CVE-2021-28799: QNAP NAS Improper Authorization Vulnerability
Status: confirmed exploited in the wild
CVE-2021-28799 appears in CISA's Known Exploited Vulnerabilities (KEV) catalog, added on 2022-03-31. Inclusion in KEV is not a severity prediction. It means CISA has reliable evidence that the vulnerability has actually been exploited against real targets, which is a stronger signal than a CVSS score on its own.
| Field | Value |
|---|---|
| CVE | CVE-2021-28799 |
| Vendor | QNAP |
| Product | Network Attached Storage (NAS) |
| Added to KEV | 2022-03-31 |
| Federal due date | 2022-04-21 |
| Ransomware campaign use | Known |
What the vulnerability is
QNAP NAS running HBS 3 contains an improper authorization vulnerability which can allow remote attackers to log in to a device.
Weakness classification
CISA classifies this entry under:
Required action
CISA's stated required action for this entry:
Apply updates per vendor instructions.
Ransomware context
CISA records this vulnerability as known to have been used in ransomware campaigns. That materially raises its priority relative to its CVSS score alone.
Federal remediation deadline
Under CISA Binding Operational Directive 22-01, U.S. federal civilian executive branch agencies were required to remediate this vulnerability by 2022-04-21. The directive does not bind private organisations, but the due date is a useful external signal: CISA sets shorter windows for what it assesses as higher risk.
Why KEV membership changes prioritisation
Most vulnerability backlogs are too large to clear, so the practical question is ordering rather than coverage. KEV is useful for that because it is evidence-based: an entry is on the list because exploitation was observed, not because a scoring formula predicted it might be.
A defensible triage rule used widely is to treat KEV membership as a promotion signal that overrides CVSS ranking, on the basis that a confirmed-exploited medium-severity issue generally warrants attention ahead of a theoretical critical one.
Determining whether you are affected
- Establish whether QNAP Network Attached Storage (NAS) is present anywhere in your estate, including indirectly and in systems not under active management.
- Identify the deployed versions and compare against the vendor advisory linked below.
- Apply the required action above, then confirm the change took effect rather than assuming it did.
Step one is where this usually fails. Several of the most costly incidents on record, including Code Red and Log4Shell, turned on organisations being unable to answer whether affected software was present at all.
How Safeguard helps
Safeguard's vulnerability management enriches findings with KEV and EPSS data, so an entry like this one surfaces as confirmed-exploited rather than sitting undifferentiated in a ranked list.
Sources
- CISA KEV catalog (version 2026.09.16): https://www.cisa.gov/known-exploited-vulnerabilities-catalog
- NVD record for CVE-2021-28799: https://nvd.nist.gov/vuln/detail/CVE-2021-28799
- Vendor and advisory references: https://nvd.nist.gov/vuln/detail/CVE-2021-28799
Catalog data retrieved from CISA KEV version 2026.09.16. Verify against the live catalog before relying on dates for compliance purposes.
