CVE-2021-25663
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 · uneditedA vulnerability has been identified in Capital Embedded AR Classic 431-422 (All versions), Capital Embedded AR Classic R20-11 (All versions < V2303), Nucleus NET (All versions), Nucleus ReadyStart V3 (All versions < V2017.02.4), Nucleus ReadyStart V4 (All versions < V4.1.0), Nucleus Source Code (All versions including affected IPv6 stack). The function that processes IPv6 headers does not check the lengths of extension header options, allowing attackers to put this function into an infinite loop with crafted length values.
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 confidenceThe IPv6 header processing function in multiple Nucleus NET and Capital Embedded products fails to validate the length fields of extension header options. Attackers can craft IPv6 packets with malicious length values that cause the parsing function to enter an infinite loop, resulting in denial of service.
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 versions< 4.1.0< 2017.02.4all 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
- 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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Identify affected Nucleus components in your systemReview your software bill of materials or dependencies to determine if any Siemens Nucleus NET, Capital Embedded, or Nucleus Readestart components are included in your deploymentAffected if Any of these components are present and IPv6 traffic is processed
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Check Nucleus Readystart versionLocate and inspect the version information for your Nucleus Readystart installation; compare against the vulnerable ranges: < 4.1.0 or < 2017.02.4Affected if The installed version is lower than 4.1.0 or lower than 2017.02.4
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Verify IPv6 processing is enabledExamine your system configuration, network settings, or runtime parameters to determine whether IPv6 protocol processing is activeAffected if IPv6 is enabled and the system processes incoming IPv6 traffic
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Inspect IPv6 extension header handlingReview network traffic logs or packet captures for incoming IPv6 packets with extension headers (such as Hop-by-Hop, Routing, Fragment, Destination Options, or Authentication Header)Affected if The system receives external IPv6 traffic with extension headers that could trigger the parsing function
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Check for infinite loop or DoS conditionsMonitor system logs, process monitoring, or network diagnostics for signs of hung processes, high CPU usage, or network stack unresponsiveness that correlate with IPv6 traffic patternsAffected if The system exhibits denial-of-service symptoms when processing IPv6 packets with extension headers
You are affected if your system runs any Siemens Nucleus NET, Capital Embedded, or Nucleus Readystart component with IPv6 enabled and processes external IPv6 traffic, particularly if the Nucleus Readystart version is below 4.1.0 or 2017.02.4.
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 data4.1.02017.02.4
Update affected products to vendor-supplied patches that add proper length validation for IPv6 extension headers. If patches are unavailable, consider filtering or rate-limiting external IPv6 traffic as a compensating control.
- Consultation4.0 h
- Implementation8.0 h
- Testing6.0 h
- Review / QA4.0 h
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Scan for this in your stack
Free · runs locallyCheck whether your project pulls in CVE-2021-25663 — 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-2021-25663 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
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