CVE-2021-3321

Firmware crash via unchecked fragment length subtraction

Published Oct 12, 2021 · Updated Sep 16, 2024

Integer underflow in IEEE 802.15.4 fragment reassembly in Zephyr RTOS allows physically proximate attackers to crash firmware via malformed fragments. The reassembly path subtracts an assumed header length from an unchecked shorter fragment, then passes the underflowed length to memmove and writes far beyond the network buffer. No privileges or user interaction are required, but access to the device's IEEE 802.15.4 link is necessary; memory corruption guarantees a crash, while code execution is unconfirmed.

CVSS severity7.5
High
EPSS probability0.76%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Integer underflow in IEEE 802.15.4 fragment reassembly in Zephyr RTOS allows physically proximate attackers to crash firmware via malformed fragments. The reassembly path subtracts an assumed header length from an unchecked shorter fragment, then passes the underflowed length to memmove and writes far beyond the network buffer. No privileges or user interaction are required, but access to the device's IEEE 802.15.4 link is necessary; memory corruption guarantees a crash, while code execution is unconfirmed.

The record

CVE
CVE-2021-3321
Published
Oct 12, 2021
Updated
Sep 16, 2024
Vendor
Zephyr Project
Product
Zephyr RTOS
Classifications
CWE-680, CWE-191, T1499
Attack vector
adjacent
Privileges
unauthenticated

Timeline

How it unfolded

  1. Oct 12, 2021CVE publishedPublication date reported by the CVE source.
  2. Sep 16, 2024Record 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=>=2.4.0 <unspecified

What conditions does exploitation require?

Attack vectoradjacent
Required privilegesunauthenticated

What is affected?

Zephyr Project · Zephyr RTOSversion=>=2.4.0 <unspecified

Published CVSS scores

8.8NIST NVDCVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
7.5Zephyr ProjectCVSS:3.1/AV:A/AC:H/PR:N/UI:N/S:C/C:L/I:L/A:H
5.8NIST NVDAV:A/AC:L/Au:N/C:P/I:P/A:P

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 exploitUnconfirmed

Weakness, pattern, technique

CWE-680Integer Overflow to Buffer Overflow
CWE-191Integer Underflow (Wrap or Wraparound)
T1499Endpoint Denial of Service

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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