CVE-2026-21386 in Mattermost
Summary
by MITRE • 03/16/2026
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
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Analysis
by VulDB Data Team • 03/21/2026
This vulnerability in Mattermost represents a critical information disclosure flaw that undermines the system's access control mechanisms through inconsistent error handling. The issue affects multiple versions including 11.3.0, 11.2.2, and 10.11.10, where the application fails to provide uniform error responses when processing the /mute command. This inconsistency creates a reconnaissance vector that allows authenticated users to perform channel enumeration attacks by observing different error messages returned for various channel types. The vulnerability specifically targets the interaction between user authentication and channel access control, exploiting the system's failure to maintain consistent security responses across different access scenarios.
The technical implementation flaw stems from the application's error handling logic during the /mute command execution process. When an authenticated team member attempts to mute a channel that does not exist, the system returns one error message format, while attempting to mute a private channel the user cannot access produces a different error response. This differential behavior creates a side-channel attack opportunity where malicious actors can distinguish between nonexistent channels and private channels they lack authorization to access. The inconsistency violates fundamental security principles of uniform error handling and can be classified under CWE-200 Information Exposure Through Error Message. The system's inability to provide consistent responses undermines its security model and creates predictable patterns that adversaries can exploit.
The operational impact of this vulnerability extends beyond simple information disclosure to potentially enable more sophisticated attacks within the Mattermost environment. An authenticated attacker can systematically enumerate private channels by making repeated mute command requests and analyzing the varying error responses, effectively mapping the channel structure without proper authorization. This enumeration capability allows threat actors to identify sensitive private channels that should remain hidden from unauthorized users, potentially leading to further privilege escalation or targeted attacks on specific channels. The vulnerability affects the confidentiality aspect of the CIA triad by enabling unauthorized information discovery and can be categorized under ATT&CK technique T1083 for File and Directory Discovery. Organizations using affected Mattermost versions face increased risk of insider threats and external reconnaissance attacks that could compromise sensitive communications channels.
Mitigation strategies should focus on implementing consistent error handling across all channel access operations and ensuring that the system provides uniform responses regardless of channel type or user authorization status. Organizations should immediately upgrade to patched versions of Mattermost that address this inconsistency in error responses, as recommended by the Mattermost Security Advisory MMSA-2026-00588. Additional defensive measures include implementing rate limiting on channel access operations to prevent systematic enumeration attempts, deploying monitoring solutions to detect unusual patterns of mute command usage, and conducting regular security assessments to identify similar inconsistencies in error handling across other system functions. The remediation process should also involve comprehensive testing of error responses to ensure that all access control violations return identical error messages, thereby eliminating the information disclosure vector that enables channel enumeration attacks.