Security readout for executives and security teams
Plain-English summary
This vulnerability affects some QNAP NAS operating systems and could let an attacker inject malicious code through the web interface. It requires an authenticated user and user interaction, but the confidentiality impact is rated high. Organizations using affected QNAP systems should confirm versions and update to the fixed builds.
Executive priority
Treat as a scheduled security update, with faster action for internet-reachable or broadly accessible NAS administration portals. The issue is not listed as actively exploited in the provided evidence, but affected NAS devices can hold sensitive business data.
Technical view
CVE-2021-28806 is a DOM-based cross-site scripting issue, CWE-79, in QNAP QTS, QuTS hero, and QuTScloud. CVSS 3.1 is 5.7 with network access, low complexity, low privileges, required user interaction, unchanged scope, and high confidentiality impact. Fixed build thresholds are listed by QNAP.
Likely exposure
Exposure is likely limited to QNAP NAS devices running QTS before 4.5.3.1652 Build 20210428, QuTS hero before h4.5.2.1638 Build 20210414, or QuTScloud before c4.5.5.1656 Build 20210503. QNAP states QTS 4.3.6 and 4.3.3 are not affected.
Exploitation context
The provided sources do not show CISA KEV listing or active exploitation. The CVSS vector indicates exploitation requires low privileges and user interaction, so risk is higher where the QNAP management interface is reachable by untrusted users.
Researcher notes
The main evidence is vendor and CVE metadata. The vulnerability is DOM-based XSS with confidentiality impact, not integrity or availability impact in the CVSS vector. The sources do not provide technical root cause detail, exploit status, or separate workaround instructions beyond fixed versions.
Mitigation direction
- Upgrade QTS to 4.5.3.1652 Build 20210428 or later.
- Upgrade QuTS hero to h4.5.2.1638 Build 20210414 or later.
- Upgrade QuTScloud to c4.5.5.1656 Build 20210503 or later.
- Check QNAP’s advisory for product-specific update guidance.
- Restrict NAS administration access to trusted networks where possible.
Validation and detection
- Inventory QNAP NAS devices and record OS family, version, and build.
- Compare versions against the affected and fixed build thresholds.
- Confirm QTS 4.3.6 or 4.3.3 systems are documented as unaffected.
- Review whether management interfaces are exposed to untrusted networks.
- Verify updates completed successfully after maintenance.
Public sources used
Generated from the cited source records. This long-tail analysis has not been individually reviewed by a named human.
Conservative CVE-to-ATT&CK context
These mappings and lookup hints may be relevant to the vulnerability behavior, CWE, affected product, or exposure path. Glexia-inferred context is not an official MITRE, ATT&CK, CWE, or CVE Program mapping.
ATT&CK lookup starting points
Use these exact CWE pages and searches to review the Glexia ATT&CK library from this CVE's weakness and description context.
CWE-79: User-session and phishing behavior lookup
Client-side and session-facing weaknesses should be reviewed alongside initial-access and user-execution behaviors. Open the exact CWE lookup page first, then review the ATT&CK searches from that MITRE weakness context. This is a Glexia lookup hint, not an official ATT&CK mapping.
Open ATT&CK lookupCVE-2021-28806 mapping review
Open the CVE-to-ATT&CK bridge for reviewed, inferred, or future official mappings tied to this CVE.
Open ATT&CK lookup- Severity
- Medium
- CVSS
- 5.7 (3.1)
- Known Exploited
- No
- Published
Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:U/C:H/I:N/A:N
CNA and ADP enrichment extracted from CVE v5
These fields come from the CVE record and ADP containers, not from Glexia's Take. They preserve time-varying source decisions such as CISA SSVC, KEV status, CVSS metrics, and provider references.
CVSS vector scores
1 official scoreWe collect every scored CVSS vector available in the official CNA and ADP containers. When more than one version is present, the table keeps the source vectors side by side instead of collapsing them into the highest score.
CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:U/C:H/I:N/A:N2.13.6Primary CVE scoreVulnerability scoring details
Base CVSS 3.1 score
5.7MediumVector: CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:U/C:H/I:N/A:N
Source materials
- CVE List V5 sourceCVE List V5
- https://www.qnap.com/zh-tw/security-advisory/qsa-21-22CVE reference · x_refsource_MISC
Products and packages named in the record
CWE details
CWE links open Glexia weakness intelligence pages with official CWE context, developer remediation guidance, and related CVE mappings.
Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')
Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting') represents a recurring weakness pattern that can create exploitable paths when design, validation, or implementation controls are missing.