# CVE-2026-31531: ipv4: nexthop: allocate skb dynamically in rtm_get_nexthop()

> A local attacker can trigger a kernel warning and potential denial of service by querying large nexthop groups via netlink.

- **Severity:** Medium 5.5 (`CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Local or same network only
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 23 Apr 2026
- **Updated:** 21 Oct 2026
- **Also known as:** AZL-83541, BELL-CVE-2026-31531, DEBIAN-CVE-2026-31531, GHSA-8q42-qfhf-592h, ROOT-OS-DEBIAN-11-CVE-2026-31531, ROOT-OS-DEBIAN-12-CVE-2026-31531, ROOT-OS-DEBIAN-13-CVE-2026-31531, ROOT-OS-UBUNTU-2204-CVE-2026-31531, ROOT-OS-UBUNTU-2404-CVE-2026-31531, UBUNTU-CVE-2026-31531

## What is CVE-2026-31531?

CVE-2026-31531 is a medium-severity denial of service vulnerability in Linux kernel, published on 23 Apr 2026. It can only be exploited from the same machine or local network, with no action from the victim.

### Official description

In the Linux kernel, the following vulnerability has been resolved:

ipv4: nexthop: allocate skb dynamically in rtm_get_nexthop()

When querying a nexthop object via RTM_GETNEXTHOP, the kernel currently
allocates a fixed-size skb using NLMSG_GOODSIZE. While sufficient for
single nexthops and small Equal-Cost Multi-Path groups, this fixed
allocation fails for large nexthop groups like 512 nexthops.

This results in the following warning splat:

 WARNING: net/ipv4/nexthop.c:3395 at rtm_get_nexthop+0x176/0x1c0, CPU#20: rep/4608
 [...]
 RIP: 0010:rtm_get_nexthop (net/ipv4/nexthop.c:3395)
 [...]
 Call Trace:
  <TASK>
  rtnetlink_rcv_msg (net/core/rtnetlink.c:6989)
  netlink_rcv_skb (net/netlink/af_netlink.c:2550)
  netlink_unicast (net/netlink/af_netlink.c:1319 net/netlink/af_netlink.c:1344)
  netlink_sendmsg (net/netlink/af_netlink.c:1894)
  ____sys_sendmsg (net/socket.c:721 net/socket.c:736 net/socket.c:2585)
  ___sys_sendmsg (net/socket.c:2641)
  __sys_sendmsg (net/socket.c:2671)
  do_syscall_64 (arch/x86/entry/syscall_64.c:63 arch/x86/entry/syscall_64.c:94)
  entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:130)
  </TASK>

Fix this by allocating the size dynamically using nh_nlmsg_size() and
using nlmsg_new(), this is consistent with nexthop_notify() behavior. In
addition, adjust nh_nlmsg_size_grp() so it calculates the size needed
based on flags passed. While at it, also add the size of NHA_FDB for
nexthop group size calculation as it was missing too.

This cannot be reproduced via iproute2 as the group size is currently
limited and the command fails as follows:

addattr_l ERROR: message exceeded bound of 1048

## Which versions are affected by CVE-2026-31531?

It affects 1 product.


Products: Linux kernel.

## How do I fix CVE-2026-31531?

Apply the vendor's patch.

## Is CVE-2026-31531 being exploited?

There is no known exploitation of CVE-2026-31531 in the wild and no public exploit code.

## Other vulnerabilities in Linux kernel

- [CVE-2026-53266](https://insights.sbomify.com/CVE-2026-53266/): Linux Kernel Out-of-Bounds Write (high 8.8, exploited in the wild)
- [CVE-2013-6282](https://insights.sbomify.com/CVE-2013-6282/): Linux Kernel Improper Input Validation (high 8.8, exploited in the wild)
- [CVE-2022-0185](https://insights.sbomify.com/CVE-2022-0185/): Linux Kernel Heap-Based Buffer Overflow (high 8.4, exploited in the wild)
- [CVE-2013-2094](https://insights.sbomify.com/CVE-2013-2094/): Linux Kernel Privilege Escalation (high 8.4, exploited in the wild)
- [CVE-2026-53362](https://insights.sbomify.com/CVE-2026-53362/): Linux Kernel Unspecified (high 7.8, exploited in the wild)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-31531 and fix it if it is affected.

What it is: CVE-2026-31531 is a medium-severity denial of service vulnerability in Linux kernel, published on 23 Apr 2026. It can only be exploited from the same machine or local network, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-31531 in the wild and no public exploit code.

Affected product: Linux kernel. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-31531/ (Markdown: https://insights.sbomify.com/CVE-2026-31531/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-31531
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-31531
- OSV: https://osv.dev/vulnerability/CVE-2026-31531

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-31531/
Summaries are generated and may contain mistakes; the official description above is the record.