# CVE-2026-63896: usb: gadget: composite: fix integer underflow in WebUSB GET_URL handling

> A malicious USB host can trigger an integer underflow in the Linux kernel WebUSB handler, causing a large out-of-bounds memory copy and potential system crash or information leak.

- **Severity:** Not scored
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Local or same network only
- **Victim action needed:** A user has to click or open something
- **Fix available:** Yes
- **Published:** 19 Jul 2026
- **Updated:** 19 Jul 2026
- **Also known as:** BELL-CVE-2026-63896, DEBIAN-CVE-2026-63896, GHSA-5hvq-fg66-prgg, UBUNTU-CVE-2026-63896

## What is CVE-2026-63896?

CVE-2026-63896 is a vulnerability in Linux, published on 19 Jul 2026. It can only be exploited from the same machine or local network, and only if a user opens a file or follows a link.

### Official description

In the Linux kernel, the following vulnerability has been resolved:

usb: gadget: composite: fix integer underflow in WebUSB GET_URL handling

The WebUSB GET_URL handler in composite_setup() narrows
landing_page_length to fit the host-supplied wLength using

	landing_page_length = w_length
		- WEBUSB_URL_DESCRIPTOR_HEADER_LENGTH + landing_page_offset;

If wLength is smaller than WEBUSB_URL_DESCRIPTOR_HEADER_LENGTH the
unsigned subtraction wraps, and the subsequent

	memcpy(url_descriptor->URL,
	       cdev->landing_page + landing_page_offset,
	       landing_page_length - landing_page_offset);

ends up copying close to UINT_MAX bytes from cdev->landing_page into
cdev->req->buf.  KASAN reports a slab-out-of-bounds in composite_setup
on the kmalloc-2k gadget_info allocation, and FORTIFY_SOURCE traps the
memcpy as a 4294967293-byte field-spanning write into
url_descriptor->URL (size 252).

A USB host can reach this from a single SETUP packet against any
gadget that has webusb/use=1 and a landingPage configured.

Handle the small-wLength case before the math: when the host requested
fewer bytes than the URL descriptor header, only the header is
meaningful and no URL bytes need to be copied.  Setting
landing_page_length to landing_page_offset makes the existing memcpy a
no-op and leaves the descriptor returned to the host unchanged for all
larger wLength values.

## Which versions are affected by CVE-2026-63896?


## How do I fix CVE-2026-63896?

Apply the vendor's patch.

## Is CVE-2026-63896 being exploited?

There is no known exploitation of CVE-2026-63896 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-63896 and fix it if it is affected.

What it is: CVE-2026-63896 is a vulnerability in Linux, published on 19 Jul 2026. It can only be exploited from the same machine or local network, and only if a user opens a file or follows a link.
Exploitation: There is no known exploitation of CVE-2026-63896 in the wild and no public exploit code.

Affected product: Linux. 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-2026-63896/ (Markdown: https://insights.sbomify.com/CVE-2026-63896/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-63896
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-63896
- OSV: https://osv.dev/vulnerability/CVE-2026-63896

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-63896/
Summaries are generated and may contain mistakes; the official description above is the record.