Security readout for executives and security teams
Plain-English summary
This CVE can let a remote unauthenticated actor disrupt affected embedded Siemens/Nucleus networking stacks by causing IPv6 packet processing to loop indefinitely. The business impact is availability: devices or services using the affected stack may stop responding or degrade until recovered.
Executive priority
Treat as a high-priority availability risk for embedded and OT assets. Prioritize systems that are network reachable, safety relevant, or difficult to restart, and require owners to confirm vendor remediation status.
Technical view
The vulnerable IPv6 Hop-by-Hop extension header option parser does not validate the header length field. Arbitrary length values can force an infinite loop. CVSS v4.0 is 8.7 with network attack vector, low complexity, no privileges, no user interaction, and high availability impact.
Likely exposure
Exposure is most likely in embedded or OT products using affected Siemens Capital Embedded AR Classic, Nucleus NET, Nucleus ReadyStart, or Nucleus Source Code with IPv6 processing reachable over a network.
Exploitation context
The source bundle does not show CISA KEV listing or other evidence of active exploitation. The issue is still operationally important because the described condition is remotely reachable and can affect availability in embedded environments.
Researcher notes
This is CWE-835 infinite loop behavior in IPv6 Hop-by-Hop extension header handling. The available evidence supports availability impact, not confidentiality or integrity compromise. Avoid assuming exploit-in-the-wild status without new KEV or vendor evidence.
Mitigation direction
- Review Siemens SSA-248289 and CISA ICSA-21-103-05 for vendor-specific remediation.
- Upgrade Nucleus ReadyStart V3 to V2017.02.4 or later where applicable.
- Upgrade Nucleus ReadyStart V4 to V4.1.0 or later where applicable.
- Upgrade Capital Embedded AR Classic R20-11 to V2303 or later where applicable.
- For products listed as all versions affected, obtain vendor guidance before accepting residual risk.
- Restrict unnecessary IPv6 exposure to affected embedded systems where operationally feasible.
Validation and detection
- Inventory products using the affected Siemens/Nucleus components and record exact versions.
- Identify systems where IPv6 packet processing is enabled or reachable.
- Compare deployed versions against the affected version ranges in the advisory.
- Check vendor advisories for product-specific fixed builds or compensating controls.
- Prioritize validation for externally reachable, safety-critical, or hard-to-recover devices.
Public sources used
Generated from the cited source records. This long-tail analysis has not been individually reviewed by a named human.
Conservative CVE-to-ATT&CK context
These mappings and lookup hints may be relevant to the vulnerability behavior, CWE, affected product, or exposure path. Glexia-inferred context is not an official MITRE, ATT&CK, CWE, or CVE Program mapping.
ATT&CK lookup starting points
Use these exact CWE pages and searches to review the Glexia ATT&CK library from this CVE's weakness and description context.
CWE-835: Exact CWE lookup
Use the exact CWE identifier as the starting point before reviewing related ATT&CK behavior. Open the exact CWE lookup page first, then review the ATT&CK searches from that MITRE weakness context. This is a Glexia lookup hint, not an official ATT&CK mapping.
Open ATT&CK lookupCVE-2021-25664 mapping review
Open the CVE-to-ATT&CK bridge for reviewed, inferred, or future official mappings tied to this CVE.
Open ATT&CK lookup- Severity
- High
- CVSS
- 8.7 (4.0)
- Known Exploited
- No
- Published
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
CNA and ADP enrichment extracted from CVE v5
These fields come from the CVE record and ADP containers, not from Glexia's Take. They preserve time-varying source decisions such as CISA SSVC, KEV status, CVSS metrics, and provider references.
CVSS vector scores
1 official scoreWe collect every scored CVSS vector available in the official CNA and ADP containers. When more than one version is present, the table keeps the source vectors side by side instead of collapsing them into the highest score.
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——Primary CVE scoreVulnerability scoring details
Base CVSS 4.0 score
8.7HighVector: 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
Source materials
- CVE List V5 sourceCVE List V5
- https://cert-portal.siemens.com/productcert/pdf/ssa-248289.pdfCVE reference · x_refsource_MISC
- https://us-cert.cisa.gov/ics/advisories/icsa-21-103-05CVE reference · x_refsource_MISC
- https://cert-portal.siemens.com/productcert/html/ssa-248289.htmlCVE reference
Products and packages named in the record
CWE details
CWE links open Glexia weakness intelligence pages with official CWE context, developer remediation guidance, and related CVE mappings.
Loop with Unreachable Exit Condition ('Infinite Loop')
Loop with Unreachable Exit Condition ('Infinite Loop') represents a recurring weakness pattern that can create exploitable paths when design, validation, or implementation controls are missing.