CVE-2021-25664Network attack vector

Remote denial of service via IPv6 loop

Published Apr 22, 2021 · Updated Mar 11, 2025

Denial-of-service in Siemens Nucleus NET IPv6 processing allows remote attackers to interrupt service via crafted Hop-by-Hop packets. The IPv6 Hop-by-Hop extension-header handler accepts arbitrary length values without validating header bounds, so packet processing can loop indefinitely. No authentication or user interaction is required, and the bounded consequence is a network-service hang rather than stated data disclosure or modification.

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

See if you're affected

Explore vulnerability checks for your environment with Hinoki.

Book a demo

Summary

What happened

Denial-of-service in Siemens Nucleus NET IPv6 processing allows remote attackers to interrupt service via crafted Hop-by-Hop packets. The IPv6 Hop-by-Hop extension-header handler accepts arbitrary length values without validating header bounds, so packet processing can loop indefinitely. No authentication or user interaction is required, and the bounded consequence is a network-service hang rather than stated data disclosure or modification.

The record

CVE
CVE-2021-25664
Published
Apr 22, 2021
Updated
Mar 11, 2025
Vendor
Siemens
Product
Capital Embedded AR Classic
Classifications
CWE-835, T1498
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

Compare your product and version with the public record. A matching version still requires validation against your environment.

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

7.5Siemens AGCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H/E:P/RL:O/RC:C
8.7Siemens AGCVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA: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 9, 2026Sep 15, 2026
Latest reporting daySep 15, 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')
T1498Network Denial of Service

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

See the directory against your own environment.

Your stack

Check the software in your environment

Book a demo to see how Hinoki identifies affected software and validates exploitability in your environment.

Book a demo