CVE-2021-3966

Heap corruption via unchecked USB HCI appends

Published Jan 11, 2023 · Updated Apr 9, 2025

Heap-based buffer overflow in Zephyr RTOS before 3.0.0 allows physically proximate attackers to corrupt memory via crafted USB HCI data. The acl_read_cb function repeatedly passes attacker-controlled HCI OUT chunks to net_buf_add_mem without checking the fixed-size buffer tailroom. A connected USB host can overflow heap memory, causing a crash, security-feature bypass, or arbitrary code execution.

CVSS severity9.6
Critical
EPSS probability0.49%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Heap-based buffer overflow in Zephyr RTOS before 3.0.0 allows physically proximate attackers to corrupt memory via crafted USB HCI data. The acl_read_cb function repeatedly passes attacker-controlled HCI OUT chunks to net_buf_add_mem without checking the fixed-size buffer tailroom. A connected USB host can overflow heap memory, causing a crash, security-feature bypass, or arbitrary code execution.

The record

CVE
CVE-2021-3966
Published
Jan 11, 2023
Updated
Apr 9, 2025
Vendor
Zephyr Project
Product
Zephyr RTOS
Classifications
CWE-122, CWE-120, T1203
Attack vector
adjacent
Privileges
unauthenticated

Timeline

How it unfolded

  1. Jan 11, 2023CVE publishedPublication date reported by the CVE source.
  2. Apr 9, 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 <=v3.0

What conditions does exploitation require?

Attack vectoradjacent
Required privilegesunauthenticated

What is affected?

Zephyr Project · Zephyr RTOSversion=unspecified <=v3.0

Published CVSS scores

8.8NIST NVDCVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
9.6Zephyr ProjectCVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:C/C:L/I:H/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.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

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 exploitUnconfirmed

Weakness, pattern, technique

CWE-122Heap-based Buffer Overflow
CWE-120Buffer Copy without Checking Size of Input
T1203Exploitation for Client Execution

Public exploit references

No public exploit references are available in this record.

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Technologies

Your stack

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Your stack

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