CVE-2021-33720Network attack vector

Service interruption via unchecked port 4443 packets

Published Sep 14, 2021 · Updated Aug 3, 2024

Classic buffer overflow in Siemens SIPROTEC 5 CP050, CP100, and CP300 relay firmware before 8.80 allows remote attackers to interrupt service. The port 4443/TCP packet handler copies data from specially crafted packets without checking its size, although Siemens has not disclosed the affected function or buffer. No authentication or user interaction is required; successful exploitation affects availability only, and public records do not specify whether the relay hangs, crashes, or reboots.

CVSS severity7.5
High
EPSS probability1.04%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Classic buffer overflow in Siemens SIPROTEC 5 CP050, CP100, and CP300 relay firmware before 8.80 allows remote attackers to interrupt service. The port 4443/TCP packet handler copies data from specially crafted packets without checking its size, although Siemens has not disclosed the affected function or buffer. No authentication or user interaction is required; successful exploitation affects availability only, and public records do not specify whether the relay hangs, crashes, or reboots.

The record

CVE
CVE-2021-33720
Published
Sep 14, 2021
Updated
Aug 3, 2024
Vendor
Siemens
Product
SIPROTEC 5 relays with CPU variant CP100
Classifications
CWE-120, T1499.004
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Sep 14, 2021CVE publishedPublication date reported by the CVE source.
  2. Aug 3, 2024Record 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 < V8.80

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Siemens · SIPROTEC 5 relays with CPU variant CP100version=All versions < V8.80
Siemens · SIPROTEC 5 relays with CPU variant CP050version=All versions < V8.80
Siemens · SIPROTEC 5 relays with CPU variant CP300version=All versions < V8.80

Published CVSS scores

5.0NIST NVDAV:N/AC:L/Au:N/C:N/I:N/A:P
7.5NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

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
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-120Buffer Copy without Checking Size of Input ('Classic Buffer Overflow')
T1499.004Endpoint Denial of Service: Application or System Exploitation

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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