Security readout for executives and security teams
Plain-English summary
This Siemens vulnerability can let a network attacker crash affected HMI or WinCC Runtime Advanced SmartVNC client functionality, causing loss of availability. It is not described as data theft or control-system manipulation in the provided sources, but outages in operator interfaces can still disrupt industrial monitoring and response.
Executive priority
Treat this as a high-priority operational resilience issue for industrial environments using affected Siemens operator interfaces. Prioritize systems supporting safety, production continuity, or remote operations, especially where VNC-related connectivity crosses less trusted network segments.
Technical view
CVE-2021-25662 is a CWE-755 improper exceptional condition handling issue in Siemens SmartVNC client handling. If execution is modified after a server packet is sent, affected SIMATIC HMI panels and WinCC Runtime Advanced versions can enter a denial-of-service condition. CVSS v3.1 is 7.5: network reachable, low complexity, no privileges, no user interaction, availability impact only.
Likely exposure
Exposure is most likely where affected Siemens SIMATIC HMI Comfort, Comfort Outdoor, KTP Mobile Panels, or WinCC Runtime Advanced V15/V16 systems can reach untrusted VNC/SmartVNC servers or networks. Products below V15.1 Update 6 or V16 Update 4 are listed as affected.
Exploitation context
The source bundle does not cite active exploitation, and the CVE is not marked CISA KEV. The CVSS vector indicates remote network attack conditions without authentication or user interaction, but the documented impact is denial of service rather than confidentiality or integrity compromise.
Researcher notes
The provided evidence supports a denial-of-service assessment only. Do not infer code execution, privilege escalation, or active exploitation. Validation should focus on exact Siemens product/version mapping, SmartVNC exposure paths, and whether vendor update thresholds have been met.
Mitigation direction
- Inventory affected Siemens HMI panels and WinCC Runtime Advanced V15/V16 installations.
- Upgrade V15 deployments to V15.1 Update 6 or later where applicable.
- Upgrade V16 deployments to V16 Update 4 or later where applicable.
- Review Siemens SSA-538778 and CISA ICSA-21-131-12 for vendor-specific guidance.
- Limit SmartVNC/VNC connectivity to trusted industrial management networks.
Validation and detection
- Confirm installed product names and major versions against the affected product list.
- Verify V15 systems are at V15.1 Update 6 or later.
- Verify V16 systems are at V16 Update 4 or later.
- Check network paths between HMI/WinCC systems and VNC/SmartVNC servers.
- Monitor affected interfaces for unexpected crashes or availability loss.
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-755: 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-2021-25662 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
- 7.5 (3.1)
- Known Exploited
- No
- Published
Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
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.
SSVC decision data
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:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H3.93.6CISA-ADPVulnerability scoring details
Base CVSS 3.1 score
7.5HighVector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Vulnerability timeline
Timeline events are normalized from CVE metadata, CNA source timelines, ADP timelines, and KEV metadata when present.
- CVE reservedCVE Program
The CVE ID was reserved by the assigning CNA.
- CVE publishedCVE Program
The CVE record was published.
- CVE updatedCVE Program
The CVE record metadata indicates this as the latest update time.
ADP provider summaries
Source materials
- CVE List V5 sourceCVE List V5
- https://cert-portal.siemens.com/productcert/pdf/ssa-538778.pdfCVE reference, siemens · x_refsource_MISC
- https://us-cert.cisa.gov/ics/advisories/icsa-21-131-12CVE reference, siemens · x_refsource_MISC
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.
Improper Handling of Exceptional Conditions
Improper Handling of Exceptional Conditions represents a recurring weakness pattern that can create exploitable paths when design, validation, or implementation controls are missing.