CVE-2023-22735: Zulip is an open-source team collaboration tool. In versions of zulip prior to commit `2f6c5a8` but after commit `04cf68b` users could upload files wi...

Description

Zulip is an open-source team collaboration tool. In versions of zulip prior to commit `2f6c5a8` but after commit `04cf68b` users could upload files with arbitrary `Content-Type` which would be served from the Zulip hostname with `Content-Disposition: inline` and no `Content-Security-Policy` header, allowing them to trick other users into executing arbitrary Javascript in the context of the Zulip application. Among other things, this enables session theft. Only deployments which use the S3 storage (not the local-disk storage) are affected, and only deployments which deployed commit 04cf68b45ebb5c03247a0d6453e35ffc175d55da, which has only been in `main`, not any numbered release. Users affected should upgrade from main again to deploy this fix. Switching from S3 storage to the local-disk storage would nominally mitigate this, but is likely more involved than upgrading to the latest `main` which addresses the issue.

Metrics

CVSS 3.1

4.6/10

CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:U/C:L/I:L/A:N

EPSS Probability

0.52%

39.8th percentile

Probability of exploitation in the next 30 days. Learn more

Weakness Enumeration

CWE-436

Affected Software

Vendor	Product	Versions
Zulip	Zulip Server	2023-01-09

References

Timeline

Published: Feb 7, 2023
Last Modified: Jun 17, 2026
Status: Modified

Frequently Asked Questions

What is CVE-2023-22735?

Zulip is an open-source team collaboration tool. In versions of zulip prior to commit `2f6c5a8` but after commit `04cf68b` users could upload files with arbitrary `Content-Type` which would be served from the Zulip hostname with `Content-Disposition: inline` and no `Content-Security-Policy` header, allowing them to trick other users into executing arbitrary Javascript in the context of the Zulip application. Among other things, this enables session theft. Only deployments which use the S3 storage (not the local-disk storage) are affected, and only deployments which deployed commit 04cf68b45ebb5c03247a0d6453e35ffc175d55da, which has only been in `main`, not any numbered release. Users affected should upgrade from main again to deploy this fix. Switching from S3 storage to the local-disk storage would nominally mitigate this, but is likely more involved than upgrading to the latest `main` which addresses the issue.

How severe is CVE-2023-22735?

CVE-2023-22735 has a CVSS score of 4.6/10 (MEDIUM severity). The EPSS model estimates a 0.52% probability of exploitation in the next 30 days.

How do I fix CVE-2023-22735?

Check the vendor references and advisories linked above for patched versions and mitigation guidance. You can also run a Strix scan to test if your systems are affected.