CVE-2021-36226Public exploitNetwork attack vector

Persistent code execution via unsigned firmware installation

Published Feb 6, 2023 · Updated Mar 26, 2025

Improper firmware verification in Western Digital NAS devices before OS5 allows remote attackers to install attacker-controlled system images. The upload_firmware executable checks firmware structure, checksums, and magic values but does not verify a vendor cryptographic signature before flashing the image. Delivery of an accepted image to the firmware-update interface is required; the documented PR4100 chain grants persistent root control after reboot.

CVSS severity9.8
Critical
EPSS probability0.81%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
Coverage for this vulnerability

See if you're affected

Explore vulnerability checks for your environment with Hinoki.

Book a demo

Summary

What happened

Improper firmware verification in Western Digital NAS devices before OS5 allows remote attackers to install attacker-controlled system images. The upload_firmware executable checks firmware structure, checksums, and magic values but does not verify a vendor cryptographic signature before flashing the image. Delivery of an accepted image to the firmware-update interface is required; the documented PR4100 chain grants persistent root control after reboot.

The record

CVE
CVE-2021-36226
Published
Feb 6, 2023
Updated
Mar 26, 2025
Vendor
Unknown vendor
Product
Unknown product
Classifications
CWE-347, T1601.001
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Feb 6, 2023CVE publishedPublication date reported by the CVE source.
  2. Mar 26, 2025Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

Compare your product and version with the public record. A matching version still requires validation against your environment.

Is a vulnerable build present?

Affected buildAffected product and version details are unavailable.

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Affected products and versions are unavailable in this record.

Published CVSS scores

9.8CISA ADPCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
9.8NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/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)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitPublished

Weakness, pattern, technique

CWE-347Improper Verification of Cryptographic Signature
T1601.001Modify System Image: Patch System Image

Public exploit references

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

Technologies

Your stack

See the directory against your own environment.

Your stack

Check the software in your environment

Book a demo to see how Hinoki identifies affected software and validates exploitability in your environment.

Book a demo