---
id: "CVE-2026-99121"
kind: "vulnerability"
aliases: ["CVE-2026-99121"]
title: "Linux kernel: IPv4 route rebalance can divide by zero and crash"
severity: "unknown"
cvss: null
epss: null
exploited: false
status: "open"
category: "infrastructure"
products: []
ecosystems: []
tags: ["infrastructure", "ipv4", "kernel", "linux", "multipath-routing", "routers"]
not_affected_if: ["single-path-routing"]
published_at: "2026-10-28T11:17:31+00:00"
first_seen_at: "2026-10-28T11:32:34+00:00"
updated_at: "2026-10-28T11:37:34+00:00"
sources: [{"source": "nvd", "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-99121", "first_seen_at": "2026-10-28T11:32:34+00:00"}]
---

# Linux kernel: IPv4 route rebalance can divide by zero and crash

A race between a route-rebalance pass and a change to the linkdown route setting can make the Linux kernel divide by zero while handling multipath IPv4 routes. The sources describe a kernel crash (denial of service), not code execution.

_Summary, check and fix written by a model from the sources; labelled as such._

## How to check

- `uname -r` and compare against the fixed kernel builds listed by your distribution's advisory for CVE-2026-99121
- unknown whether the sources list affected kernel version ranges

## Fix

- Install the kernel build from your distribution or vendor that includes the upstream fix (IPv4 fib_rebalance divide-by-zero guard, mirroring the IPv6 commit d2c26c2911dd)
- Reboot into the patched kernel
- unknown which exact kernel versions are fixed, as the sources do not say

## Timeline

- 2026-10-28T11:32:34+00:00 nvd https://nvd.nist.gov/vuln/detail/CVE-2026-99121

## From the sources

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

ipv4: avoid divide by zero in fib_rebalance

fib_rebalance() computes the total eligible nexthop weight in one pass
and programs upper bounds in a second pass. A concurrent change to
ignore_routes_with_linkdown can make the first pass return zero while
the second pass sees an eligible nexthop, resulting in division by zero.

If the first pass reports a zero total, set each nexthop upper bound to
-1 and skip the division. This matches the IPv6 fix in commit
d2c26c2911dd ("ipv6: avoid divide by zero in rt6_multipath_rebalance")
and preserves the lock-free rebalance path.

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_Free to read at a human pace. An API with keys and an MCP server are in preparation; early access: hello@infrastack.ai_