CVE-2021-31889Network attack vector

Service interruption via corrupted TCP SACK arithmetic

Published Nov 9, 2021 · Updated Mar 11, 2025

Integer underflow in Nucleus NET in affected Siemens embedded products allows remote attackers to interrupt service with malformed TCP SACK packets. The TCP SACK-option parser performs arithmetic on corrupted option values that can underflow while processing acknowledgment ranges. No authentication or user interaction is required; Siemens scores the documented product impact as availability-only despite also describing information leaks.

CVSS severity7.5
High
EPSS probability2.17%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Integer underflow in Nucleus NET in affected Siemens embedded products allows remote attackers to interrupt service with malformed TCP SACK packets. The TCP SACK-option parser performs arithmetic on corrupted option values that can underflow while processing acknowledgment ranges. No authentication or user interaction is required; Siemens scores the documented product impact as availability-only despite also describing information leaks.

The record

CVE
CVE-2021-31889
Published
Nov 9, 2021
Updated
Mar 11, 2025
Vendor
Siemens
Product
Capital Embedded AR Classic
Classifications
CWE-191, T1499
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Nov 9, 2021CVE publishedPublication date reported by the CVE source.
  2. Mar 11, 2025Record 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=0 <*
  2. Affected versionversion=0 <V2303

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Siemens · Capital Embedded AR Classicversion=0 <*; version=0 <V2303
Siemens · PLUSCONTROL 1st Genversion=All versions
Siemens · SIMOTICS CONNECT 400version=All versions < V0.5.0.0

Published CVSS scores

7.5Siemens ProductCERTCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/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-191Integer Underflow (Wrap or Wraparound)
T1499Endpoint Denial of Service

Public exploit references

No public exploit references are available in this record.

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Technologies

Your stack

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Your stack

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