db64205 |
Why the CNA scored it this way
Quoted from the kernel CNA's own scoring file, word for word. A highlighted metric carries the most severe value CVSS v3.1 defines for it, which is what pushes the score up. source
- AV:NAttack vector, network
- fib_lookup() is on the IPv4 receive and transmit datapaths (ip_route_input_rcu, ip_route_output_key, forwarding) reached by remote packets on internet-facing routers, gateways, cloud hosts, and container nodes without any local syscall.
- AC:LAttack complexity, low
- Once main/local holds an error route and default has an overlapping prefix (reproduced in the fix commit with basic ip-route commands), any matching packet or socket lookup deterministically hits the bypass; no race or attacker-independent memory layout is required.
- PR:NPrivileges required, none
- Overlapping table layout is a deployment/CAP_NET_ADMIN prerequisite, but exploitation only requires sending traffic or using sockets; the remote attacker needs no credentials or privileges on the victim once the vulnerable routing state exists.
- UI:NUser interaction, none
- Route resolution runs automatically in the kernel networking stack during normal packet forwarding and connection setup; no victim mount, click, or file open is needed beyond ordinary network use.
- S:CScope, changed
- Bypassing unreachable/prohibit/blackhole routes can defeat network-namespace/pod egress blackholes and segmentation boundaries on multi-tenant hosts and routers, allowing access outside the intended isolated security scope.
- C:HConfidentiality impact, high
- Ignored error routes let traffic reach destinations deliberately isolated (internal networks, metadata endpoints, sinkholed ranges), exposing data that routing policy was designed to keep inaccessible.
- I:HIntegrity impact, high
- Packets that should be dropped or rejected are instead forwarded via the default-table route, enabling injection of traffic into networks marked unreachable, prohibited, or blackhole.
- A:NAvailability impact, none
- The defect is incorrect route selection only; it does not corrupt memory, panic the kernel, deadlock, or inherently exhaust resources, so availability impact is none.