CVE-2021-25663Network attack vector

Service hang via unchecked IPv6 extension lengths

Published Apr 22, 2021 · Updated Mar 11, 2025

Infinite-loop denial of service in Siemens Nucleus NET-based products allows remote attackers to hang devices with crafted IPv6 packets. The IPv6 header-processing function fails to validate extension-option lengths and repeatedly processes attacker-controlled length values without reaching an exit. An affected device must expose the IPv6 stack to network traffic; exploitation requires no authentication or user interaction and interrupts availability without reported data access or modification.

CVSS severity8.7
High
EPSS probability1.88%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Infinite-loop denial of service in Siemens Nucleus NET-based products allows remote attackers to hang devices with crafted IPv6 packets. The IPv6 header-processing function fails to validate extension-option lengths and repeatedly processes attacker-controlled length values without reaching an exit. An affected device must expose the IPv6 stack to network traffic; exploitation requires no authentication or user interaction and interrupts availability without reported data access or modification.

The record

CVE
CVE-2021-25663
Published
Apr 22, 2021
Updated
Mar 11, 2025
Vendor
Siemens
Product
Capital Embedded AR Classic
Classifications
CWE-835, T1499.004
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Apr 22, 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 · Nucleus Source Codeversion=0 <*
Siemens · Nucleus ReadyStart V4version=0 <V4.1.0
Siemens · Nucleus NET for Nucleus PLUSversion=All versions
Siemens · Nucleus ReadyStart V3version=0 <V2017.02.4

Published CVSS scores

7.5NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
7.5Siemens ProductCERTCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
8.7Siemens ProductCERTCVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N
5.0NIST NVDAV:N/AC:L/Au:N/C:N/I:N/A:P

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-835Loop with Unreachable Exit Condition ('Infinite Loop')
T1499.004Application or System Exploitation

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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