CVE-2025-20621
Published: 16 January 2025
Summary
CVE-2025-20621 is a medium-severity Improper Validation of Specified Type of Input (CWE-1287) vulnerability in Mattermost Mattermost Server. Its CVSS base score is 6.5 (Medium).
Operationally, exploitation aligns with the MITRE ATT&CK technique Exploit Public-Facing Application (T1190); ranked in the top 39.9% of CVEs by exploit likelihood; it is not currently listed in the CISA KEV catalog.
The strongest mitigations our analysis identified are NIST 800-53 SI-10 (Information Input Validation) and SI-11 (Error Handling).
Threat & Defense at a Glance
Threat & Defense Details
Mitigating Controls (NIST 800-53 r5)AI
SI-11 requires proper error handling for unhandled exceptions like casting failures in attachment fields, directly preventing webapp crashes from malformed posts.
SI-10 mandates validation of information inputs such as attachment fields in posts, blocking malformed data that cannot be cast to strings from causing denial-of-service.
SI-2 ensures timely identification, reporting, and correction of flaws like this unhandled casting error in Mattermost, mitigating the vulnerability through patching.
MITRE ATT&CK Enterprise TechniquesAI
Why these techniques?
Malformed post attachment triggers unhandled exception leading to webapp crash (DoS); directly matches exploitation of public-facing app and application/system exploitation for availability impact.
NVD 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…
more
via creating and sending such a post to a channel.
Deeper analysisAI
CVE-2025-20621 is a vulnerability in Mattermost versions 10.2.x up to and including 10.2.0, 9.11.x up to 9.11.5, 10.0.x up to 10.0.3, and 10.1.x up to 10.1.3. It arises from the webapp's failure to properly handle posts containing attachments with fields that cannot be cast to a String. An attacker can exploit this by creating and sending such a post to a channel, resulting in a crash of the webapp. The issue carries a CVSS v3.1 base score of 6.5 (AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H) and is linked to CWE-1287.
The attack requires network access and low privileges, such as those of an authenticated user, with low complexity and no user interaction. By posting a malformed attachment to any channel, the attacker triggers an unhandled casting error that crashes the Mattermost webapp, leading to a denial-of-service condition that disrupts availability for all users without compromising confidentiality or integrity.
Mattermost has published details on mitigations in their security updates, available at https://mattermost.com/security-updates.
Details
- CWE(s)