CVE-2021-3329Public exploit

Bluetooth host crash via zero ACL MTU

Published Feb 26, 2023 · Updated Mar 11, 2025

Improper HCI initialization in Zephyr RTOS 2.4 and earlier allows attackers controlling the Bluetooth controller to crash the host stack. When a non-BR/EDR controller reports a zero ACL MTU, hci_init leaves bt_dev.le.acl_pkts uninitialized instead of rejecting startup. The flaw requires CONFIG_BT_CONN with BR/EDR disabled and a controller that supplies the zero length; ordinary Bluetooth traffic then dereferences the uninitialized semaphore.

CVSS severity9.6
Critical
EPSS probability0.62%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Improper HCI initialization in Zephyr RTOS 2.4 and earlier allows attackers controlling the Bluetooth controller to crash the host stack. When a non-BR/EDR controller reports a zero ACL MTU, hci_init leaves bt_dev.le.acl_pkts uninitialized instead of rejecting startup. The flaw requires CONFIG_BT_CONN with BR/EDR disabled and a controller that supplies the zero length; ordinary Bluetooth traffic then dereferences the uninitialized semaphore.

The record

CVE
CVE-2021-3329
Published
Feb 26, 2023
Updated
Mar 11, 2025
Vendor
Zephyr Project
Product
Zephyr RTOS
Classifications
CWE-665, CWE-703, T1499
Attack vector
adjacent
Privileges
unauthenticated

Timeline

How it unfolded

  1. Feb 26, 2023CVE publishedPublication date reported by the CVE source.
  2. Mar 11, 2025Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=unspecified <=v2.4

What conditions does exploitation require?

Attack vectoradjacent
Required privilegesunauthenticated

What is affected?

Zephyr Project · Zephyr RTOSversion=unspecified <=v2.4

Published CVSS scores

9.6Zephyr ProjectCVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:C/C:L/I:H/A:H
6.5NISTCVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

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No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectoradjacent
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitPublished

Weakness, pattern, technique

CWE-665Improper Initialization
CWE-703Improper Check or Handling of Exceptional Conditions
T1499Endpoint Denial of Service

Public exploit references

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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