CVE-2021-21003Network attack vector

Management-service interruption via fragmented TCP packets

Published Jun 25, 2021 · Updated Sep 17, 2024

Denial of service in Phoenix Contact FL SWITCH 4.70 and earlier and FL NAT 4.63 and earlier allows remote attackers to interrupt management services. Fragmented TCP packets make the firmware stop serving web, SNMP, and ICMP Echo requests; the underlying resource-handling defect is undisclosed. Unauthenticated network access is sufficient, but the interruption does not affect the devices' packet-switching functionality.

CVSS severity5.3
Medium
EPSS probability0.95%
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 SWITCH 4.70 and earlier and FL NAT 4.63 and earlier allows remote attackers to interrupt management services. Fragmented TCP packets make the firmware stop serving web, SNMP, and ICMP Echo requests; the underlying resource-handling defect is undisclosed. Unauthenticated network access is sufficient, but the interruption does not affect the devices' packet-switching functionality.

The record

CVE
CVE-2021-21003
Published
Jun 25, 2021
Updated
Sep 17, 2024
Vendor
Unknown vendor
Product
Unknown product
Classifications
CWE-404, T1499
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?

Affected buildAffected product and version details are unavailable.

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Affected products and versions are unavailable in this record.

Published CVSS scores

5.3CERT VDECVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L
5.3Phoenix ContactCVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L
5.0NIST NVDAV: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-404Improper Resource Shutdown or Release
T1499Endpoint Denial of Service

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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