CVE-2020-27738Network attack vector

Service interruption via unchecked DNS pointer offsets

Published Apr 22, 2021 · Updated Aug 4, 2024

Out-of-bounds read in Siemens Nucleus DNS clients allows man-in-the-middle attackers to cause a service interrupt with malformed DNS replies. The DNS record decompressor trusts compressed-name pointer offsets and reads beyond the allocated response structure when an offset is out of range. Exploitation requires control of a DNS response path; successful parsing can crash or otherwise halt the embedded service, with no reported data modification.

CVSS severity6.5
Medium
EPSS probability3.66%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Out-of-bounds read in Siemens Nucleus DNS clients allows man-in-the-middle attackers to cause a service interrupt with malformed DNS replies. The DNS record decompressor trusts compressed-name pointer offsets and reads beyond the allocated response structure when an offset is out of range. Exploitation requires control of a DNS response path; successful parsing can crash or otherwise halt the embedded service, with no reported data modification.

The record

CVE
CVE-2020-27738
Published
Apr 22, 2021
Updated
Aug 4, 2024
Vendor
Siemens
Product
TALON TC Compact (BACnet)
Classifications
CWE-119, CWE-788, T1499.004
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Apr 22, 2021CVE publishedPublication date reported by the CVE source.
  2. Aug 4, 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=All versions < V3.5.5

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Siemens · TALON TC Compact (BACnet)version=All versions < V3.5.5
Siemens · APOGEE PXC Modular (P2 Ethernet)version=All versions < V2.8.20
Siemens · Nucleus Source Codeversion=Versions including affected DNS modules
Siemens · SIMOTICS CONNECT 400version=All versions < V0.5.0.0
Siemens · Nucleus ReadyStart V4version=All versions < V4.1.0
Siemens · Nucleus NET for Nucleus PLUSversion=All versions
Siemens · Nucleus ReadyStart V3version=All versions < V2017.02.3
Siemens · APOGEE PXC Compact (BACnet)version=All versions < V3.5.5
Siemens · TALON TC Modular (BACnet)version=All versions < V3.5.5
Siemens · APOGEE PXC Modular (BACnet)version=All versions < V3.5.5
Siemens · APOGEE PXC Compact (P2 Ethernet)version=All versions < V2.8.20

Published CVSS scores

5.8NIST NVDAV:N/AC:M/Au:N/C:P/I:N/A:P
7.4NIST NVDCVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:N/A:H
6.5Siemens AGCVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:N/A:H

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.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

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-119Improper Restriction of Operations within the Bounds of a Memory Buffer
CWE-788Access of Memory Location After End of Buffer
T1499.004Application or System Exploitation

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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