CVE-2025-21088

Mattermost Server 9.11.0 – 9.11.6

Published

15 January 2025

Modified

30 September 2025

Patch / advisory

View advisory

CVSS Score v3.1 6.5

Click a component to see what it means

Raw vector

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

EPSS Score 0.0056 44th percentile

Risk Priority 51 floored blend · peak EPSS

CVSS and EPSS are reproduced from their sources (NVD, FIRST EPSS). Risk Priority is our own derived reading, not an NVD score.

Summary

CVE-2025-21088 is a medium-severity Incorrect Type Conversion or Cast (CWE-704) 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 44th 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 SA-11 (Developer Testing and Evaluation) and SA-15 (Development Process, Standards, and Tools) — see the control section below for these in your framework.

EU & UK References

🇪🇺 ENISA EUVD: EUVD-2025-0093

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 validate the style of proto supplied to an action's style in post.props.attachments, which allows an attacker to crash the frontend via crafted malicious…

input.

CWE(s): CWE-704

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 →

T1687 Exploitation for Defense Impairment Defense Impairment

Adversaries may exploit vulnerabilities in security software, infrastructure, or defensive components to degrade, disable, or otherwise continue to impair their ability to prevent, detect, or respond to malicious activity.

attack.mitre.org →

Derived from this CVE’s CWE(s) via the direct CWE→ATT&CK cross-walk.

CVEs Like This One

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CVE-2025-20086Same product: Mattermost Mattermost Server

CVE-2025-20033Same product: Mattermost Mattermost Server

CVE-2025-20621Same product: Mattermost Mattermost Server

CVE-2026-4646Same product: Mattermost Mattermost Server

CVE-2025-13352Same product: Mattermost Mattermost Server

CVE-2026-25783Same 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

Mitigating Controls (NIST 800-53 r5) AI

SA-11 Developer Testing and Evaluation good match

Developer testing and evaluation (static analysis, fuzzing, unit tests) directly finds incorrect type conversions or casts.

SA-15 Development Process, Standards, and Tools partial match

Requiring documented development standards and tools can mandate safe typing, casting rules, and compiler checks that stop the weakness from being introduced.

SA-8 Security and Privacy Engineering Principles partial match

Security engineering principles can require type-safe design and casting practices that structurally avoid incorrect conversions.

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 prevent type-conversion flaws via coding standards, reviews, and testing.

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 uncover type-conversion defects before release.

A.8.25 Secure development life cycle partial match

prevents

Secure development lifecycle includes type-safety reviews that reduce incorrect casts.

A.8.26 Application security requirements partial match

prevents

Application security requirements can mandate strong typing and safe casting rules.

A.8.27 Secure system architecture and engineering principles partial match

degrades

Secure architecture principles discourage unsafe type conversions in design.