Mattermost 11.3.0, 11.2.2, 10.11.10: /mute ErrResp Enables Private Disclosure
CVE-2026-21386 Published on March 16, 2026
Private channel enumeration via /mute slash command
Mattermost versions 11.3.x <= 11.3.0, 11.2.x <= 11.2.2, 10.11.x <= 10.11.10 fail to use consistent error responses when handling the /mute command which allows an authenticated team member to enumerate private channels they are not authorized to know about via differing error messages for nonexistent versus private channels. Mattermost Advisory ID: MMSA-2026-00588
Vulnerability Analysis
CVE-2026-21386 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.
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-2026-21386 has been classified to as a Side Channel Attack vulnerability or weakness.
Products Associated with CVE-2026-21386
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Affected Versions
Mattermost:- Version 11.3.0, <= 11.3.0 is affected.
- Version 11.2.0, <= 11.2.2 is affected.
- Version 10.11.0, <= 10.11.10 is affected.
- Version 11.4.0 is unaffected.
- Version 11.3.1 is unaffected.
- Version 11.2.3 is unaffected.
- Version 10.11.11 is unaffected.
Exploit Probability
EPSS (Exploit Prediction Scoring System) scores estimate the probability that a vulnerability will be exploited in the wild within the next 30 days. The percentile shows you how this score compares to all other vulnerabilities.