# CVE-2026-74583: net/sched: cls_route: fix fastmap use-after-free on filter

> A race condition in the Linux kernel's route classifier allows a use-after-free, potentially enabling local attackers to execute code or escalate privileges.

- **Severity:** High 7.8 (`CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Local or same network only
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 21 Aug 2026
- **Updated:** 16 Oct 2026
- **Also known as:** AZL-96998, BELL-CVE-2026-74583, DEBIAN-CVE-2026-74583, GHSA-qm4p-6qr9-f3w6, UBUNTU-CVE-2026-74583

## What is CVE-2026-74583?

CVE-2026-74583 is a high-severity use after free vulnerability in Linux, published on 21 Aug 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: fix fastmap use-after-free on filter

The route4 classifier maintains a 16-slot fastmap cache that stores raw
struct route4_filter pointers indexed by (id, iif). The reader
(route4_classify) populates this cache via route4_set_fastmap() for every
classified packet that hits a filter. The writer (route4_delete,
route4_change) clears the cache via route4_reset_fastmap() before
RCU-deferred kfree of the filter.

This creates a UAF race:
 1. Reader walks the RCU-protected bucket chain, finds filter f
 2. Writer unlinks f, calls route4_reset_fastmap(), then tcf_queue_work()
 3. Reader calls route4_set_fastmap() and writes f into the cache
    *after* the writer's reset, caching a pointer about to be freed
 4. After the RCU grace period, kfree(f) executes
 5. Next classified packet on the same (id, iif) tuple hits the stale
    fastmap entry and reads f->res from freed memory

Reproduced with an mdelay(100) accelerator in route4_set_fastmap() and a
concurrent add/delete stress test (provided by both zdi and Santosh).
Both triggered KASAN slab-use-after-free reports in the route4 fastmap
paths.

Fix:
Introduce a per-filter boolean dying flag to suppress stale fastmap
republishing by in-flight readers.

## Which versions are affected by CVE-2026-74583?


## How do I fix CVE-2026-74583?

Apply the vendor's patch.

## Is CVE-2026-74583 being exploited?

There is no known exploitation of CVE-2026-74583 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-74583 and fix it if it is affected.

What it is: CVE-2026-74583 is a high-severity use after free vulnerability in Linux, published on 21 Aug 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-2026-74583 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-2026-74583/ (Markdown: https://insights.sbomify.com/CVE-2026-74583/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-74583
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-74583
- OSV: https://osv.dev/vulnerability/CVE-2026-74583

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-74583/
Summaries are generated and may contain mistakes; the official description above is the record.