CVE-2021-27393Network attack vector

DNS cache poisoning via predictable UDP ports

Published Apr 22, 2021 · Updated Aug 3, 2024

Insufficient port randomization in Siemens Nucleus DNS clients allows remote attackers to poison DNS caches via spoofed replies. The DNS client selects request source ports without sufficient randomization, reducing the entropy required to forge a matching DNS response. No authentication or user interaction is required; successful spoofing can redirect hostname resolution but does not directly disclose data or interrupt service.

CVSS severity5.3
Medium
EPSS probability0.75%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Insufficient port randomization in Siemens Nucleus DNS clients allows remote attackers to poison DNS caches via spoofed replies. The DNS client selects request source ports without sufficient randomization, reducing the entropy required to forge a matching DNS response. No authentication or user interaction is required; successful spoofing can redirect hostname resolution but does not directly disclose data or interrupt service.

The record

CVE
CVE-2021-27393
Published
Apr 22, 2021
Updated
Aug 3, 2024
Vendor
Siemens
Product
Nucleus Source Code
Classifications
CWE-330, T1659
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Apr 22, 2021CVE publishedPublication date reported by the CVE source.
  2. Aug 3, 2024Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=Versions including affected DNS modules

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Siemens · Nucleus Source Codeversion=Versions including affected DNS modules
Siemens · Nucleus NET for Nucleus PLUSversion=All versions
Siemens · Nucleus ReadyStart V3version=All versions < V2013.08

Published CVSS scores

5.3Siemens ProductCERTCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

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No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-330Use of Insufficiently Random Values
T1659Content Injection

Public exploit references

No public exploit references are available in this record.

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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