CVE-2021-3967Network attack vector

API credential acquisition via cookie-authenticated regeneration

Published Feb 26, 2022 · Updated Aug 3, 2024

Improper access control in Zulip Server before 4.10 allows remote attackers with a valid session cookie to obtain a replacement API key. The API-key regeneration handler was registered under the cookie-authenticated web namespace, allowing a logged-in browser session to invoke the handler without presenting the existing API key. A valid browser session was the only gate; successful exploitation returned a new persistent API credential and invalidated the previous key.

CVSS severity8.8
High
EPSS probability0.84%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
Coverage for this vulnerability

See if you're affected

Explore vulnerability checks for your environment with Hinoki.

Book a demo

Summary

What happened

Improper access control in Zulip Server before 4.10 allows remote attackers with a valid session cookie to obtain a replacement API key. The API-key regeneration handler was registered under the cookie-authenticated web namespace, allowing a logged-in browser session to invoke the handler without presenting the existing API key. A valid browser session was the only gate; successful exploitation returned a new persistent API credential and invalidated the previous key.

The record

CVE
CVE-2021-3967
Published
Feb 26, 2022
Updated
Aug 3, 2024
Vendor
Zulip
Product
Zulip Server
Classifications
CWE-284, T1098
Attack vector
network
Privileges
authenticated

Timeline

How it unfolded

  1. Feb 26, 2022CVE publishedPublication date reported by the CVE source.
  2. Aug 3, 2024Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

Compare your product and version with the public record. A matching version still requires validation against your environment.

Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=unspecified <4.10

What conditions does exploitation require?

Attack vectornetwork
Required privilegesauthenticated

What is affected?

Zulip · Zulip Serverversion=unspecified <4.10

Published CVSS scores

8.8NIST NVDCVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
6.5NIST NVDAV:N/AC:L/Au:S/C:P/I:P/A:P
4.1huntr.devCVSS:3.0/AV:N/AC:H/PR:H/UI:N/S:U/C:L/I:L/A:L

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-284Improper Access Control
T1098Account Manipulation

Public exploit references

No public exploit references are available in this record.

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

See the directory against your own environment.

Your stack

Check the software in your environment

Book a demo to see how Hinoki identifies affected software and validates exploitability in your environment.

Book a demo