CVE-2026-98393 net: ipv4: Fix UDP length overflow with PMTU discover and big MTU
A local attacker can trigger a UDP length integer overflow in the Linux kernel by sending large packets with specific MTU settings, causing a warning and potential denial of service.
- Exploited in the wild
- Not known
- Login needed
- Any signed-in user
- Attackable
- Local or same network only
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-98393?
CVE-2026-98393 is a vulnerability in Linux, published on 14 Oct 2026. It can only be exploited from the same machine or local network, with no action from the victim.
Official description
In the Linux kernel, the following vulnerability has been resolved:
net: ipv4: Fix UDP length overflow with PMTU discover and big MTU
This commit bounds cork->base.fragsize to IP_MAX_MTU to avoid a possible overflow of UDP length that triggers a WARN in udp_set_len_short when setsockopt IP_MTU_DISCOVER is set to IP_PMTUDISC_PROBE, and a large packet is sent over a netdev with an unusually large MTU.
Steps to reproduce:
1. Set device MTU bigger than IP_MAX_MTU + 20. cork->base.fragsize will be set to that MTU in ip_setup_cork. 2. Set IP_MTU_DISCOVER to IP_PMTUDISC_PROBE. It lets maxnonfragsize be set to device MTU (cork->fragsize) in __ip_append_data, rather than to IP_MAX_MTU. 3. Send 65528 bytes of payload (+8 bytes of UDP header, +20 bytes of IPv4 header). Device MTU allows it (it's only one byte bigger than IP_MAX_MTU + IPv4 header, and the device MTU is bigger than that). 4. The UDP length in the built packet is 65536, which overflows the 16-bit length field and triggers the WARN in udp_set_len_short.
Note: IP_PMTUDISC_DO with IPv4 is safe, because ip_dst_mtu_maybe_forward always clamps at IP_MAX_MTU, unlike ip6_dst_mtu_maybe_forward.
The Fixes tag points at the first commit where I could reproduce the overflow with IPv4 and IP_PMTUDISC_PROBE.
Which versions are affected by CVE-2026-98393?
How do I fix CVE-2026-98393?
Apply the vendor's patch.
Is CVE-2026-98393 being exploited?
There is no known exploitation of CVE-2026-98393 in the wild and no public exploit code.
Sources
Also known as AZL-108105, BELL-CVE-2026-98393, DEBIAN-CVE-2026-98393, GHSA-m5wc-gf88-8h7m, UBUNTU-CVE-2026-98393.