CVE-2026-81009 io_uring/query: cap user size passed to copy_struct_to_user
A local attacker can cause a denial of service by requesting excessive memory zeroing in the Linux kernel io_uring query interface.
- 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
What is CVE-2026-81009?
CVE-2026-81009 is a denial of service vulnerability in Linux, published on 11 Sep 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:
io_uring/query: cap user size passed to copy_struct_to_user
io_handle_query_entry() clamps hdr.size for the inbound copy_from_user() but keeps the original user value as usize. copy_struct_to_user() uses that usize and, when it is larger than the kernel result, clear_user()s the trailing bytes.
As hdr.size is a __u32, a query can request nearly 4 GiB of zeroing, including on the error path where res_size stays 0. The interface is reachable without a ring via IORING_REGISTER_QUERY.
Reject sizes larger than PAGE_SIZE, as recommended for copy_struct_* interfaces.
Which versions are affected by CVE-2026-81009?
How do I fix CVE-2026-81009?
Apply the vendor's patch.
Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.
Is CVE-2026-81009 being exploited?
There is no known exploitation of CVE-2026-81009 in the wild and no public exploit code.
Sources
Also known as BELL-CVE-2026-81009, DEBIAN-CVE-2026-81009, GHSA-w44c-cv6c-x94c, UBUNTU-CVE-2026-81009.