# CVE-2026-64539: Bluetooth: eir: Fix stack OOB write when prepending the Flags AD

> A local attacker with CAP_NET_ADMIN can trigger a stack out-of-bounds write in the Linux kernel Bluetooth subsystem, potentially leading to code execution or privilege escalation.

- **Severity:** High 7.8 (`CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/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:** 27 Jul 2026
- **Updated:** 21 Oct 2026
- **Also known as:** AZL-94061, BELL-CVE-2026-64539, DEBIAN-CVE-2026-64539, GHSA-rxg5-jr9r-8pc7, ROOT-OS-DEBIAN-12-CVE-2026-64539, UBUNTU-CVE-2026-64539

## What is CVE-2026-64539?

CVE-2026-64539 is a high-severity memory corruption vulnerability in Linux, published on 27 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:

Bluetooth: eir: Fix stack OOB write when prepending the Flags AD

eir_create_adv_data() builds the advertising data into a fixed-size
buffer ("size", 31 for the legacy path). It may prepend a 3-byte "Flags"
AD structure (LE_AD_NO_BREDR on an LE-only controller) and then copies
the per-instance data without checking that it still fits:

	memcpy(ptr, adv->adv_data, adv->adv_data_len);

tlv_data_max_len() only reserves those 3 bytes when the user-supplied
flags carry a managed-flags bit, so an instance added with flags == 0 is
accepted with adv_data_len up to the full buffer. At advertise time the
flags are still prepended, and the memcpy() writes 3 + adv_data_len
bytes into the size-byte buffer:

  BUG: KASAN: stack-out-of-bounds in eir_create_adv_data (net/bluetooth/eir.c:301)
  Write of size 31 at addr ffff88800a547bdc by task kworker/u9:0/65
  Workqueue: hci0 hci_cmd_sync_work
   __asan_memcpy (mm/kasan/shadow.c:106)
   eir_create_adv_data (net/bluetooth/eir.c:301)
   hci_update_adv_data_sync (net/bluetooth/hci_sync.c:1310)
   hci_schedule_adv_instance_sync (net/bluetooth/hci_sync.c:1817)
   hci_cmd_sync_work (net/bluetooth/hci_sync.c:332)
  This frame has 1 object:
   [32, 64) 'cp'

The "Flags" structure is added by the kernel, not requested by
userspace, so only prepend it when it fits together with the instance
advertising data; when there is no room for both, drop the flags rather
than the user-provided data.

Reachable by a local user with CAP_NET_ADMIN owning an LE-only
controller on the legacy advertising path.

Weakness: CWE-121 (Stack-based Buffer Overflow)

## Which versions are affected by CVE-2026-64539?


## How do I fix CVE-2026-64539?

Apply the vendor's patch.

## Is CVE-2026-64539 being exploited?

There is no known exploitation of CVE-2026-64539 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-64539 and fix it if it is affected.

What it is: CVE-2026-64539 is a high-severity memory corruption vulnerability in Linux, published on 27 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-2026-64539 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-2026-64539/ (Markdown: https://insights.sbomify.com/CVE-2026-64539/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-64539
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-64539
- OSV: https://osv.dev/vulnerability/CVE-2026-64539

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-64539/
Summaries are generated and may contain mistakes; the official description above is the record.