# CVE-2026-97959: net/sched: cls_route: free emptied bucket on filter move

> A memory leak in the Linux kernel's route classifier allows local attackers to exhaust kernel memory, causing denial of service.

- **Severity:** Not scored
- **Exploited in the wild:** Not known
- **Login needed:** An administrator
- **Attackable:** Local or same network only
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 25 Sep 2026
- **Updated:** 19 Oct 2026
- **Also known as:** AZL-104640, BELL-CVE-2026-97959, DEBIAN-CVE-2026-97959, GHSA-q5gc-9c5f-rc6r, UBUNTU-CVE-2026-97959

## What is CVE-2026-97959?

CVE-2026-97959 is a memory corruption vulnerability in Linux, published on 25 Sep 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:

net/sched: cls_route: free emptied bucket on filter move

route4_change can move an existing filter to a different top-level
bucket: route4_set_parms recomputes the handle from TCA_ROUTE4_TO/
FROM/IIF, and the handle-mismatch check is gated on the 'new' flag, so
for an existing filter the new handle may differ from the old one and
land in a different bucket. When this happens, the filter is unlinked
from the old bucket, but the bucket itself is never freed once it goes
empty. The stale empty bucket remains in head->table[], causing
route4_delete to report *last=false even after the last live filter is
gone. That pins the empty tcf_proto and causes a leak.

Fix this by refcounting the filters linked to a bucket and freeing the
bucket when the count drops to zero. The existing scan in route4_delete
goes away with it.

The count is updated at all sites that link or unlink a filter during add,
change and delete, and the bucket is dropped from head->table[] as soon as
it reaches zero.

Conditions to recreate the bug:
  CONFIG_NET_CLS_ROUTE4=y, CONFIG_NET_SCH_INGRESS=y, CONFIG_NET_CLS_ACT=y.

  tc qdisc replace dev lo clsact
  tc filter add dev lo ingress protocol ip pref 100 route from 1 to 1
  tc filter change dev lo ingress protocol ip pref 100 handle 0x10001 \
    route from 1 to 2
  tc filter del dev lo ingress protocol ip pref 100 handle 0x10002 \
    route from 1 to 2
  tc filter show dev lo ingress | grep -c 'pref 100 route chain 0 '

## Which versions are affected by CVE-2026-97959?


## How do I fix CVE-2026-97959?

Apply the vendor's patch.

## Is CVE-2026-97959 being exploited?

There is no known exploitation of CVE-2026-97959 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-97959 and fix it if it is affected.

What it is: CVE-2026-97959 is a memory corruption vulnerability in Linux, published on 25 Sep 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-2026-97959 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-2026-97959/ (Markdown: https://insights.sbomify.com/CVE-2026-97959/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-97959
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-97959
- OSV: https://osv.dev/vulnerability/CVE-2026-97959

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-97959/
Summaries are generated and may contain mistakes; the official description above is the record.