What is CVE-2021-3967?

CVE-2021-3967 is a high-severity vulnerability in Zulip, classified under Improper Access Control. CVSS score: 8.8/10. Published 2022-02-26.

How severe is CVE-2021-3967?

High severity. CVSS v3 base score is 8.8 out of 10.

Is CVE-2021-3967 known to be exploited?

2 public proof-of-concept repositories are indexed. Not currently listed in the CISA KEV catalog.

Vulnerability in Zulip

CVE-2021-3967

Improper Access Control in GitHub repository zulip/zulip prior to 4.10.

Published 2022-02-26 · last modified 2026-06-17.

CVSS v3 metric

CVSS v3 base score 8.8 (High). Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H.

EPSS exploit prediction

EPSS: 0.008 (57.3th percentile), scored 2026-10-23.

Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 57th percentile — 57.3% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.

EPSS over last 30 days · oldest: 0.008 · newest: 0.008 · change: 0.000

Affected products

Zulip
Zulip Zulip/zulip — versions unspecified

Weakness classification (CWE)

CWE-284 — Improper Access Control

Public proof-of-concept exploits

References

huntr.dev/bounties/2928a625-0467-4a0a-b4e2-e27322786686 (Exploit, Third Party Advisory)
github.com/zulip/zulip/commit/d5db254ca8167995a1654d1c45ffc74b2fade39a (Patch, Third Party Advisory)

Frequently asked questions

What is CVE-2021-3967?: CVE-2021-3967 is a high-severity vulnerability in Zulip, classified under Improper Access Control. CVSS score: 8.8/10. Published 2022-02-26.
How severe is CVE-2021-3967?: High severity. CVSS v3 base score is 8.8 out of 10.
Is CVE-2021-3967 known to be exploited?: 2 public proof-of-concept repositories are indexed. Not currently listed in the CISA KEV catalog.