# CVE-2026-64527: drm/hyperv: validate VMBus packet size in receive callback

> A missing size validation in the Hyper-V DRM driver allows reading stale memory or causing a buffer overflow, potentially leading to information disclosure or system crashes.

- **Severity:** Not scored
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Local or same network only
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 25 Jul 2026
- **Updated:** 25 Jul 2026
- **Also known as:** BELL-CVE-2026-64527, DEBIAN-CVE-2026-64527, GHSA-86px-9vhr-gg2j, ROOT-OS-UBUNTU-2204-CVE-2026-64527, UBUNTU-CVE-2026-64527

## What is CVE-2026-64527?

CVE-2026-64527 is a memory corruption vulnerability in Linux, published on 25 Jul 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:

drm/hyperv: validate VMBus packet size in receive callback

hyperv_receive_sub() reads msg->vid_hdr.type and dispatches into one
of four message-type branches without knowing how many bytes the host
wrote into hv->recv_buf. The completion path then runs
memcpy(hv->init_buf, msg, VMBUS_MAX_PACKET_SIZE), so the consumer that
wakes on wait_for_completion_timeout() can read up to 16 KiB of
residue from a prior message as if it were the response payload.

Pass bytes_recvd into hyperv_receive_sub() and reject any packet that
does not cover the pipe + synthvid header. A single switch on
msg->vid_hdr.type then computes the type-specific payload size: the
three completion-driving types (SYNTHVID_VERSION_RESPONSE,
SYNTHVID_RESOLUTION_RESPONSE, SYNTHVID_VRAM_LOCATION_ACK) fall through
to a shared exit that requires that size before memcpy/complete, while
SYNTHVID_FEATURE_CHANGE validates its own payload and returns before
reading is_dirt_needed. Unknown types are dropped.

SYNTHVID_RESOLUTION_RESPONSE is variable length: the host fills
resolution_count entries, not the full SYNTHVID_MAX_RESOLUTION_COUNT
array. Validate the fixed prefix first so resolution_count can be
read, bound it against the array, then require only the count-sized
array, so the shorter responses the host actually sends are accepted.

Only run the sub-handler when vmbus_recvpacket() returned success. The
memcpy length is bytes_recvd, which is bounded by VMBUS_MAX_PACKET_SIZE
only on a successful receive; on -ENOBUFS vmbus_recvpacket() instead
reports the required length, which can exceed hv->recv_buf, so copying
bytes_recvd would read and write past the 16 KiB buffers. Gating on the
success return keeps the copy bounded. The nonzero-return path is itself
a malformed-message case and is now logged rather than silently skipped;
channel recovery is not attempted.

Rejected packets are reported via drm_err_ratelimited() rather than
silently dropped, matching the CoCo-hardened pattern in
hv_kvp_onchannelcallback().

## Which versions are affected by CVE-2026-64527?


## How do I fix CVE-2026-64527?

Apply the vendor's patch.

## Is CVE-2026-64527 being exploited?

There is no known exploitation of CVE-2026-64527 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-64527 and fix it if it is affected.

What it is: CVE-2026-64527 is a memory corruption vulnerability in Linux, published on 25 Jul 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-2026-64527 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-2026-64527/ (Markdown: https://insights.sbomify.com/CVE-2026-64527/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-64527
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-64527
- OSV: https://osv.dev/vulnerability/CVE-2026-64527

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-64527/
Summaries are generated and may contain mistakes; the official description above is the record.