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.
- 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
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.
Sources
Also known as BELL-CVE-2026-63896, DEBIAN-CVE-2026-63896, GHSA-5hvq-fg66-prgg, UBUNTU-CVE-2026-63896.