CVE-2025-20621: mattermost server — Fix

Description

Mattermost versions 10.2.x <= 10.2.0, 9.11.x <= 9.11.5, 10.0.x <= 10.0.3, 10.1.x <= 10.1.3 fail to properly handle posts with attachments containing fields that cannot be cast to a String, which allows an attacker to cause the webapp to crash via creating and sending such a post to a channel.

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CVSS v3.1 Score

6.5

MEDIUM

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

Weakness Type (CWE)

CWE-1287 CWE-1287

Affected Products

Vendor	Product	Versions
mattermost	mattermost_server	9.11.0 — 9.11.6
mattermost	mattermost_server	10.0.0 — 10.0.4
mattermost	mattermost_server	10.1.0 — 10.1.4
mattermost	mattermost_server	All

References

Advisories & Patches

https://mattermost.com/security-updates

Frequently Asked Questions

What is CVE-2025-20621? +

Mattermost versions 10.2.x <= 10.2.0, 9.11.x <= 9.11.5, 10.0.x <= 10.0.3, 10.1.x <= 10.1.3 fail to properly handle posts with attachments containing fields that cannot be cast to a String, which allows an attacker to cause the webapp to crash via creating and sending such a post to a channel. It has a CVSS v3.1 base score of 6.5 (MEDIUM).

How severe is CVE-2025-20621? +

CVE-2025-20621 has a CVSS v3.1 score of 6.5 out of 10, rated MEDIUM. This is a medium-severity vulnerability that should be remediated as part of regular maintenance.

What products are affected by CVE-2025-20621? +

CVE-2025-20621 affects products from mattermost, specifically: mattermost_server. Check the affected products table above for specific version ranges.

How do I check if I'm vulnerable to CVE-2025-20621? +

You can use Secably's free Website Scanner to check your website for known vulnerabilities. For infrastructure scanning, use the Port Scanner to identify exposed services that may be affected. Check the vendor advisories linked above for specific patch and version information.

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