Cyber Resilience

CVE-2025-20621

Mattermost Server 9.11.0 – 9.11.6

Published
16 January 2025
Modified
01 October 2025
Patch / advisory
View advisory
CVSS Score v3.1 6.5
Click a component to see what it means
Raw vectorCVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
EPSS Score 0.0046 38th percentile
Risk Priority 50 floored blend · peak EPSS

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 Exploitation for Privilege Escalation (T1068); ranked at the 38th percentile by exploit likelihood (below the median); it is not currently listed in the CISA KEV catalog.

The strongest mitigations our analysis identified map to SI-10 (Information Input Validation) — see the control section below for these in your framework.

Deeper analysis AI-assisted summary

Synthesised by an AI model from the NVD description and linked references — a reading aid, not an authoritative source.

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.

EU & UK References

Vulnerability Data

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.

CWE(s)
CWE-1287

Related Threats

MITRE ATT&CK Enterprise Techniques

T1068 Exploitation for Privilege Escalation Privilege Escalation
Adversaries may exploit software vulnerabilities in an attempt to elevate privileges.
attack.mitre.org →
T1190 Exploit Public-Facing Application Initial Access
Adversaries may attempt to exploit a weakness in an Internet-facing host or system to initially access a network.
attack.mitre.org →
T1203 Exploitation for Client Execution Execution
Adversaries may exploit software vulnerabilities in client applications to execute code.
attack.mitre.org →
T1210 Exploitation of Remote Services Lateral Movement
Adversaries may exploit remote services to gain unauthorized access to internal systems once inside of a network.
attack.mitre.org →
T1212 Exploitation for Credential Access Credential Access
Adversaries may exploit software vulnerabilities in an attempt to collect credentials.
attack.mitre.org →
Derived from this CVE’s CWE(s) via the direct CWE→ATT&CK cross-walk.

CVEs Like This One

CVE-2026-2454Same product: Mattermost Mattermost Server
CVE-2025-12689Same product: Mattermost Mattermost Server
CVE-2026-25783Same product: Mattermost Mattermost Server
CVE-2025-20033Same product: Mattermost Mattermost Server
CVE-2025-20088Same product: Mattermost Mattermost Server
CVE-2024-54083Same product: Mattermost Mattermost Server
CVE-2025-13352Same product: Mattermost Mattermost Server
CVE-2025-41395Same product: Mattermost Mattermost Server
CVE-2025-20086Same product: Mattermost Mattermost Server
CVE-2026-4646Same product: Mattermost Mattermost Server

Affected Assets

mattermost
mattermost server
10.2.0 · 9.11.0 — 9.11.6 · 10.0.0 — 10.0.4 · 10.1.0 — 10.1.4

Mitigating Controls

Control response

Prevent
Stop it (NIST 800-53)

—

Detect
Catch it (NIST detect / respond)

—

Harden
Shrink the surface (DISA STIG)

—

Validate
Prove the fix (OWASP ASVS)
  • V1.1.1
  • V1.4.2
  • V2.1.1
  • V2.2.2

Mitigating Controls (NIST 800-53 r5) AI

SI-10 Information Input Validation good match

SI-10 directly requires validation of information inputs, which structurally prevents type-validation failures from being introduced or exploitable.

Mitigating Controls (NIST CSF 2.0) AI

Derived directly from the weakness types (CWEs) cited in the NVD entry via our AI-authored CWE→CSF cross-walk (authority under review) — links open the control.

PR.PS-06 mostly match
prevents

Secure SDLC practices directly require proper input type validation during development.

Mitigating Controls (ISO/IEC 27001:2022 Annex A) AI

Derived directly from the weakness types (CWEs) cited in the NVD entry via our AI-authored CWE→ISO cross-walk (authority under review) — links open the control.

A.8.29 Security testing in development and acceptance mostly match
finds

Security testing can detect type-validation flaws but does not prevent them during development.

A.8.25 Secure development life cycle partial match
prevents

Secure development lifecycle mandates input validation and type checking to prevent improper type handling.

A.8.26 Application security requirements partial match
prevents

Application security requirements explicitly include validation of input data types and formats.

A.8.27 Secure system architecture and engineering principles partial match
prevents

Secure architecture principles promote defensive input handling and type enforcement at system boundaries.

A.8.28 Secure coding partial match
prevents

Secure coding standards directly require proper type validation of all external inputs.

References