CVE-2021-25662Network attack vector

Service interruption via mishandled SmartVNC exception

Published May 12, 2021 · Updated Jun 2, 2026

SmartVNC client exception handling in affected Siemens SIMATIC HMI and WinCC products allows remote authenticated users to interrupt service. After a server sends a packet, a change to the program execution process raises an exception that the SmartVNC client fails to handle properly. The attacker must reach the SmartVNC service on TCP port 5900 with low privileges; the reported consequence is limited to denial of service.

CVSS severity7.5
High
EPSS probability1.64%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

SmartVNC client exception handling in affected Siemens SIMATIC HMI and WinCC products allows remote authenticated users to interrupt service. After a server sends a packet, a change to the program execution process raises an exception that the SmartVNC client fails to handle properly. The attacker must reach the SmartVNC service on TCP port 5900 with low privileges; the reported consequence is limited to denial of service.

The record

CVE
CVE-2021-25662
Published
May 12, 2021
Updated
Jun 2, 2026
Vendor
Siemens
Product
SIMATIC WinCC Runtime Advanced
Classifications
CWE-755, T1499
Attack vector
network
Privileges
authenticated

Timeline

How it unfolded

  1. May 12, 2021CVE publishedPublication date reported by the CVE source.
  2. 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.

  1. Affected versionversion=All versions < V15.1 Update 6
  2. Affected versionversion=All versions < V16 Update 4

What conditions does exploitation require?

Attack vectornetwork
Required privilegesauthenticated

What is affected?

Siemens · SIMATIC WinCC Runtime Advancedversion=All versions < V15.1 Update 6; version=All versions < V16 Update 4
Siemens · SIMATIC HMI Comfort Outdoor Panelsversion=All versions < V15.1 Update 6; version=All versions < V16 Update 4
Siemens · SIMATIC HMI Comfort Panelsversion=All versions < V15.1 Update 6; version=All versions < V16 Update 4
Siemens · SIMATIC HMI KTP Mobile Panelsversion=All versions < V15.1 Update 6; version=All versions < V16 Update 4

Published CVSS scores

5.0NVDAV:N/AC:L/Au:N/C:N/I:N/A:P
7.5NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
5.9Siemens ProductCERTCVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:L/I:N/A:H/E:P/RL:O/RC:C

CVSS describes severity. EPSS estimates exploitation probability.

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.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-755Improper Handling of Exceptional Conditions
T1499Endpoint Denial of Service

Public exploit references

No public exploit references are available in this record.

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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