CVE-2023-22735Network attack vector

Session theft via unsafe inline S3 uploads

Published Feb 7, 2023 · Updated Mar 10, 2025

Stored cross-site scripting in Zulip Server main after 04cf68b and before 2f6c5a8 allows remote authenticated users to execute JavaScript. The S3 upload proxy trusted a browser-supplied Content-Type while deriving Content-Disposition only from the filename, so HTML disguised with an image extension was rendered inline without a protective policy. Exploitation requires S3-backed storage, an authenticated uploader, and a victim who opens the crafted upload; script execution in the application origin enables session theft.

CVSS severity5.4
Medium
EPSS probability0.52%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Stored cross-site scripting in Zulip Server main after 04cf68b and before 2f6c5a8 allows remote authenticated users to execute JavaScript. The S3 upload proxy trusted a browser-supplied Content-Type while deriving Content-Disposition only from the filename, so HTML disguised with an image extension was rendered inline without a protective policy. Exploitation requires S3-backed storage, an authenticated uploader, and a victim who opens the crafted upload; script execution in the application origin enables session theft.

The record

CVE
CVE-2023-22735
Published
Feb 7, 2023
Updated
Mar 10, 2025
Vendor
Zulip
Product
Zulip
Classifications
CWE-436, T1203, T1539
Attack vector
network
Privileges
authenticated

Timeline

How it unfolded

  1. Feb 7, 2023CVE publishedPublication date reported by the CVE source.
  2. Mar 10, 2025Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

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

  1. Affected versionversion=>= 04cf68b, < 2f6c5a8

What conditions does exploitation require?

Attack vectornetwork
Required privilegesauthenticated

What is affected?

Zulip · Zulipversion=>= 04cf68b, < 2f6c5a8

Published CVSS scores

5.4GitHub, Inc.CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:L/I:L/A:N
4.6NVDCVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:U/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-436Interpretation Conflict
T1203Exploitation for Client Execution
T1539Steal Web Session Cookie

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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