Cyber Resilience

CVE-2026-3473

Medium

Published: 22 May 2026

Published
22 May 2026
Modified
22 May 2026
KEV Added
—
Patch
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CVSS Score v3.1 5.9 CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:H/I:L/A:N
EPSS Score 0.0015 4.5th percentile
Risk Priority 12 60% EPSS · 20% KEV · 20% CVSS

Summary

CVE-2026-3473 is a medium-severity Authorization Bypass Through User-Controlled Key (CWE-639) vulnerability in Mattermost Mattermost Server. Its CVSS base score is 5.9 (Medium).

Operationally, exploitation aligns with the MITRE ATT&CK technique Data from Local System (T1005); ranked at the 4.5th percentile by exploit likelihood (below the median); it is not currently listed in the CISA KEV catalog.

EU & UK References

Vulnerability details

Mattermost versions 11.6.x <= 11.6.0, 11.5.x <= 11.5.3, 11.4.x <= 11.4.4, 10.11.x <= 10.11.14 fail to validate file ownership and access control, which allows an authenticated user to access and download files belonging to other users or teams via crafted…

more

Boards API requests using valid file IDs.. Mattermost Advisory ID: MMSA-2026-00620

CWE(s)
CWE-639

Related Threats

MITRE ATT&CK Enterprise TechniquesAI

T1005 Data from Local System Collection
Adversaries may search local system sources, such as file systems, configuration files, local databases, virtual machine files, or process memory, to find files of interest and sensitive data prior to Exfiltration.
attack.mitre.org →
Why these techniques?

IDOR authz bypass directly enables unauthorized file reads/downloads from the system.

Confidence: MEDIUM · MITRE ATT&CK Enterprise v18.1

Affected Assets

mattermost
mattermost server
10.11.0 — 10.11.15 · 11.4.0 — 11.4.5 · 11.5.0 — 11.5.4

Mitigating Controls

Likely Mitigating Controls AI

Per-CVE control mapping for this CVE has not run yet; the list below is derived from the weakness types (CWEs) cited in the NVD entry.

AC-24 Access Control Decisions
addresses: CWE-639

Per-request decision making makes it harder to bypass authorization using user-controlled keys without proper validation in the decision process.

AC-3 Access Enforcement
addresses: CWE-639

Consistent enforcement of approved authorizations makes bypassing via user-controlled keys ineffective.

References