CVE-2021-21002Network attack vector

Temporary denial of service via invalid Modbus response

Published Jun 25, 2021 · Updated Sep 17, 2024

Denial of service in Phoenix Contact FL COMSERVER UNI before 2.40 allows remote attackers to interrupt Modbus communications. Processing an invalid Modbus exception response as a query stops Modbus communication and leaves the appliance unresponsive for several minutes. Network access to the Modbus communication partner is sufficient; the reported consequence is a temporary service interruption rather than persistent loss of function.

CVSS severity7.5
High
EPSS probability0.96%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Denial of service in Phoenix Contact FL COMSERVER UNI before 2.40 allows remote attackers to interrupt Modbus communications. Processing an invalid Modbus exception response as a query stops Modbus communication and leaves the appliance unresponsive for several minutes. Network access to the Modbus communication partner is sufficient; the reported consequence is a temporary service interruption rather than persistent loss of function.

The record

CVE
CVE-2021-21002
Published
Jun 25, 2021
Updated
Sep 17, 2024
Vendor
Phoenix Contact
Product
FL COMSERVER
Classifications
CWE-772, T1499.004
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Jun 25, 2021CVE publishedPublication date reported by the CVE source.
  2. Sep 17, 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=UNI 232/422/485 (2313452) <2.40
  2. Affected versionversion=UNI 232/422/485-T (2904817) <2.40

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Phoenix Contact · FL COMSERVERversion=UNI 232/422/485 (2313452) <2.40; version=UNI 232/422/485-T (2904817) <2.40

Published CVSS scores

7.5CERT VDECVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
7.5Phoenix ContactCVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
5.0NVDAV:N/AC:L/Au:N/C:N/I:N/A:P

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-772Missing Release of Resource after Effective Lifetime
T1499.004Application 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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