CVE-2021-20776
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 · uneditedImproper authentication vulnerability in SCT-40CM01SR and AT-40CM01SR allows an attacker to bypass access restriction and execute an arbitrary command via telnet.
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 · high confidenceImproper authentication in the telnet service on SCT-40CM01SR and AT-40CM01SR devices allows unauthenticated attackers to bypass access controls and execute arbitrary commands, achieving full system compromise.
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 dataall versionsall versionsCVSS 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
- High
- Integrity
- High
- Availability
- High
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/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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Identify device modelCheck the device label, web interface, or system information to confirm the exact model is SCT-40CM01SR or AT-40CM01SRAffected if Device model is SCT-40CM01SR or AT-40CM01SR - these are affected in all firmware versions
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Check if telnet service is enabledAccess device administration interface or run 'telnet localhost' or 'netstat -an | grep 23' from the device console to see if telnet port 23 is listeningAffected if Telnet service is running and listening on any interface (port 23) - the vulnerability exists when telnet is enabled regardless of version since all versions are affected
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Verify telnet network exposureCheck device network settings, firewall rules, or run 'iptables -L' or check router configuration to determine if telnet port 23 is accessible from untrusted networksAffected if Telnet is accessible from external/untrusted networks (WAN or non-localhost) - the bypass allows unauthenticated access when reachable
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Test telnet authenticationConnect to the device telnet port (23) from a remote system and observe if it provides immediate shell access without prompting for credentialsAffected if Telnet grants shell access without requiring any username/password - indicates the authentication bypass is present
User is affected if they use an SCT-40CM01SR or AT-40CM01SR device with telnet service enabled and accessible on any network interface, as all firmware versions contain the authentication bypass vulnerability.
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 dataDisable telnet access if unnecessary; otherwise restrict telnet to trusted networks via firewall rules and monitor for unauthorized access attempts. Contact vendor for firmware updates.
- Consultation8.0 h
- Implementation16.0 h
- Testing8.0 h
- Review / QA4.0 h
An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $10,240.
Scan for this in your stack
Free · runs locallyCheck whether your project pulls in CVE-2021-20776 — 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-2021-20776 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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