CVE-2021-20856
Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.
NVD · uneditedCross-site scripting vulnerability in ELECOM LAN routers (WRH-733GBK firmware v1.02.9 and prior and WRH-733GWH firmware v1.02.9 and prior) allows a remote authenticated attacker to inject an arbitrary script via unspecified vectors.
Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.
dbcve analysis · moderate confidenceAuthenticated cross-site scripting (XSS) vulnerability in ELECOM LAN routers WRH-733GBK and WRH-733GWH (firmware v1.02.9 and prior). A remote attacker with valid admin credentials can inject arbitrary JavaScript through unspecified input vectors, potentially enabling session hijacking, cookie theft, or malicious actions performed as the authenticated user.
Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.
Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.
NVD · CPE data<= 1.02.9<= 1.02.9CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.
From the vector- Attack vector
- Network
- Complexity
- Low
- Privileges
- Low
- User interaction
- Required
- Scope
- Changed
- Confidentiality
- Low
- Integrity
- Low
- Availability
- None
CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:L/I:L/A:N
Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.
dbcve checksWork through these to decide whether this CVE applies to you.
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Identify router modelLocate the physical device or access the admin interface and verify the model number is WRH-733GBK or WRH-733GWH. This information is typically printed on a label on the device casing or displayed in the router status page.Affected if Model is WRH-733GBK or WRH-733GWH and falls within the affected product line
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Check installed firmware versionLog into the router admin interface (usually via web browser at 192.168.1.1 or similar) and navigate to the System or Firmware status page. Record the exact firmware version number displayed.Affected if Firmware version is 1.02.9 or prior (versions higher than 1.02.9 are not affected)
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Confirm admin credentials are configuredVerify that an administrator account exists on the router. Check if admin login is enabled in the User Accounts or Administration settings of the router web interface.Affected if Admin credentials are active - the vulnerability requires a valid admin account to exploit
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Assess admin interface network exposureDetermine if the router admin web interface is accessible from external networks or untrusted segments. Check router firewall settings and port forwarding rules to see if HTTP/HTTPS ports (80/443) are exposed to WAN or guest networks.Affected if Admin interface is reachable from untrusted networks - this increases exposure but the vulnerability can also be exploited by anyone with valid admin credentials on the local network
The environment is affected if the router is model WRH-733GBK or WRH-733GWH running firmware version 1.02.9 or earlier, and an administrator account exists with access to the router web interface.
Generated from the published advisory. Verify against your own configuration.
Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.
From vendor dataUpdate router firmware to v1.03 or later if available. If no vendor patch exists, restrict administrative interface access to trusted IPs only using firewall rules, and consider network segmentation or device replacement.
- Consultation2.0 h
- Implementation3.0 h
- Testing2.0 h
- Review / QA1.0 h
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Free · runs locallyCheck whether your project pulls in CVE-2021-20856 — or any other known-vulnerable package — straight from your lock files. Free and open source; it runs locally and uploads nothing.
References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.
Primary sourcesPractitioner notes
ContributedPeer-ranked notes from engineers who’ve handled CVE-2021-20856 in production — separate from our analysis above.
The advisory tells you what broke. It rarely tells you what actually worked. If you’ve dealt with this one, that detail is what the next engineer is searching for.
- The version that genuinely resolved it — not the one the vendor claimed
- A config change or rule that shut the vector down
- A gotcha in the upgrade path that cost you an afternoon
No notes yet
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- Verified mitigations, workarounds, and config changes
- Version or environment caveats, and links to real fixes
- No weaponised exploit code, or anything meant to cause harm
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