CVE
CVE-2021-28799
QNAP NAS Improper Authorization Vulnerability
An improper authorization vulnerability has been reported to affect QNAP NAS running HBS 3 (Hybrid Backup Sync. ) If exploited, the vulnerability allows remote attackers to log in to a device. This issue affects: QNAP Systems Inc. HBS 3 versions prior to v16.0.0415 on QTS 4.5.2.
versions prior to v3.0.210412 on QTS 4.3.6.
versions prior to v3.0.210411 on QTS 4.3.4.
versions prior to v3.0.210411 on QTS 4.3.3.
versions prior to v16.0.0419 on QuTS hero h4.5.1.
versions prior to v16.0.0419 on QuTScloud c4.5.1~c4.5.4. This issue does not affect: QNAP Systems Inc. HBS 2 . QNAP Systems Inc. HBS 1.3 .
CRITICAL · CVSS 10
⚠ CISA KEV
EPSS 0.92368
Ransomware: known
Act now
- Listed on CISA KEV (known exploited in the wild)
- Linked to known ransomware campaigns
- SSVC exploitation status: active
- EPSS ≥ 0.50 - high probability of exploitation in the next 30 days
- EPSS percentile: top 0% of all CVEs by exploitation likelihood
- Public exploit or PoC is available
- SSVC automatable: yes - attacks can be scripted at scale
- CVSS base score ≥ 7.0
Sigma rules0
YARA rules0
Look this up elsewhere - one-click external pivots
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How to read a CVE - triage first, then detect and patch
This page is every public fact about CVE-2021-28799, cross-linked. Its job is to answer one question fast - does this need my attention now? - and then hand you the two things you do about it. Here is how an analyst reads it.
Triage: should I act now? Four signals, and they are not interchangeable:
CVSSseverity - how bad it is IF exploited, 0-10. A high CVSS alone is not urgency; a flaw can be a perfect 10 and never actually be attacked.
EPSSprobability - a model’s estimate of the chance it is exploited in the next 30 days, 0-1. This is the “will it actually happen” signal.
CISA KEVconfirmed - it is being exploited in the wild right now. The strongest signal on the page; KEV beats any score.
Weaponisedavailability - public exploits / PoCs, and especially Metasploit modules rated Excellent / Great. Reliable, packaged exploit code means low-skill attackers can use it today.
How they combine: KEV, or a dependable Metasploit module, means patch now regardless of CVSS. High CVSS + low EPSS + no exploit is real but not an emergency - schedule it. Low CVSS but KEV-listed still gets patched now. The verdict above already weighed these for you; this is how it got there.
Then what - two workflows:
Detectwhen you cannot patch today, follow this CVE to the ATT&CK techniques it enables, then Build a SIEM detection (the green button) - author a rule, test it in Atomic, deploy it. That buys visibility while the patch waits.
PatchAffected products / packages tell you if you are exposed; Fixed versions by distribution and Vendor advisories give the exact version that closes it.
Reading order for the panels below: verdict + badges, then Public exploits / Metasploit (is it weaponised), then ATT&CK techniques + Sigma / IDS rules (can I detect it), then Affected products / packages + Fixed versions (am I exposed, what patches it), then Threat actors / IOCs (who uses it), then Scoring & timeline / references (the evidence).
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Required Remediation
Apply updates per vendor instructions.
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ATT&CK techniques
12Techniques this CVE enables - linked via CWECAPECATT&CK. High◆ = named directly in ATT&CK or Nuclei templates.
T1005 · Data from Local System T1012 · Query Registry T1083 · File and Directory Discovery T1134.001 · Token Impersonation/Theft T1550.004 · Web Session Cookie T1552.002 · Credentials in Registry T1562.003 · Impair Command History Logging T1565.002 · Transmitted Data Manipulation T1574.005 · Executable Installer File Permissions Weakness T1574.006 · Dynamic Linker Hijacking T1574.007 · Path Interception by PATH Environment Variable T1574.010 · Services File Permissions Weakness
▤ Build a SIEM detection for these techniques
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CAPEC attack patterns
12Attack patterns this CVE enables - the bridge from weakness to ATT&CK technique.
CAPEC-CAPEC-1 · Accessing Functionality Not Properly Constrained by ACLs CAPEC-CAPEC-104 · Cross Zone Scripting CAPEC-CAPEC-127 · Directory Indexing CAPEC-CAPEC-13 · Subverting Environment Variable Values CAPEC-CAPEC-17 · Using Malicious Files CAPEC-CAPEC-39 · Manipulating Opaque Client-based Data Tokens CAPEC-CAPEC-402 · Bypassing ATA Password Security CAPEC-CAPEC-45 · Buffer Overflow via Symbolic Links CAPEC-CAPEC-5 · Blue Boxing CAPEC-CAPEC-51 · Poison Web Service Registry CAPEC-CAPEC-59 · Session Credential Falsification through Prediction CAPEC-CAPEC-60 · Reusing Session IDs (aka Session Replay)
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Weakness Classification
CWE-285Improper Authorization
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Affected Products & Versions
4qnap hybrid backup sync< 16.0.0415
qnap hybrid backup sync< 3.0.210412
qnap hybrid backup sync< 3.0.210411
qnap hybrid backup sync< 16.0.0419
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Public Exploits & PoCs
5These PoC and exploit links come from public sources and are not verified to be safe or functional. Review the code before running anything, and treat unverified entries as untrusted.
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Scoring & Timeline
10
CRITICAL · CVSS v3.1 · [email protected]
Attack Vector
Network
Adjacent
Local
Physical
Attack Complexity
Low
High
Privileges Required
None
Low
High
User Interaction
None
Required
Scope
Unchanged
Changed
Confidentiality
None
Low
High
Integrity
None
Low
High
Availability
None
Low
High
SSVC triage · cisa-vulnrichment
Exploitation
active
Automatable
yes
Technical impact
total
SSVC asks the questions that actually drive patch urgency: is it being exploited, can attacks be automated, and how total is the impact.
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Vendor Advisories
1🔗
References & Sources
2Source URLs (vendor pages, mailing lists, write-ups). Exploit/PoC links are in their own section above to avoid duplication.
https://www.qnap.com/en/security-advisory/QSA-21-13Vendor Advisory
https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2021-28799US Government Resource