CVE-2026-97984 net: ipv6: Fix UDP length overflow with PMTU discover and big MTU
A local attacker with network privileges can trigger a UDP length integer overflow in the Linux kernel IPv6 stack, causing a warning and potential denial of service.
- Exploited in the wild
- Not known
- Login needed
- Any signed-in user
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-97984?
CVE-2026-97984 is a vulnerability in Linux, published on 25 Sep 2026. Any signed-in user can exploit it over the network, with no action from the victim.
Official description
In the Linux kernel, the following vulnerability has been resolved:
net: ipv6: Fix UDP length overflow with PMTU discover and big MTU
This commit bounds cork->base.fragsize to IP6_MAX_MTU for UDP sockets to avoid a possible overflow of UDP length that triggers a WARN in udp_set_len_short when setsockopt IPV6_MTU_DISCOVER is set to IPV6_PMTUDISC_DO or IPV6_PMTUDISC_PROBE, and a large packet is sent over a netdev with an unusually large MTU.
Steps to reproduce (included in the new selftest):
1. Set device MTU bigger than IP6_MAX_MTU. cork->base.fragsize will be set to that MTU in ip6_setup_cork. 2. Set IPV6_MTU_DISCOVER to IPV6_PMTUDISC_PROBE or IPV6_PMTUDISC_DO. It lets maxnonfragsize be set to device MTU (cork->fragsize) in __ip6_append_data, rather than to IP6_MAX_MTU. 3. Send 65528 bytes of payload (+8 bytes of UDP header, +40 bytes of IPv6 header). Device MTU allows it (it's only one byte bigger than IP6_MAX_MTU, 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.
To avoid breaking sending UDP jumbograms over raw IPv6 sockets, limit the change to UDP sockets only.
The original overflow bug with IPv6 and IPV6_PMTUDISC_DO seems to predate git history (verified reproduction on 2.6.21), was fixed later, and then reappeared in commit 427faee167bc ("net: ipv6: introduce ip6_dst_mtu_maybe_forward"), which is chosen as the Fixes tag here. The overflow with IPV6_PMTUDISC_PROBE reproduces since its introduction in commit 628a5c561890 ("[INET]: Add IP(V6)_PMTUDISC_RPOBE").
Which versions are affected by CVE-2026-97984?
How do I fix CVE-2026-97984?
Apply the vendor's patch.
Is CVE-2026-97984 being exploited?
There is no known exploitation of CVE-2026-97984 in the wild and no public exploit code.
Sources
Also known as AZL-104355, BELL-CVE-2026-97984, DEBIAN-CVE-2026-97984, GHSA-5m3w-7wwr-wqm2, UBUNTU-CVE-2026-97984.