# CVE-2026-89790: ipv6: avoid divide by zero in rt6_multipath_rebalance

> A race condition in the Linux kernel IPv6 routing code can cause a divide-by-zero error, leading to a system crash.

- **Severity:** Not scored
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 16 Sep 2026
- **Updated:** 19 Oct 2026
- **Also known as:** BELL-CVE-2026-89790, DEBIAN-CVE-2026-89790, GHSA-8437-c9wr-2522, UBUNTU-CVE-2026-89790

## What is CVE-2026-89790?

CVE-2026-89790 is a denial of service vulnerability in Linux, published on 16 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

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

ipv6: avoid divide by zero in rt6_multipath_rebalance

rt6_multipath_rebalance() calculates the total eligible nexthop weight
in one pass and programs upper bounds in a second pass. Since
RTM_NEWROUTE is RTNL-free, a concurrent
ignore_routes_with_linkdown update can make the first pass return zero
while the second sees an eligible nexthop, causing
rt6_upper_bound_set() to divide by zero.

UBSAN: division-overflow in net/ipv6/route.c:4845:17
Oops: divide error: 0000 [#1] SMP KASAN NOPTI
  rt6_upper_bound_set() net/ipv6/route.c:4845
  rt6_multipath_rebalance()
  fib6_add_rt2node()
  ip6_route_multipath_add()
  inet6_rtm_newroute()

Skip upper-bound calculation when the first pass reports a zero total.
This respects the lock-free performance considerations here and solves
insecure scenarios.

## Which versions are affected by CVE-2026-89790?


## How do I fix CVE-2026-89790?

Apply the vendor's patch.

## Is CVE-2026-89790 being exploited?

There is no known exploitation of CVE-2026-89790 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-89790 and fix it if it is affected.

What it is: CVE-2026-89790 is a denial of service vulnerability in Linux, published on 16 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-89790 in the wild and no public exploit code.

Affected product: Linux. 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-2026-89790/ (Markdown: https://insights.sbomify.com/CVE-2026-89790/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-89790
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-89790
- OSV: https://osv.dev/vulnerability/CVE-2026-89790

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-89790/
Summaries are generated and may contain mistakes; the official description above is the record.