# CVE-2025-21653: net_sched: cls_flow: validate TCA_FLOW_RSHIFT attribute

> A missing validation in the Linux kernel's cls_flow scheduler allows a large shift value to cause an undefined behavior, potentially leading to a system crash.

- **Severity:** Medium 5.5 (`CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/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:** 19 Jan 2025
- **Updated:** 15 Oct 2026
- **Also known as:** BELL-CVE-2025-21653, DEBIAN-CVE-2025-21653, GHSA-478m-w5gm-f3cr, UBUNTU-CVE-2025-21653

## What is CVE-2025-21653?

CVE-2025-21653 is a medium-severity vulnerability in Linux kernel, published on 19 Jan 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: cls_flow: validate TCA_FLOW_RSHIFT attribute

syzbot found that TCA_FLOW_RSHIFT attribute was not validated.
Right shitfing a 32bit integer is undefined for large shift values.

UBSAN: shift-out-of-bounds in net/sched/cls_flow.c:329:23
shift exponent 9445 is too large for 32-bit type 'u32' (aka 'unsigned int')
CPU: 1 UID: 0 PID: 54 Comm: kworker/u8:3 Not tainted 6.13.0-rc3-syzkaller-00180-g4f619d518db9 #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 09/13/2024
Workqueue: ipv6_addrconf addrconf_dad_work
Call Trace:
 <TASK>
  __dump_stack lib/dump_stack.c:94 [inline]
  dump_stack_lvl+0x241/0x360 lib/dump_stack.c:120
  ubsan_epilogue lib/ubsan.c:231 [inline]
  __ubsan_handle_shift_out_of_bounds+0x3c8/0x420 lib/ubsan.c:468
  flow_classify+0x24d5/0x25b0 net/sched/cls_flow.c:329
  tc_classify include/net/tc_wrapper.h:197 [inline]
  __tcf_classify net/sched/cls_api.c:1771 [inline]
  tcf_classify+0x420/0x1160 net/sched/cls_api.c:1867
  sfb_classify net/sched/sch_sfb.c:260 [inline]
  sfb_enqueue+0x3ad/0x18b0 net/sched/sch_sfb.c:318
  dev_qdisc_enqueue+0x4b/0x290 net/core/dev.c:3793
  __dev_xmit_skb net/core/dev.c:3889 [inline]
  __dev_queue_xmit+0xf0e/0x3f50 net/core/dev.c:4400
  dev_queue_xmit include/linux/netdevice.h:3168 [inline]
  neigh_hh_output include/net/neighbour.h:523 [inline]
  neigh_output include/net/neighbour.h:537 [inline]
  ip_finish_output2+0xd41/0x1390 net/ipv4/ip_output.c:236
  iptunnel_xmit+0x55d/0x9b0 net/ipv4/ip_tunnel_core.c:82
  udp_tunnel_xmit_skb+0x262/0x3b0 net/ipv4/udp_tunnel_core.c:173
  geneve_xmit_skb drivers/net/geneve.c:916 [inline]
  geneve_xmit+0x21dc/0x2d00 drivers/net/geneve.c:1039
  __netdev_start_xmit include/linux/netdevice.h:5002 [inline]
  netdev_start_xmit include/linux/netdevice.h:5011 [inline]
  xmit_one net/core/dev.c:3590 [inline]
  dev_hard_start_xmit+0x27a/0x7d0 net/core/dev.c:3606
  __dev_queue_xmit+0x1b73/0x3f50 net/core/dev.c:4434

## Which versions are affected by CVE-2025-21653?

It affects 1 product.


Products: Linux kernel.

## How do I fix CVE-2025-21653?

Apply the vendor's patch.

## Is CVE-2025-21653 being exploited?

There is no known exploitation of CVE-2025-21653 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-2025-21653 and fix it if it is affected.

What it is: CVE-2025-21653 is a medium-severity vulnerability in Linux kernel, published on 19 Jan 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-2025-21653 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-2025-21653/ (Markdown: https://insights.sbomify.com/CVE-2025-21653/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2025-21653
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2025-21653
- OSV: https://osv.dev/vulnerability/CVE-2025-21653

---
From sbomify Insights: https://insights.sbomify.com/CVE-2025-21653/
Summaries are generated and may contain mistakes; the official description above is the record.