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Mattermost doesn't escape some variables that could contain malicious content during error page composition

Low severity GitHub Reviewed Published May 18, 2026 to the GitHub Advisory Database • Updated Jun 1, 2026

Package

gomod github.com/mattermost/mattermost-server (Go)

Affected versions

< 5.3.2-0.20260310115442-5a1ea95044dc

Patched versions

5.3.2-0.20260310115442-5a1ea95044dc
gomod github.com/mattermost/mattermost/server/v8 (Go)
>= 10.11.0, < 10.11.14
>= 11.5.0, < 11.5.2
< 8.0.0-20260310115442-5a1ea95044d
10.11.14
11.5.2
8.0.0-20260310115442-5a1ea95044d

Description

Mattermost versions 11.5.x <= 11.5.1, 10.11.x <= 10.11.13 fail to escape some variables that could contain malicious content during error page composition which allows an attacker with access to edit some site configuration to execute some malicious code via injecting some JS as part of those values.. Mattermost Advisory ID: MMSA-2026-00622

References

Published by the National Vulnerability Database May 18, 2026
Published to the GitHub Advisory Database May 18, 2026
Reviewed Jun 1, 2026
Last updated Jun 1, 2026

Severity

Low

CVSS overall score

This score calculates overall vulnerability severity from 0 to 10 and is based on the Common Vulnerability Scoring System (CVSS).
/ 10

CVSS v3 base metrics

Attack vector
Network
Attack complexity
Low
Privileges required
High
User interaction
None
Scope
Unchanged
Confidentiality
Low
Integrity
Low
Availability
None

CVSS v3 base metrics

Attack vector: More severe the more the remote (logically and physically) an attacker can be in order to exploit the vulnerability.
Attack complexity: More severe for the least complex attacks.
Privileges required: More severe if no privileges are required.
User interaction: More severe when no user interaction is required.
Scope: More severe when a scope change occurs, e.g. one vulnerable component impacts resources in components beyond its security scope.
Confidentiality: More severe when loss of data confidentiality is highest, measuring the level of data access available to an unauthorized user.
Integrity: More severe when loss of data integrity is the highest, measuring the consequence of data modification possible by an unauthorized user.
Availability: More severe when the loss of impacted component availability is highest.
CVSS:3.1/AV:N/AC:L/PR:H/UI:N/S:U/C:L/I:L/A:N

EPSS score

Exploit Prediction Scoring System (EPSS)

This score estimates the probability of this vulnerability being exploited within the next 30 days. Data provided by FIRST.
(9th percentile)

Weaknesses

Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')

The product does not neutralize or incorrectly neutralizes user-controllable input before it is placed in output that is used as a web page that is served to other users. Learn more on MITRE.

CVE ID

CVE-2026-3495

GHSA ID

GHSA-jx93-pf6x-874r

Source code

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