CVE-2021-25677Network attack vector

DNS poisoning via predictable transaction identifiers

Published Apr 22, 2021 · Updated Aug 3, 2024

Insufficient randomness in Siemens Nucleus RTOS DNS clients allows remote attackers to poison DNS caches or spoof name resolution. The DNS client generates transaction identifiers without enough randomness, allowing forged replies to match outstanding queries and be accepted as legitimate responses. Network access sufficient to race a DNS reply is the only reported gate, and successful exploitation redirects subsequent connections without directly reading data or interrupting service.

CVSS severity5.3
Medium
EPSS probability1.06%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Insufficient randomness in Siemens Nucleus RTOS DNS clients allows remote attackers to poison DNS caches or spoof name resolution. The DNS client generates transaction identifiers without enough randomness, allowing forged replies to match outstanding queries and be accepted as legitimate responses. Network access sufficient to race a DNS reply is the only reported gate, and successful exploitation redirects subsequent connections without directly reading data or interrupting service.

The record

CVE
CVE-2021-25677
Published
Apr 22, 2021
Updated
Aug 3, 2024
Vendor
Siemens
Product
TALON TC Compact (BACnet)
Classifications
CWE-330, T1557
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=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; version=All versions >= V0.5.0.0 < V1.0.0.0
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; version=All versions < V2017.02.4
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

5.3Siemens AGCVSS: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.

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-330Use of Insufficiently Random Values
T1557Adversary-in-the-Middle

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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