# CVE-2025-21702: pfifo_tail_enqueue: Drop new packet when sch->limit == 0

> A logic error in the Linux kernel packet scheduler allows local attackers to escalate privileges by manipulating queue lengths under specific configurations.

- **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:** 18 Feb 2025
- **Updated:** 15 Oct 2026
- **Also known as:** BELL-CVE-2025-21702, DEBIAN-CVE-2025-21702, GHSA-c82f-pmfx-x3vv, UBUNTU-CVE-2025-21702

## What is CVE-2025-21702?

CVE-2025-21702 is a high-severity vulnerability in Linux kernel, published on 18 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:

pfifo_tail_enqueue: Drop new packet when sch->limit == 0

Expected behaviour:
In case we reach scheduler's limit, pfifo_tail_enqueue() will drop a
packet in scheduler's queue and decrease scheduler's qlen by one.
Then, pfifo_tail_enqueue() enqueue new packet and increase
scheduler's qlen by one. Finally, pfifo_tail_enqueue() return
`NET_XMIT_CN` status code.

Weird behaviour:
In case we set `sch->limit == 0` and trigger pfifo_tail_enqueue() on a
scheduler that has no packet, the 'drop a packet' step will do nothing.
This means the scheduler's qlen still has value equal 0.
Then, we continue to enqueue new packet and increase scheduler's qlen by
one. In summary, we can leverage pfifo_tail_enqueue() to increase qlen by
one and return `NET_XMIT_CN` status code.

The problem is:
Let's say we have two qdiscs: Qdisc_A and Qdisc_B.
 - Qdisc_A's type must have '->graft()' function to create parent/child relationship.
   Let's say Qdisc_A's type is `hfsc`. Enqueue packet to this qdisc will trigger `hfsc_enqueue`.
 - Qdisc_B's type is pfifo_head_drop. Enqueue packet to this qdisc will trigger `pfifo_tail_enqueue`.
 - Qdisc_B is configured to have `sch->limit == 0`.
 - Qdisc_A is configured to route the enqueued's packet to Qdisc_B.

Enqueue packet through Qdisc_A will lead to:
 - hfsc_enqueue(Qdisc_A) -> pfifo_tail_enqueue(Qdisc_B)
 - Qdisc_B->q.qlen += 1
 - pfifo_tail_enqueue() return `NET_XMIT_CN`
 - hfsc_enqueue() check for `NET_XMIT_SUCCESS` and see `NET_XMIT_CN` => hfsc_enqueue() don't increase qlen of Qdisc_A.

The whole process lead to a situation where Qdisc_A->q.qlen == 0 and Qdisc_B->q.qlen == 1.
Replace 'hfsc' with other type (for example: 'drr') still lead to the same problem.
This violate the design where parent's qlen should equal to the sum of its childrens'qlen.

Bug impact: This issue can be used for user->kernel privilege escalation when it is reachable.

## Which versions are affected by CVE-2025-21702?

It affects 1 product.


Products: Linux kernel.

## How do I fix CVE-2025-21702?

Apply the vendor's patch.

## Is CVE-2025-21702 being exploited?

There is no known exploitation of CVE-2025-21702 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-2025-21702 and fix it if it is affected.

What it is: CVE-2025-21702 is a high-severity vulnerability in Linux kernel, published on 18 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-2025-21702 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-2025-21702/ (Markdown: https://insights.sbomify.com/CVE-2025-21702/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2025-21702
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2025-21702
- OSV: https://osv.dev/vulnerability/CVE-2025-21702

---
From sbomify Insights: https://insights.sbomify.com/CVE-2025-21702/
Summaries are generated and may contain mistakes; the official description above is the record.