# CVE-2026-97991: vdpa_sim_blk: reject out-of-range sector starts

> A guest VM can trigger out-of-bounds reads and writes in the Linux kernel's vdpa_sim_blk driver, potentially leading to code execution or data exposure.

- **Severity:** High 7.8 (`CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:C/C:H/I:H/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:** 25 Sep 2026
- **Updated:** 19 Oct 2026
- **Also known as:** AZL-104381, BELL-CVE-2026-97991, DEBIAN-CVE-2026-97991, GHSA-hprw-93rc-4v5g, UBUNTU-CVE-2026-97991

## What is CVE-2026-97991?

CVE-2026-97991 is a high-severity memory corruption vulnerability in Linux, published on 25 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:

vdpa_sim_blk: reject out-of-range sector starts

vdpasim_blk_check_range() logs an invalid start sector but continues
validating the request.  The subsequent unsigned capacity subtraction can
underflow and let an out-of-range buffer offset reach the data path.

The invalid offset is used by three request paths.  VIRTIO_BLK_T_OUT
copies guest data to blk->buffer + offset through
vringh_iov_pull_iotlb(), causing an out-of-bounds write in
_copy_from_iter() or memcpy().  VIRTIO_BLK_T_IN copies from
blk->buffer + offset to the guest through vringh_iov_push_iotlb(),
causing an out-of-bounds read in _copy_to_iter().
VIRTIO_BLK_T_WRITE_ZEROES passes blk->buffer + offset to memset(),
causing an out-of-bounds write.

Reject starts at or beyond the capacity before the subtraction.  Treat the
capacity boundary as invalid because the IN and OUT paths round byte counts
down to sectors for validation but later copy the original byte counts.  A
sub-sector request at the capacity boundary would otherwise still access
past the end of the buffer.

I found this bug myself, though the patch was written with AI assistance.

## Which versions are affected by CVE-2026-97991?


## How do I fix CVE-2026-97991?

Apply the vendor's patch.

## Is CVE-2026-97991 being exploited?

There is no known exploitation of CVE-2026-97991 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-97991 and fix it if it is affected.

What it is: CVE-2026-97991 is a high-severity memory corruption vulnerability in Linux, published on 25 Sep 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-2026-97991 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-2026-97991/ (Markdown: https://insights.sbomify.com/CVE-2026-97991/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-97991
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-97991
- OSV: https://osv.dev/vulnerability/CVE-2026-97991

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-97991/
Summaries are generated and may contain mistakes; the official description above is the record.