How severe is CVE-2021-21005?

CVE-2021-21005 has a CVSS 3.x base score of 7.5, rated high severity. It is exploitable over network with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is none, integrity none, and availability high.

Is CVE-2021-21005 being actively exploited?

It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 1% (48th percentile), an estimate of the probability of exploitation in the next 30 days.

What products are affected by CVE-2021-21005?

CVE-2021-21005 primarily affects Phoenixcontact Fl Switch Smcs 16tx Firmware. In total, 15 product configurations (CPEs) are listed as vulnerable; see the affected-products list for the exact versions.

How do I fix CVE-2021-21005?

Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its high severity, prioritise patching exposed systems.

When was CVE-2021-21005 published?

CVE-2021-21005 was published on 2021-06-25 and last updated on 2026-06-17.

CVE-2021-21005

CVE-2021-21005 is a high-severity vulnerability in Phoenixcontact Fl Switch Smcs 16tx Firmware with a CVSS 3.x base score of 7.5. It is not currently listed as actively exploited by CISA, and its EPSS exploit-prediction score is low. The underlying weakness is classified as CWE-362.

Key facts

Severity: High (CVSS 3.x base score 7.5)
CVSS v2: 7.8
EPSS exploit prediction: 1% (48th percentile)
Actively exploited: Not listed in CISA KEV
Weakness: CWE-362
Affected product: Phoenixcontact Fl Switch Smcs 16tx Firmware
Published: 2021-06-25
Last modified: 2026-06-17

Description

In Phoenix Contact FL SWITCH SMCS series products in multiple versions if an attacker sends a hand-crafted TCP-Packet with the Urgent-Flag set and the Urgent-Pointer set to 0, the network stack will crash. The device needs to be rebooted afterwards.

Frequently asked questions

What is CVE-2021-21005?: In Phoenix Contact FL SWITCH SMCS series products in multiple versions if an attacker sends a hand-crafted TCP-Packet with the Urgent-Flag set and the Urgent-Pointer set to 0, the network stack will crash. The device needs to be rebooted afterwards.
How severe is CVE-2021-21005?: CVE-2021-21005 has a CVSS 3.x base score of 7.5, rated high severity. It is exploitable over network with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is none, integrity none, and availability high.
Is CVE-2021-21005 being actively exploited?: It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 1% (48th percentile), an estimate of the probability of exploitation in the next 30 days.
What products are affected by CVE-2021-21005?: CVE-2021-21005 primarily affects Phoenixcontact Fl Switch Smcs 16tx Firmware. In total, 15 product configurations (CPEs) are listed as vulnerable; see the affected-products list for the exact versions.
How do I fix CVE-2021-21005?: Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its high severity, prioritise patching exposed systems.
When was CVE-2021-21005 published?: CVE-2021-21005 was published on 2021-06-25 and last updated on 2026-06-17.

References

https://cert.vde.com/en-us/advisories/vde-2021-023

Affected products (15)

cpe:2.3:o:phoenixcontact:fl_switch_smcs_16tx_firmware:*:*:*:*:*:*:*:*
cpe:2.3:o:phoenixcontact:fl_switch_smcs_14tx\/2fx_firmware:*:*:*:*:*:*:*:*
cpe:2.3:o:phoenixcontact:fl_switch_smcs_14tx\/2fx-sm_firmware:*:*:*:*:*:*:*:*
cpe:2.3:o:phoenixcontact:fl_switch_smcs_8gt_firmware:*:*:*:*:*:*:*:*
cpe:2.3:o:phoenixcontact:fl_switch_smcs_6gt\/2sfp_firmware:*:*:*:*:*:*:*:*
cpe:2.3:o:phoenixcontact:fl_switch_smcs_8tx-pn_firmware:*:*:*:*:*:*:*:*
cpe:2.3:o:phoenixcontact:fl_switch_smcs_4tx-pn_firmware:*:*:*:*:*:*:*:*
cpe:2.3:o:phoenixcontact:fl_switch_smcs_8tx_firmware:*:*:*:*:*:*:*:*
cpe:2.3:o:phoenixcontact:fl_switch_smcs_6tx\/2sfp_firmware:*:*:*:*:*:*:*:*
cpe:2.3:o:phoenixcontact:fl_switch_smn_6tx\/2pof-pn_firmware:*:*:*:*:*:*:*:*
cpe:2.3:o:phoenixcontact:fl_switch_smn_8tx-pn_firmware:*:*:*:*:*:*:*:*
cpe:2.3:o:phoenixcontact:fl_switch_smn_6tx\/2fx_firmware:*:*:*:*:*:*:*:*
cpe:2.3:o:phoenixcontact:fl_switch_smn_6tx\/2fx_sm_firmware:*:*:*:*:*:*:*:*
cpe:2.3:o:phoenixcontact:fl_nat_smn_8tx_firmware:*:*:*:*:*:*:*:*
cpe:2.3:o:phoenixcontact:fl_nat_smn_8tx-m_firmware:*:*:*:*:*:*:*:*

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