Mattermost <=11.10.0/11.9.1/11.8.5 Perms Bypass API FlaggedPost Inference
CVE-2026-18560 Published on October 16, 2026

Improper order of authorisations in Mattermost GetFile API leads to an authenticated user to determine a post’s quarantined status
Mattermost versions 11.10.x <= 11.10.0, 11.7.x <= 11.7.9, 11.9.x <= 11.9.1, 11.8.x <= 11.8.5 fail to validate users permissions before other validations which allows non-content reviewers to infer a posts flagged status via direct API call.. Mattermost Advisory ID: MMSA-2026-00754

Vendor Advisory NVD

Vulnerability Analysis

CVE-2026-18560 is exploitable with network access, and requires small amount of user privileges. This vulnerability is considered to have a low attack complexity. The potential impact of an exploit of this vulnerability is considered to have a small impact on confidentiality, a small impact on integrity and availability.

Attack Vector:
NETWORK
Attack Complexity:
LOW
Privileges Required:
LOW
User Interaction:
NONE
Scope:
UNCHANGED
Confidentiality Impact:
LOW
Integrity Impact:
NONE
Availability Impact:
NONE

Weakness Type

What is a Side Channel Attack Vulnerability?

The product behaves differently or sends different responses under different circumstances in a way that is observable to an unauthorized actor, which exposes security-relevant information about the state of the product, such as whether a particular operation was successful or not. Discrepancies can take many forms, and variations may be detectable in timing, control flow, communications such as replies or requests, or general behavior. These discrepancies can reveal information about the product's operation or internal state to an unauthorized actor. In some cases, discrepancies can be used by attackers to form a side channel.

CVE-2026-18560 has been classified to as a Side Channel Attack vulnerability or weakness.


Products Associated with CVE-2026-18560

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MatterMost
 

Affected Versions

Mattermost: