CVE-2020-28388Network attack vector

TCP session hijacking via predictable sequence numbers

Published Feb 9, 2021 · Updated Jun 2, 2026

Predictable TCP sequence numbers in Siemens Nucleus NET before 5.2 allow remote attackers to hijack or spoof TCP sessions. The TCP/IP stack derives initial sequence numbers from an insufficiently random source, making current and future values predictable. A network path to the target and sufficient sequence prediction are required; successful use can inject traffic or terminate a session.

CVSS severity6.5
Medium
EPSS probability1.35%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Predictable TCP sequence numbers in Siemens Nucleus NET before 5.2 allow remote attackers to hijack or spoof TCP sessions. The TCP/IP stack derives initial sequence numbers from an insufficiently random source, making current and future values predictable. A network path to the target and sufficient sequence prediction are required; successful use can inject traffic or terminate a session.

The record

CVE
CVE-2020-28388
Published
Feb 9, 2021
Updated
Jun 2, 2026
Vendor
Siemens
Product
TALON TC Compact (BACnet)
Classifications
CWE-342, T1565.002
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Feb 9, 2021CVE publishedPublication date reported by the CVE source.
  2. Jun 2, 2026Record 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 · PLUSCONTROL 1st Genversion=All versions
Siemens · Nucleus Source Codeversion=All versions
Siemens · Nucleus NET for Nucleus PLUSversion=All versions < V5.2
Siemens · Nucleus ReadyStart V3version=All versions < V2012.12
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

5.3NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N
6.5Siemens AGCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:L
5.0NIST NVDAV:N/AC:L/Au:N/C:N/I:P/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-342Predictable Exact Value from Previous Values
T1565.002Transmitted Data Manipulation

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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