CVE-2025-0503 in Mattermost
Summary
by MITRE • 02/14/2025
Mattermost versions 9.11.x <= 9.11.6 fail to filter out DMs from the deleted channels endpoint which allows an attacker to infer user IDs and other metadata from deleted DMs if someone had manually marked DMs as deleted in the database.
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Analysis
by VulDB Data Team • 09/30/2025
The vulnerability identified as CVE-2025-0503 affects Mattermost server versions 9.11.x through 9.11.6 and represents a significant information disclosure flaw within the platform's channel management system. This issue stems from insufficient input validation and access control mechanisms within the deleted channels endpoint, which should theoretically only return information about channels that have been properly deleted through the application's standard procedures. The flaw allows unauthorized access to direct message metadata that should remain protected, creating potential privacy and security risks for users within the Mattermost environment.
The technical implementation of this vulnerability occurs at the application layer where the deleted channels endpoint fails to properly filter out direct messages from its response data. When administrators or users manually mark direct messages as deleted within the database, the system's API endpoint continues to expose metadata about these deleted DMs, including user identifiers and other sensitive information that would normally be restricted. This represents a classic case of improper access control where the system does not adequately verify whether the requesting user has legitimate authorization to access the specific metadata being exposed. The vulnerability operates under the principle of least privilege violation, where sensitive information is accessible beyond intended authorization boundaries.
The operational impact of this vulnerability extends beyond simple information disclosure to potentially enable advanced reconnaissance attacks by threat actors. Attackers can leverage this flaw to infer user relationships, communication patterns, and potentially identify high-value targets within the organization's communication network. The exposure of user IDs through deleted DMs creates opportunities for social engineering attacks, targeted phishing campaigns, and network mapping activities that could lead to further compromise. This vulnerability directly impacts the confidentiality and integrity aspects of the information security triad, as it allows unauthorized parties to gather intelligence about user activities and relationships within the platform. The issue is particularly concerning in enterprise environments where Mattermost serves as a primary communication channel for sensitive business operations.
Mitigation strategies for CVE-2025-0503 should prioritize immediate patching of affected Mattermost versions to the latest available releases that contain the necessary security fixes. Organizations should implement network segmentation and access controls to limit exposure of the affected endpoints, while also monitoring for suspicious API activity that might indicate exploitation attempts. The fix should include proper input validation and access control checks that ensure only authorized users can access deleted channel information, with special consideration for direct message metadata. Security teams should conduct comprehensive audits of their Mattermost installations to identify any potential unauthorized modifications to database entries, and implement logging controls that can detect attempts to access restricted information through the deleted channels endpoint. This vulnerability aligns with CWE-200 (Information Disclosure) and could be categorized under ATT&CK technique T1566 (Phishing) or T1071.1 (Application Layer Protocol: Web Protocols) depending on how attackers might exploit the information gathered. Organizations should also consider implementing additional security controls such as API rate limiting and enhanced monitoring of API access patterns to prevent automated exploitation attempts.