CVE-2026-80793 ipv4: reject undersized MTUs in ip_do_fragment()

Q: Is CVE-2026-80793 being exploited?

There is no known exploitation of CVE-2026-80793 in the wild and no public exploit code.

Not scored Published 4 Sep 2026 Updated 19 Oct 2026

A remote attacker can cause a kernel softlockup and denial of service by sending packets that trigger infinite fragmentation loops due to undersized MTUs.

Exploited in the wild: Not known
Login needed: Not needed
Attackable: Over the internet
Victim action needed: Not needed
Fix: Available

What is CVE-2026-80793?

CVE-2026-80793 is a denial of service vulnerability in Linux, published on 4 Sep 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: reject undersized MTUs in ip_do_fragment()

ip_do_fragment() subtracts the IPv4 header length from the effective MTU and passes the resulting payload MTU to ip_frag_next().

If the effective MTU is smaller than hlen + 8, ip_frag_next() rounds the fragment payload length down to zero. The fragmentation state then never makes forward progress: state->left, state->ptr and state->offset stay unchanged while ip_do_fragment() keeps allocating and transmitting header-only fragments until the softlockup detector fires.

This is reproducible with a route installed using "mtu lock 20", but it is also reproducible without route MTU lock, for example by forwarding a packet to a device whose MTU is 20.

Fix it in ip_do_fragment() by rejecting mtu < hlen + 8 with -EMSGSIZE, matching the existing IPv6 fragmentation check.

Which versions are affected by CVE-2026-80793?

How do I fix CVE-2026-80793?

Apply the vendor's patch.

Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.

Is CVE-2026-80793 being exploited?