CVE-2021-21005Network attack vector

Network-stack crash via zero urgent pointer

Published Jun 25, 2021 · Updated Sep 17, 2024

Denial of service in Phoenix Contact FL SWITCH and FL NAT firmware allows remote attackers to crash the network stack with a crafted TCP packet. The network stack mishandles a TCP segment whose Urgent flag is set while its Urgent Pointer is zero, terminating network processing. No authentication or user interaction is required; network access is sufficient, and recovery requires a device reboot.

CVSS severity7.5
High
EPSS probability0.68%
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 and FL NAT firmware allows remote attackers to crash the network stack with a crafted TCP packet. The network stack mishandles a TCP segment whose Urgent flag is set while its Urgent Pointer is zero, terminating network processing. No authentication or user interaction is required; network access is sufficient, and recovery requires a device reboot.

The record

CVE
CVE-2021-21005
Published
Jun 25, 2021
Updated
Sep 17, 2024
Vendor
Unknown vendor
Product
Unknown product
Classifications
CWE-362, T0814
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

Compare your product and version with the public record. A matching version still requires validation against your environment.

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

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
7.8NVDAV:N/AC:L/Au:N/C:N/I:N/A: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 9, 2026Sep 15, 2026
Latest reporting daySep 15, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-362Concurrent Execution using Shared Resource with Improper Synchronization ('Race Condition')
T0814Denial 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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