Mattermost fails to validate team-specific upload_file permissions

CVE-2026-4265 → View on GitHub ↗ View on CVE.org ↗ View on NVD ↗

Description

Mattermost versions 11.3.x <= 11.3.0, 11.2.x <= 11.2.2, 10.11.x <= 10.11.10 fail to validate team-specific upload_file permissions which allows a guest user to post files in channels where they lack upload_file permission via uploading files in a team where they have permission and reusing the file metadata in a POST request to a different team. Mattermost Advisory ID: MMSA-2025-00553

Basic information

Type
reviewed
Severity
medium
Advisory on GitHub
Open advisory ↗
Repository advisory
—
Source code
Browse source ↗
Published (advisory)
2026-03-16 15:30:46 UTC
Updated
2026-03-18 16:09:04 UTC
GitHub reviewed
2026-03-18 16:09:03 UTC
NVD published
2026-03-16

EPSS Score

Score Percentile
0.03% 8.23%

CVSS Scores

Base score Version Severity Vector
4.3 3.1 —
CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:L/A:N Click to expand
Attack vector (AV:N)
Could be attacked over the internet or any normal routed network—not just someone sitting at the machine.
Attack complexity (AC:L)
Once they can reach the bug, pulling it off is straightforward—no weird race conditions or rare setup.
Privileges required (PR:L)
A normal user session is enough; they don’t have to be admin.
User interaction (UI:N)
Nobody has to click “OK” or open a trap file; it can work without a victim helping.
Scope (S:U)
Damage stays in the same “trust bubble” as the broken component—no big spill into unrelated systems.
Confidentiality (C:N)
Doesn’t really leak secrets in a meaningful way.
Integrity (I:L)
Attackers could change some data, but it’s limited—not everything goes.
Availability (A:N)
Service keeps running; no real outage angle.

Identifiers

Type Value
GHSA GHSA-xpvf-6qcc-9jqc ↗
CVE CVE-2026-4265 ↗

CWEs

CWE id Name
CWE-863 Incorrect Authorization

Affected packages (5)

Vulnerable version ranges and first patched releases as published by GitHub.

Ecosystem Package Vulnerable range First patched Vulnerable functions
go github.com/mattermost/mattermost/server/v8 < 8.0.0-20260107144005-c7f6efdfb035 8.0.0-20260107144005-c7f6efdfb035 —
go github.com/mattermost/mattermost-server < 5.3.2-0.20260107144005-c7f6efdfb035 5.3.2-0.20260107144005-c7f6efdfb035 —
go github.com/mattermost/mattermost-server >= 10.11.0-rc1, < 10.11.11 10.11.11 —
go github.com/mattermost/mattermost-server >= 11.2.0-rc1, < 11.2.3 11.2.3 —
go github.com/mattermost/mattermost-server >= 11.3.0-rc1, < 11.3.1 11.3.1 —

References

cvelogic Threat Intelligence