Security readout for executives and security teams
Plain-English summary
This vulnerability can let an unauthenticated network attacker make affected Siemens embedded networking components stop processing by trapping IPv6 header handling in an infinite loop. The main business risk is availability loss in systems that rely on these embedded stacks.
Executive priority
Treat as high priority for systems where downtime affects operations, safety, or contractual service levels. Focus first on internet-connected, partner-connected, or flat-network deployments, then coordinate vendor-supported upgrades or compensating controls.
Technical view
CVE-2021-25663 is a CWE-835 infinite loop in IPv6 extension header option length processing. Crafted length values can cause the parser to loop indefinitely. CVSS v4.0 is 8.7 with network attack vector, low complexity, no privileges, no user interaction, and high vulnerable-system availability impact.
Likely exposure
Exposure is most likely in environments using affected Siemens Capital Embedded AR Classic, Nucleus NET, Nucleus ReadyStart, or Nucleus Source Code with the affected IPv6 stack. Risk is higher where those systems are reachable over IPv6-capable networks.
Exploitation context
The provided bundle does not show CISA KEV listing or cited evidence of active exploitation. The condition is network reachable and unauthenticated, but evidence here only supports potential denial of service, not observed exploitation in the wild.
Researcher notes
The evidence supports an IPv6 parser availability flaw, not confidentiality or integrity compromise. Version boundaries are explicit for some products, but the bundle leaves remediation for products listed as all versions affected dependent on Siemens guidance.
Mitigation direction
- Review Siemens SSA-248289 and CISA ICSA-21-103-05 for product-specific guidance.
- 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 current Siemens remediation guidance.
Validation and detection
- Inventory Siemens embedded products and Nucleus-derived components in operational environments.
- Confirm versions against the affected ranges in Siemens SSA-248289.
- Determine whether the affected IPv6 stack is enabled or reachable on relevant networks.
- Check whether compensating network controls limit untrusted IPv6 access.
- Avoid malformed-packet testing on production systems without vendor-approved procedures.
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-2021-25663 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_CONFIRM
- 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.