CVE-2025-20621: Webapp crash via object that can't be cast to String in Attachment Field

Severity: high (CVSS 7.5 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.5% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2025-01-16 · updated 2026-06-17
Weakness: CWE-1287

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.

Affected products

Mattermost: from 9.11.0 before 9.11.6; from 10.0.0 before 10.0.4; from 10.1.0 before 10.1.4; 10.2.0 — fixed in 10.0.4, 10.1.4, 9.11.6

How to fix

Upgrade to a fixed version: Mattermost 10.0.4, 10.1.4, 9.11.6. Check the vendor advisory for mitigations.

References

https://mattermost.com/security-updates

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