# CVE-2026-72421: ipv4: fib: Don't ignore error route in local/main tables.

> A logic error in Linux kernel IPv4 routing allows bypassing unreachable route restrictions when multiple tables are enabled but no rules are configured.

- **Severity:** Critical 10.0 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 15 Aug 2026
- **Updated:** 16 Oct 2026
- **Also known as:** BELL-CVE-2026-72421, DEBIAN-CVE-2026-72421, GHSA-66j4-c66h-jm7x, UBUNTU-CVE-2026-72421

## What is CVE-2026-72421?

CVE-2026-72421 is a critical vulnerability in Linux, published on 15 Aug 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:

ipv4: fib: Don't ignore error route in local/main tables.

When CONFIG_IP_MULTIPLE_TABLES is enabled but no rule is added,
fib_lookup() performs route lookup directly on two tables.

Since the first lookup does not properly bail out, the result
of an error route in the merged local/main table could be
overwritten by another route in the default table:

  # unshare -n
  # ip link set lo up
  # ip route add 192.168.0.0/24 dev lo table 253
  # ip route add unreachable 192.168.0.0/24
  # ip route get 192.168.0.1
  192.168.0.1 dev lo table default uid 0
      cache <local>

Once a random rule is added, the error route is respected:

  # ip rule add table 0
  # ip rule del table 0
  # ip route get 192.168.0.1
  RTNETLINK answers: No route to host

Let's fix the inconsistent behaviour.

## Which versions are affected by CVE-2026-72421?


## How do I fix CVE-2026-72421?

Apply the vendor's patch.

## Is CVE-2026-72421 being exploited?

There is no known exploitation of CVE-2026-72421 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-72421 and fix it if it is affected.

What it is: CVE-2026-72421 is a critical vulnerability in Linux, published on 15 Aug 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-2026-72421 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-2026-72421/ (Markdown: https://insights.sbomify.com/CVE-2026-72421/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-72421
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-72421
- OSV: https://osv.dev/vulnerability/CVE-2026-72421

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-72421/
Summaries are generated and may contain mistakes; the official description above is the record.