CVE-2021-25811
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 · uneditedMERCUSYS Mercury X18G 1.0.5 devices allow Denial of service via a crafted value to the POST listen_http_lan parameter. Upon subsequent device restarts after this vulnerability is exploted the device will not be able to access the webserver unless the listen_http_lan parameter to uhttpd.json is manually fixed.
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 confidenceMERCUSYS Mercury X18G 1.0.5 devices contain a denial of service vulnerability in the webserver configuration handling. An attacker can send a crafted POST request with a malicious value to the listen_http_lan parameter, which corrupts the uhttpd webserver configuration. Upon device reboot, the webserver fails to start and becomes inaccessible, requiring manual editing of the uhttpd.json configuration file to recover.
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= 1.0.5CVSS 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
- None
- User interaction
- None
- Scope
- Unchanged
- Confidentiality
- None
- Integrity
- None
- Availability
- High
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
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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Confirm device model and firmware versionAccess the router web interface or use Nmap/curl to check the HTTP response headers for the firmware version string. Alternatively, check the MERCUSYS support page for the X18G model and compare against version 1.0.5.Affected if The device is a Mercusys Mercury X18G running firmware version 1.0.5 exactly.
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Inspect uhttpd.json configuration fileIf you have shell access or can access the device filesystem, navigate to /etc/config/ or /etc/ and open uhttpd.json. Look for the listen_http_lan parameter and check if its value appears corrupted, unusually long, or contains unexpected characters.Affected if The listen_http_lan parameter contains a malformed or corrupted value that differs from a valid port number (typically 80 or 8080).
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Test webserver accessibility after rebootReboot the device (if safe to do so in a lab environment) and attempt to access the web interface via HTTP on the LAN IP address. Note whether the page loads or times out.Affected if After a reboot, the web interface becomes completely inaccessible and the device does not respond to HTTP requests on the LAN interface.
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Check for uhttpd process statusIf SSH/telnet access is available, run 'ps | grep uhttpd' or check /proc/processes to verify if the uhttpd process is running.Affected if The uhttpd process is not running or has terminated unexpectedly, especially after a configuration change to listen_http_lan.
A user is affected if they have a Mercusys Mercury X18G device running firmware 1.0.5 and the listen_http_lan parameter in uhttpd.json has been corrupted, causing the webserver to fail after reboot.
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 dataApply any available vendor firmware update. Until a patch is released, network segmentation should be implemented to limit exposure to untrusted networks, and manual recovery via uhttpd.json configuration file editing should be documented as a contingency procedure.
- Consultation4.0 h
- Implementation2.0 h
- Testing3.0 h
- Review / QA2.0 h
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Free · runs locallyCheck whether your project pulls in CVE-2021-25811 — 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-2021-25811 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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- Version or environment caveats, and links to real fixes
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