CVE-2021-35954

Firmware compromise via exposed SWD port

Published Dec 26, 2022 · Updated Apr 14, 2025

Improper access control in Fastrack Reflex 2.0 firmware W307S_REFLEX_v90.89 allows physically proximate attackers to alter firmware via SWD. The activity tracker's PCB leaves the Serial Wire Debug port enabled, permitting unrestricted firmware readout and reflashing. Physical access to the PCB is required; exploitation exposes the installed firmware, permits persistent malicious replacement, or renders the tracker inoperable.

CVSS severity8.1
High
EPSS probability0.33%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Improper access control in Fastrack Reflex 2.0 firmware W307S_REFLEX_v90.89 allows physically proximate attackers to alter firmware via SWD. The activity tracker's PCB leaves the Serial Wire Debug port enabled, permitting unrestricted firmware readout and reflashing. Physical access to the PCB is required; exploitation exposes the installed firmware, permits persistent malicious replacement, or renders the tracker inoperable.

The record

CVE
CVE-2021-35954
Published
Dec 26, 2022
Updated
Apr 14, 2025
Vendor
Unknown vendor
Product
Unknown product
Classifications
T1495, T1542.001
Attack vector
physical
Privileges
unauthenticated

Timeline

How it unfolded

  1. Dec 26, 2022CVE publishedPublication date reported by the CVE source.
  2. Apr 14, 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?

Affected buildAffected product and version details are unavailable.

What conditions does exploitation require?

Attack vectorphysical
Required privilegesunauthenticated

What is affected?

Affected products and versions are unavailable in this record.

Published CVSS scores

8.1NVDCVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:H
8.1CISA-ADPCVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:U/C:N/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)
Vectorphysical
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

T1495Firmware Corruption
T1542.001Pre-OS Boot: System Firmware

Public exploit references

No public exploit references are available in this record.

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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