Service interruption via out-of-bounds SmartVNC client access
Published May 12, 2021 · Updated Jun 2, 2026
Out-of-bounds memory access in SmartVNC across affected Siemens SIMATIC HMI and WinCC releases allows remote attackers to interrupt service. The SmartVNC client accesses memory beyond a buffer boundary while processing data supplied by a VNC server. Network access to TCP port 5900 and valid low-privileged VNC access are required, and the documented consequence is limited to denial of service.
Summary
What happened
Out-of-bounds memory access in SmartVNC across affected Siemens SIMATIC HMI and WinCC releases allows remote attackers to interrupt service. The SmartVNC client accesses memory beyond a buffer boundary while processing data supplied by a VNC server. Network access to TCP port 5900 and valid low-privileged VNC access are required, and the documented consequence is limited to denial of service.
The record
- CVE
- CVE-2021-25661
- Published
- May 12, 2021
- Updated
- Jun 2, 2026
- Vendor
- Siemens
- Product
- SIMATIC WinCC Runtime Advanced
- Classifications
- CWE-788, T1499.004
- Attack vector
- network
- Privileges
- authenticated
Timeline
How it unfolded
- May 12, 2021CVE publishedPublication date reported by the CVE source.
- Jun 2, 2026Record updatedLatest update available in the CVE record.
Exploitability
Present is not the same as exploitable
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Is a vulnerable build present?
Compare these published version ranges with your installed build and any vendor patches.
- Affected versionversion=All versions < V15.1 Update 6
- Affected versionversion=All versions < V16 Update 4
What conditions does exploitation require?
What is affected?
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.
Weakness, pattern, technique
Public exploit references
No public exploit references are available in this record.
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Technologies
Your stack
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Your stack
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