CVE-2026-72421 ipv4: fib: Don't ignore error route in local/main tables.
A logic error in Linux kernel IPv4 routing allows bypassing unreachable route restrictions when multiple tables are enabled but no rules are configured.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-72421?
CVE-2026-72421 is a critical vulnerability in Linux, published on 15 Aug 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official description
In the Linux kernel, the following vulnerability has been resolved:
ipv4: fib: Don't ignore error route in local/main tables.
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.
Which versions are affected by CVE-2026-72421?
How do I fix CVE-2026-72421?
Apply the vendor's patch.
Is CVE-2026-72421 being exploited?
There is no known exploitation of CVE-2026-72421 in the wild and no public exploit code.
Sources
Also known as BELL-CVE-2026-72421, DEBIAN-CVE-2026-72421, GHSA-66j4-c66h-jm7x, UBUNTU-CVE-2026-72421.