CVE-2021-37186

Connection spoofing via predictable TCP sequence numbers

Published Sep 14, 2021 · Updated Aug 4, 2024

Weak TCP sequence generation in Siemens LOGO! CMR and SIMATIC RTU devices allows adjacent attackers to spoof connections and access sensitive data. The underlying TCP/IP stack generates insufficiently random initial sequence numbers, making established TCP traffic predictable enough to impersonate an endpoint. Exploitation requires unauthenticated access to the device LAN; it permits traffic interference and sensitive-data disclosure without a reported availability impact.

CVSS severity5.4
Medium
EPSS probability0.36%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Weak TCP sequence generation in Siemens LOGO! CMR and SIMATIC RTU devices allows adjacent attackers to spoof connections and access sensitive data. The underlying TCP/IP stack generates insufficiently random initial sequence numbers, making established TCP traffic predictable enough to impersonate an endpoint. Exploitation requires unauthenticated access to the device LAN; it permits traffic interference and sensitive-data disclosure without a reported availability impact.

The record

CVE
CVE-2021-37186
Published
Sep 14, 2021
Updated
Aug 4, 2024
Vendor
Siemens
Product
SIMATIC RTU3030C
Classifications
CWE-330, T1557
Attack vector
adjacent
Privileges
unauthenticated

Timeline

How it unfolded

  1. Sep 14, 2021CVE publishedPublication date reported by the CVE source.
  2. Aug 4, 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 < V4.0.9

What conditions does exploitation require?

Attack vectoradjacent
Required privilegesunauthenticated

What is affected?

Siemens · SIMATIC RTU3030Cversion=All versions < V4.0.9
Siemens · SIMATIC RTU3041Cversion=All versions < V4.0.9
Siemens · LOGO! CMR2040version=All versions < V2.2
Siemens · LOGO! CMR2020version=All versions < V2.2
Siemens · SIMATIC RTU3010Cversion=All versions < V4.0.9
Siemens · SIMATIC RTU3031Cversion=All versions < V4.0.9

Published CVSS scores

5.4NIST NVDCVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N
4.8NIST NVDAV:A/AC:L/Au:N/C:P/I:P/A:N

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 9, 2026Sep 15, 2026
Latest reporting daySep 15, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectoradjacent
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-330Use of Insufficiently Random Values
T1557Adversary-in-the-Middle

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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