How severe is CVE-2021-28166?

CVE-2021-28166 has a CVSS 3.x base score of 6.5, rated medium severity. It is exploitable over network with low attack complexity, requires low privileges and no user interaction. Impact on confidentiality is none, integrity none, and availability high.

Is CVE-2021-28166 being actively exploited?

It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 1% (58th percentile), an estimate of the probability of exploitation in the next 30 days.

What products are affected by CVE-2021-28166?

CVE-2021-28166 affects Eclipse Mosquitto. See the affected-products list for the exact vulnerable versions.

How do I fix CVE-2021-28166?

Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround.

When was CVE-2021-28166 published?

CVE-2021-28166 was published on 2021-04-07 and last updated on 2026-06-17.

CVE-2021-28166

CVE-2021-28166 is a medium-severity vulnerability in Eclipse Mosquitto with a CVSS 3.x base score of 6.5. 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-476.

Key facts

Severity: Medium (CVSS 3.x base score 6.5)
CVSS v2: 4.0
EPSS exploit prediction: 1% (58th percentile)
Actively exploited: Not listed in CISA KEV
Weakness: CWE-476
Affected product: Eclipse Mosquitto
Published: 2021-04-07
Last modified: 2026-06-17

Description

In Eclipse Mosquitto version 2.0.0 to 2.0.9, if an authenticated client that had connected with MQTT v5 sent a crafted CONNACK message to the broker, a NULL pointer dereference would occur.

Frequently asked questions

What is CVE-2021-28166?: In Eclipse Mosquitto version 2.0.0 to 2.0.9, if an authenticated client that had connected with MQTT v5 sent a crafted CONNACK message to the broker, a NULL pointer dereference would occur.
How severe is CVE-2021-28166?: CVE-2021-28166 has a CVSS 3.x base score of 6.5, rated medium severity. It is exploitable over network with low attack complexity, requires low privileges and no user interaction. Impact on confidentiality is none, integrity none, and availability high.
Is CVE-2021-28166 being actively exploited?: It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 1% (58th percentile), an estimate of the probability of exploitation in the next 30 days.
What products are affected by CVE-2021-28166?: CVE-2021-28166 affects Eclipse Mosquitto. See the affected-products list for the exact vulnerable versions.
How do I fix CVE-2021-28166?: Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround.
When was CVE-2021-28166 published?: CVE-2021-28166 was published on 2021-04-07 and last updated on 2026-06-17.

References

https://bugs.eclipse.org/bugs/show_bug.cgi?id=572608

Affected products (1)

cpe:2.3:a:eclipse:mosquitto:*:*:*:*:*:*:*:*

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