CVE-2026-72421: Undefined Security Weakness

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.