How severe is CVE-2021-39509?

CVE-2021-39509 has a CVSS 3.x base score of 9.8, rated critical severity. It is exploitable over network with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is high, integrity high, and availability high.

Is CVE-2021-39509 being actively exploited?

It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 5% (91st percentile), an estimate of the probability of exploitation in the next 30 days.

What products are affected by CVE-2021-39509?

CVE-2021-39509 affects Dlink Dir-816 Firmware. See the affected-products list for the exact vulnerable versions.

How do I fix CVE-2021-39509?

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

When was CVE-2021-39509 published?

CVE-2021-39509 was published on 2021-08-24 and last updated on 2026-06-17.

CVE-2021-39509

CVE-2021-39509 is a critical-severity vulnerability in Dlink Dir-816 Firmware with a CVSS 3.x base score of 9.8. 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-77.

Key facts

Severity: Critical (CVSS 3.x base score 9.8)
CVSS v2: 7.5
EPSS exploit prediction: 5% (91st percentile)
Actively exploited: Not listed in CISA KEV
Weakness: CWE-77
Affected product: Dlink Dir-816 Firmware
Published: 2021-08-24
Last modified: 2026-06-17

Description

An issue was discovered in D-Link DIR-816 DIR-816A2_FWv1.10CNB05_R1B011D88210 The HTTP request parameter is used in the handler function of /goform/form2userconfig.cgi route, which can construct the user name string to delete the user function. This can lead to command injection through shell metacharacters.

Frequently asked questions

What is CVE-2021-39509?: An issue was discovered in D-Link DIR-816 DIR-816A2_FWv1.10CNB05_R1B011D88210 The HTTP request parameter is used in the handler function of /goform/form2userconfig.cgi route, which can construct the user name string to delete the user function. This can lead to command injection through shell metacharacters.
How severe is CVE-2021-39509?: CVE-2021-39509 has a CVSS 3.x base score of 9.8, rated critical severity. It is exploitable over network with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is high, integrity high, and availability high.
Is CVE-2021-39509 being actively exploited?: It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 5% (91st percentile), an estimate of the probability of exploitation in the next 30 days.
What products are affected by CVE-2021-39509?: CVE-2021-39509 affects Dlink Dir-816 Firmware. See the affected-products list for the exact vulnerable versions.
How do I fix CVE-2021-39509?: Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its critical severity, prioritise patching exposed systems.
When was CVE-2021-39509 published?: CVE-2021-39509 was published on 2021-08-24 and last updated on 2026-06-17.

References

Affected products (1)

cpe:2.3:o:dlink:dir-816_firmware:1.10cnb05_r1b011d88210:*:*:*:*:*:*:*

More vulnerabilities in Dlink Dir-816 Firmware

CVE-2026-4184 — Critical (CVSS 9.8): A vulnerability was detected in D-Link DIR-816 1.10CNB05. Affected by this vulnerability is an unknown functionality of…
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CVE-2026-4182 — Critical (CVSS 9.8): A weakness has been identified in D-Link DIR-816 1.10CNB05. This impacts an unknown function of the file…
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