CVE-2021-3866Network attack vector

Stored script execution via unescaped stream names

Published Jan 20, 2022 · Updated Aug 3, 2024

Stored XSS in Zulip Server development builds allows remote authenticated users to execute JavaScript through crafted stream names. The stream-name rendering path places attacker-controlled names into page output without neutralizing executable markup. Exploitation requires permission to create or rename streams and affects other users when their browsers render the stored name; official releases were unaffected.

CVSS severity6.8
Medium
EPSS probability0.89%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Stored XSS in Zulip Server development builds allows remote authenticated users to execute JavaScript through crafted stream names. The stream-name rendering path places attacker-controlled names into page output without neutralizing executable markup. Exploitation requires permission to create or rename streams and affects other users when their browsers render the stored name; official releases were unaffected.

The record

CVE
CVE-2021-3866
Published
Jan 20, 2022
Updated
Aug 3, 2024
Vendor
Zulip
Product
Zulip Server
Classifications
CWE-79, T1203
Attack vector
network
Privileges
authenticated

Timeline

How it unfolded

  1. Jan 20, 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=44f935695d452cc3fb16845a0c6af710438b153d <unspecified
  2. Affected versionversion=unspecified <3eb2791c3e9695f7d37ffe84e0c2184fae665cb6

What conditions does exploitation require?

Attack vectornetwork
Required privilegesauthenticated

What is affected?

Zulip · Zulip Serverversion=44f935695d452cc3fb16845a0c6af710438b153d <unspecified; version=unspecified <3eb2791c3e9695f7d37ffe84e0c2184fae665cb6

Published CVSS scores

6.8@huntrdevCVSS:3.0/AV:N/AC:H/PR:H/UI:R/S:C/C:H/I:L/A:L
3.5NIST NVDAV:N/AC:M/Au:S/C:N/I:P/A:N
5.4NIST NVDCVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:L/I:L/A:N

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-79Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')
T1203Exploitation for Client Execution

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

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Your stack

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