CVE-2021-25664
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 the Hop-by-Hop extension header in IPv6 packets and its options lacks any checks against the length field of the header, allowing attackers to put the function into an infinite loop by supplying arbitrary 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 function processing the IPv6 Hop-by-Hop extension header and its options lacks validation of the header's length field. By supplying malicious arbitrary length values in crafted IPv6 packets, attackers can trigger an infinite loop in the affected function, causing 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< 2017.02.4< 4.1.0all 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 the installed Siemens productDetermine which Siemens Nucleus or Capital product is deployed in your environment. Check system documentation, software inventory, or installed packages to confirm the product name and version.Affected if The product is any version of Siemens Capital Vstar, Siemens Nucleus Net, Siemens Nucleus Source Code, Nucleus Readystart V3 (< 2017.02.4), or Nucleus Readystart V4 (< 4.1.0).
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Check IPv6 stack usageReview system configuration, network settings, and running processes to determine if IPv6 networking is enabled and active in the environment.Affected if IPv6 is enabled and the system processes IPv6 traffic, as the vulnerability is triggered by malicious IPv6 packets with crafted Hop-by-Hop extension headers.
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Review network exposureAudit network interfaces, firewall rules, and access controls to determine if the affected system accepts IPv6 traffic from untrusted sources.Affected if The system accepts IPv6 packets from network segments that include untrusted or external sources, enabling delivery of malicious packets.
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Verify patch levelCompare the installed product version against the fixed versions: Nucleus Readystart V3 should be 2017.02.4 or later, Nucleus Readystart V4 should be 4.1.0 or later. For Capital Vstar and Nucleus Net, check for vendor patches V2303 or V2017.02.4 respectively.Affected if The installed version is below the fixed releases and no vendor patch has been applied.
You are affected if your environment uses any version of the listed Siemens products with IPv6 enabled and network exposure to untrusted IPv6 traffic, and the version is below the patched releases.
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
Apply vendor-supplied patches: V2303 for Capital Embedded AR Classic R20-11, V2017.02.4 for Nucleus ReadyStart V3, V4.1.0 for Nucleus ReadyStart V4. For unpatched versions, implement network segmentation to restrict untrusted IPv6 traffic.
- Consultation2.0 h
- Implementation4.0 h
- Testing6.0 h
- Review / QA3.0 h
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Scan for this in your stack
Free · runs locallyCheck whether your project pulls in CVE-2021-25664 — 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-2021-25664 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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A place for practitioners to share what actually worked: a mitigation you’ve tested, a configuration change, a version- or environment-specific caveat, or a link to a verified patch. The most useful notes rise to the top as peers upvote them, so the signal stays high.
- 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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