CVE-2026-97984
UnknownSummary
The Linux kernel has a UDP length overflow when sending large packets over IPv6 UDP sockets with IPV6_MTU_DISCOVER set to IPV6_PMTUDISC_DO or IPV6_PMTUDISC_PROBE and a network device with an unusually large MTU. The cork->base.fragsize value is not bounded to IP6_MAX_MTU, causing the 16-bit UDP length field to overflow and triggering a WARN in udp_set_len_short. The issue affects UDP sockets only and was fixed by limiting fragsize to IP6_MAX_MTU.
Risk Assessment
This vulnerability can trigger a kernel WARN and potentially destabilize the system when sending large UDP packets over IPv6 on interfaces with very large MTU. In production environments, it may cause network service disruptions or unpredictable network stack behavior.
Recommendation
Update the Linux kernel to a version containing the fix that bounds cork->base.fragsize to IP6_MAX_MTU for UDP sockets. Until the update is applied, avoid configuring interface MTU larger than IP6_MAX_MTU and avoid IPV6_PMTUDISC_DO/PROBE settings on affected systems.
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Original NVD description (English source)
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").
Vulnerability data from NVD (NIST) · CISA KEV · EPSS

