# CVE-2026-31617: usb: gadget: f_ncm: validate minimum block_len in ncm_unwrap_ntb()

> A malicious USB host can trigger an integer underflow in the Linux kernel NCM gadget driver, leading to out-of-bounds memory reads and potential code execution.

- **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:** 24 Apr 2026
- **Updated:** 15 Oct 2026
- **Also known as:** BELL-CVE-2026-31617, DEBIAN-CVE-2026-31617, GHSA-v24g-wpq8-cwqx, ROOT-OS-DEBIAN-11-CVE-2026-31617, ROOT-OS-DEBIAN-12-CVE-2026-31617, ROOT-OS-DEBIAN-13-CVE-2026-31617, ROOT-OS-UBUNTU-2204-CVE-2026-31617, ROOT-OS-UBUNTU-2404-CVE-2026-31617, UBUNTU-CVE-2026-31617

## What is CVE-2026-31617?

CVE-2026-31617 is a medium-severity vulnerability in Linux kernel, published on 24 Apr 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_ncm: validate minimum block_len in ncm_unwrap_ntb()

The block_len read from the host-supplied NTB header is checked against
ntb_max but has no lower bound. When block_len is smaller than
opts->ndp_size, the bounds check of:
	ndp_index > (block_len - opts->ndp_size)
will underflow producing a huge unsigned value that ndp_index can never
exceed, defeating the check entirely.

The same underflow occurs in the datagram index checks against block_len
- opts->dpe_size.  With those checks neutered, a malicious USB host can
choose ndp_index and datagram offsets that point past the actual
transfer, and the skb_put_data() copies adjacent kernel memory into the
network skb.

Fix this by rejecting block lengths that cannot hold at least the NTB
header plus one NDP.  This will make block_len - opts->ndp_size and
block_len - opts->dpe_size both well-defined.

Commit 8d2b1a1ec9f5 ("CDC-NCM: avoid overflow in sanity checking") fixed
a related class of issues on the host side of NCM.

Weakness: CWE-191 (Integer Underflow (Wrap or Wraparound))

## Which versions are affected by CVE-2026-31617?

It affects 1 product.


Products: Linux kernel.

## How do I fix CVE-2026-31617?

Apply the vendor's patch.

## Is CVE-2026-31617 being exploited?

There is no known exploitation of CVE-2026-31617 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-2026-31617 and fix it if it is affected.

What it is: CVE-2026-31617 is a medium-severity vulnerability in Linux kernel, published on 24 Apr 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-2026-31617 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-2026-31617/ (Markdown: https://insights.sbomify.com/CVE-2026-31617/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-31617
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-31617
- OSV: https://osv.dev/vulnerability/CVE-2026-31617

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-31617/
Summaries are generated and may contain mistakes; the official description above is the record.