No exploitation reportedFix available

CVE-2021-25663

Q: Is CVE-2021-25663 actively exploited?

Active exploitation of CVE-2021-25663 has not been confirmed. Its EPSS score was 1.9% on 2026-10-22, the estimated probability of exploitation in the next 30 days.

Q: What is the CVSS score for CVE-2021-25663?

CVE-2021-25663 has a CVSS v3 base score of 7.5 (HIGH severity), with vector string CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H.

No exploitation reported by the sources we read.

Published2021-04-22
Fixed in2017.02.4 · 4.1.0
WeaknessCWE-835

Am I affected?

With a domain, we read the version its public headers and pages expose. A distribution build can carry the fix under an older number.

Description

A vulnerability has been identified in Capital Embedded AR Classic 431-422 (All versions), Capital Embedded AR Classic R20-11 (All versions < V2303), Nucleus NET (All versions), Nucleus ReadyStart V3 (All versions < V2017.02.4), Nucleus ReadyStart V4 (All versions < V4.1.0), Nucleus Source Code (All versions including affected IPv6 stack). The function that processes IPv6 headers does not check the lengths of extension header options, allowing attackers to put this function into an infinite loop with crafted length values.

Timeline

dates in UTC

2021-04-2221:15CVE publishedNVD

Observed activity

isMalicious corpus · updated 2026-10-23 18:48 UTC

Indicators tagged with this CVE, per day0 indicators over 7 days

Block these indicators

Firewall list (EDL, text): https://api.ismalicious.com/edl/cve/CVE-2021-25663?format=txt
CSV export: https://api.ismalicious.com/edl/cve/CVE-2021-25663?format=csv
TAXII collection of exploited CVEs
The EDL and CSV take an API key on the Pro plan.

Versions per branch

NVD

Siemens nucleus readystart

Branch	Affected	Fixed in	Source
2017.2	< 2017.02.4	2017.02.4	NVD
4.1	< 4.1.0	4.1.0	NVD

Siemens capital vstar

Branch	Affected	Fixed in	Source
—	all versions	no fix published	NVD

Siemens nucleus net

Branch	Affected	Fixed in	Source
—	all versions	no fix published	NVD

1 more product in the sources.

as publishedRanges as NVD and OSV publish them; a distribution’s build can be fixed under an older number.

Distribution builds and vendor advisories

Packages from OSV.dev and products from the vendors' CSAF advisories.

version not verifiedVersions not verified: read as OSV.dev and the vendors publish them, not checked against each vendor's release list.

Vendor advisories

ICSA-21-103-05 CISA6 known affected
Siemens Nucleus Products IPv6 Stack
Capital Embedded AR Classic 431-422 · Capital Embedded AR Classic R20-11 · Nucleus NET · +3 more

Full record on NVD

Fix

Update Siemens nucleus readystart to 2017.02.4 · 4.1.0
Vendor fix CISA ICSA-21-103-05 · 1 productUpdate to V2017.02.4 or later version
Vendor fix CISA ICSA-21-103-05 · 1 productUpdate to V2303 or later version
Vendor fix CISA ICSA-21-103-05 · 1 productUpdate to V4.1.0 or later version
Vendor fix CISA ICSA-21-103-05 · 1 productContact customer support to receive patch and update information
Mitigation from the vendor CISA ICSA-21-103-05 · 1 productContact customer support or your local Nucleus Sales team for mitigation advice
Mitigation from the vendor CISA ICSA-21-103-05 · 2 productsDisable IPv6 functionality, if feature not used, by deselecting the TcpIpIpV6General/IpV6Enabled Pre-Compile configuration option

Advisories and patches

cert-portal.siemens.comVendor Advisory

Detection

Nuclei template: none so far
Exploit-DB: no entry
Proof-of-concept repositories: none listed

Sources

CNACVSS 4.08.7read 2026-10-23
CNACVSS 3.17.5read 2026-10-23
NVDCVSS 3.17.5read 2026-10-23
NVDCVSS 2.05read 2026-10-23
CNAweakness (CWE)CWE-835read 2026-10-23
NVDweakness (CWE)CWE-835read 2026-10-23
FIRST EPSSEPSS score1.9 %2026-10-22

Technical details

CVSS v3 Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
CVSS 4.0 vector: CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X
Published: 2021-04-22
Last Modified: 2026-06-17
NVD status: Modified

Frequently asked questions

What is CVE-2021-25663?

Is CVE-2021-25663 actively exploited?

Active exploitation of CVE-2021-25663 has not been confirmed. Its EPSS score was 1.9% on 2026-10-22, the estimated probability of exploitation in the next 30 days.

What is the CVSS score for CVE-2021-25663?

CVE-2021-25663 has a CVSS v3 base score of 7.5 (HIGH severity), with vector string CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H.

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