# CVE-2026-31721: usb: gadget: f_hid: move list and spinlock inits from bind to alloc

> A race condition in the Linux kernel USB HID gadget driver can cause list corruption and system crashes when unbinding and rebinding the UDC while file descriptors are active.

- **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:** 1 May 2026
- **Updated:** 11 May 2026
- **Also known as:** BELL-CVE-2026-31721, DEBIAN-CVE-2026-31721, GHSA-8r63-5629-3mpc, ROOT-OS-DEBIAN-11-CVE-2026-31721, ROOT-OS-UBUNTU-2204-CVE-2026-31721, ROOT-OS-UBUNTU-2404-CVE-2026-31721, UBUNTU-CVE-2026-31721

## What is CVE-2026-31721?

CVE-2026-31721 is a medium-severity vulnerability in Linux kernel, published on 1 May 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:

usb: gadget: f_hid: move list and spinlock inits from bind to alloc

There was an issue when you did the following:
- setup and bind an hid gadget
- open /dev/hidg0
- use the resulting fd in EPOLL_CTL_ADD
- unbind the UDC
- bind the UDC
- use the fd in EPOLL_CTL_DEL

When CONFIG_DEBUG_LIST was enabled, a list_del corruption was reported
within remove_wait_queue (via ep_remove_wait_queue). After some
debugging I found out that the queues, which f_hid registers via
poll_wait were the problem. These were initialized using
init_waitqueue_head inside hidg_bind. So effectively, the bind function
re-initialized the queues while there were still items in them.

The solution is to move the initialization from hidg_bind to hidg_alloc
to extend their lifetimes to the lifetime of the function instance.

Additionally, I found many other possibly problematic init calls in the
bind function, which I moved as well.

## Which versions are affected by CVE-2026-31721?

It affects 1 product.


Products: Linux kernel.

## How do I fix CVE-2026-31721?

Apply the vendor's patch.

## Is CVE-2026-31721 being exploited?

There is no known exploitation of CVE-2026-31721 in the wild and no public exploit code.

Public exploit code:

- Backports of three published f_hid fixes (incl. CVE-2026-31721, CVE-2026-31606) to an EOL Linux 4.14.190 Android vendor kernel, with on-device verification records. (github_poc): https://github.com/JakeStone594/f_hid-4.14-backports

## 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-2026-31721 and fix it if it is affected.

What it is: CVE-2026-31721 is a medium-severity vulnerability in Linux kernel, published on 1 May 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-2026-31721 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-2026-31721/ (Markdown: https://insights.sbomify.com/CVE-2026-31721/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-31721
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-31721
- OSV: https://osv.dev/vulnerability/CVE-2026-31721

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-31721/
Summaries are generated and may contain mistakes; the official description above is the record.