How severe is CVE-2021-3866?

CVE-2021-3866 has a CVSS 3.x base score of 5.4, rated medium severity. It is exploitable over network with low attack complexity, requires low privileges and user interaction. Impact on confidentiality is low, integrity low, and availability none.

Is CVE-2021-3866 being actively exploited?

It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 1% (55th percentile), an estimate of the probability of exploitation in the next 30 days.

What products are affected by CVE-2021-3866?

CVE-2021-3866 affects Zulip. See the affected-products list for the exact vulnerable versions.

How do I fix CVE-2021-3866?

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

When was CVE-2021-3866 published?

CVE-2021-3866 was published on 2022-01-20 and last updated on 2026-06-17.

CVE-2021-3866

CVE-2021-3866 is a medium-severity vulnerability in Zulip with a CVSS 3.x base score of 5.4. 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-79.

Key facts

Severity: Medium (CVSS 3.x base score 5.4)
CVSS v2: 3.5
EPSS exploit prediction: 1% (55th percentile)
Actively exploited: Not listed in CISA KEV
Weakness: CWE-79
Affected product: Zulip
Published: 2022-01-20
Last modified: 2026-06-17

Description

Cross-site Scripting (XSS) - Stored in GitHub repository zulip/zulip more than and including 44f935695d452cc3fb16845a0c6af710438b153d and prior to 3eb2791c3e9695f7d37ffe84e0c2184fae665cb6.

Frequently asked questions

What is CVE-2021-3866?: Cross-site Scripting (XSS) - Stored in GitHub repository zulip/zulip more than and including 44f935695d452cc3fb16845a0c6af710438b153d and prior to 3eb2791c3e9695f7d37ffe84e0c2184fae665cb6.
How severe is CVE-2021-3866?: CVE-2021-3866 has a CVSS 3.x base score of 5.4, rated medium severity. It is exploitable over network with low attack complexity, requires low privileges and user interaction. Impact on confidentiality is low, integrity low, and availability none.
Is CVE-2021-3866 being actively exploited?: It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 1% (55th percentile), an estimate of the probability of exploitation in the next 30 days.
What products are affected by CVE-2021-3866?: CVE-2021-3866 affects Zulip. See the affected-products list for the exact vulnerable versions.
How do I fix CVE-2021-3866?: Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround.
When was CVE-2021-3866 published?: CVE-2021-3866 was published on 2022-01-20 and last updated on 2026-06-17.

References

Affected products (1)

cpe:2.3:a:zulip:zulip:*:*:*:*:*:*:*:*

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