# CVE-2026-90251: Bluetooth: MSFT: validate evt_prefix_len against the response length

> A malicious Bluetooth controller can cause the Linux kernel to read uninitialized memory, potentially disclosing sensitive kernel data to the attacker.

- **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:** 17 Sep 2026
- **Updated:** 19 Oct 2026
- **Also known as:** AZL-102443, BELL-CVE-2026-90251, DEBIAN-CVE-2026-90251, GHSA-hmjm-jgv8-x76g, UBUNTU-CVE-2026-90251

## What is CVE-2026-90251?

CVE-2026-90251 is a vulnerability in Linux, published on 17 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:

Bluetooth: MSFT: validate evt_prefix_len against the response length

read_supported_features() only checks that the response covers the fixed
part of struct msft_rp_read_supported_features, which is 11 bytes:

	if (skb->len < sizeof(*rp)) {
		bt_dev_err(hdev, "MSFT supported features length mismatch");
		goto failed;
	}

evt_prefix[] is a flexible array member and rp->evt_prefix_len is an
unvalidated u8 taken straight out of that response, so

	msft->evt_prefix = kmemdup(rp->evt_prefix, rp->evt_prefix_len,
				   GFP_KERNEL);

copies up to 255 bytes from a reply that may have carried none of them.
What is copied is data the controller never sent, and it is then used to
match incoming vendor events in msft_vendor_evt().

This is not an out-of-bounds access. An skb data allocation always has
at least SKB_DATA_ALIGN(sizeof(struct skb_shared_info)) bytes past the
payload, which is more than the 255 byte maximum, so the read stays
inside the allocation and KASAN does not report it. It is still a read
of bytes the host was never given, with the length fully controlled by
the controller.

Reject a response that is too short for the prefix it declares.

Verified with an emulated controller over /dev/vhci on a KASAN kernel,
with vhci made to advertise an MSFT opcode the way btintel, btqca, btmtk
and btrtl do unconditionally. A reply of exactly 11 bytes declaring
evt_prefix_len = 255 reaches kmemdup and copies 255 bytes
("skb->len=11 evt_prefix_len=255", with the copied buffer dumped); since
the reply ends at the fixed part, all 255 come from past the end of the
response. No KASAN report is produced, as expected from the allocation
slack described above. With this patch the response is rejected with
"MSFT event prefix length mismatch" and msft->evt_prefix is left unset.

## Which versions are affected by CVE-2026-90251?


## How do I fix CVE-2026-90251?

Apply the vendor's patch.

## Is CVE-2026-90251 being exploited?

There is no known exploitation of CVE-2026-90251 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-90251 and fix it if it is affected.

What it is: CVE-2026-90251 is a vulnerability in Linux, published on 17 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-2026-90251 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-2026-90251/ (Markdown: https://insights.sbomify.com/CVE-2026-90251/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-90251
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-90251
- OSV: https://osv.dev/vulnerability/CVE-2026-90251

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-90251/
Summaries are generated and may contain mistakes; the official description above is the record.