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Nucleus NetApplication · Siemens

CVE-2021-27393

MEDIUM · 5.3 CVSS v3.1 Published 2021-04-22
Fix available
A fix is available. Upgrade to 2013.08 or later.
See remediation →
62/100
Remediation priority · Elevated
Remotely reachable No privileges Zero-click

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 · unedited
A vulnerability has been identified in Nucleus NET (All versions), Nucleus ReadyStart V3 (All versions < V2013.08), Nucleus Source Code (Versions including affected DNS modules). The DNS client does not properly randomize UDP port numbers of DNS requests. That could allow an attacker to poison the DNS cache or spoof DNS resolving.

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 confidence

The DNS client in Nucleus NET and Nucleus ReadyStart V3 does not properly randomize UDP source port numbers when sending DNS queries. This predictable port selection enables an attacker to perform DNS cache poisoning or DNS spoofing attacks by guessing the source port and crafting malicious DNS responses.

MitigationImplement cryptographically secure random UDP source port selection for all DNS queries. Use a secure random number generator and ensure ports are selected from the full ephemeral port range to prevent prediction.

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
Nucleus NetApplication
Affected:all versions
Nucleus Readystart V3Application
Affected:< 2013.08
Nucleus Source CodeApplication
Affected:all versions

CVSS 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
Low
Availability
None

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/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 checks

Work through these to decide whether this CVE applies to you.

  1. Identify Nucleus NET or Nucleus ReadyStart usage
    Determine if your embedded system or software uses Siemens Nucleus NET or Nucleus ReadyStart as its network stack. Check product documentation, software BOM, or system components for these components.
    Affected if The system uses any version of Siemens Nucleus NET, Nucleus ReadyStart V3, or Nucleus Source Code as its network stack.
  2. Check Nucleus ReadyStart V3 version
    For systems using Nucleus ReadyStart V3, identify the installed version by reviewing the software release notes, version file, or checking the binary metadata. Compare against the affected range: versions prior to 2013.08.
    Affected if The system runs Nucleus ReadyStart V3 with a version earlier than 2013.08.
  3. Verify DNS client is enabled
    Determine if the DNS client functionality is active in the Nucleus NET stack. Check the system configuration files, network initialization code, or runtime settings for DNS client module activation.
    Affected if The DNS client module is enabled and the system performs DNS queries using the Nucleus NET stack.
  4. Inspect DNS query source port behavior
    Capture network traffic during DNS resolution operations. Observe the UDP source port used for outgoing DNS queries. Check if ports follow a predictable pattern or are selected from a limited range.
    Affected if DNS queries use predictable or sequential UDP source ports instead of random selection from the ephemeral port range.
  5. Review DNS client source code or configuration
    If source code access is available, examine the DNS client implementation for the source port selection logic. Look for use of secure random number generation versus sequential or fixed port assignment.
    Affected if The code uses non-random or predictable source port selection for DNS queries.

A system is affected if it uses Siemens Nucleus NET or Nucleus ReadyStart with DNS client functionality enabled and exhibits predictable UDP source port selection during DNS queries.

Generated from the published advisory. Verify against your own configuration.

Check your environment

Paste your version and any relevant configuration and it will be compared against the affected criteria above. Do not include secrets or credentials.

AI-assisted, checked against the advisory. Informational, not a guarantee.

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 data
Upgrade available Upgrade to 2013.08 or later
Fixed in 2013.08
Interim mitigation

Implement cryptographically secure random UDP source port selection for all DNS queries. Use a secure random number generator and ensure ports are selected from the full ephemeral port range to prevent prediction.

Fix this in Nucleus Net Scoped from the published advisory
  • Consultation4.0 h
  • Implementation8.0 h
  • Testing6.0 h
  • Review / QA4.0 h
22.0 hours of engineering $3,860
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References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2021-27393 in production — separate from our analysis above.

No notes yet

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What this is

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.

What belongs here
  • 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
  • No spam, self-promotion, credentials, or personal data