CVE-2026-91181 in Mattermost
Summary
by MITRE • 09/15/2026
Mattermost versions 11.9.x <= 11.9.0, 11.8.x <= 11.8.4, 11.7.x <= 11.7.7, 10.11.x <= 10.11.22 Fail to sanitize Team objects returned by the data retention teams endpoint which allows an authenticated user holding only the read-only Data Retention Policy permission to obtain a private team's secret invite_id and email, and use it to join the team without authorization, via GET /api/v4/data_retention/policies/{policy_id}/teams.. Mattermost Advisory ID: MMSA-2026-00702
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Analysis
by VulDB Data Team • 09/15/2026
The vulnerability identified in Mattermost versions 11.9.x through 11.8.4, as well as earlier branches including 11.7.x up to 11.7.7 and 10.11.x up to 10.11.22, represents a critical authorization bypass rooted in improper input validation and data exposure within the application's API layer. Specifically, this flaw resides in the endpoint responsible for retrieving teams associated with specific data retention policies. The core technical deficiency is that the server fails to sanitize or filter Team objects before returning them via the GET /api/v4/data_retention/policies/{policy_id}/teams interface. This lack of sanitization means that sensitive metadata attached to team configurations, which should be restricted based on user permissions and visibility settings, is inadvertently included in the response payload regardless of whether the requesting user has explicit access rights to those specific teams.
From an authorization perspective, this vulnerability allows for a significant escalation of privileges despite strict role-based controls. An authenticated user who possesses only the read-only Data Retention Policy permission can exploit this endpoint to enumerate private team identifiers and associated email addresses. In standard Mattermost architecture, joining a private or restricted team typically requires either direct invitation by an administrator or explicit approval from existing members. However, because the API leaks the secret invite_id along with the target email address, the attacker can bypass these access controls entirely. By capturing the leaked invite_id, the user can subsequently issue requests to join the targeted teams without any further authorization checks, effectively gaining unauthorized access to private channels and restricted communications intended for other groups within the organization.
The operational impact of this vulnerability is severe, particularly in enterprise environments where data retention policies are used to manage compliance and archival requirements rather than as a mechanism for team discovery or enumeration. The leakage of email addresses combined with valid invite links facilitates not only unauthorized access but also potential phishing campaigns or social engineering attacks against employees whose contact information has been exposed. Furthermore, the ability to join private teams without authorization compromises the confidentiality of internal communications, potentially leading to intellectual property theft, violation of regulatory compliance standards such as GDPR or HIPAA if sensitive data is accessed, and a general erosion of trust in the platform's security model. This flaw undermines the principle of least privilege by allowing low-level permissions to yield high-impact access outcomes through indirect API manipulation rather than direct permission grants.
This vulnerability aligns with Common Weakness Enumeration (CWE) categories such as CWE-209, which describes the generation of an error or exception message that reveals implementation details, and more critically CWE-862, missing authorization checks in critical business logic. In terms of the MITRE ATT&CK framework, this behavior corresponds to T1530 Data from Cloud Storage Object Discovery, where attackers enumerate accessible resources, followed by T1078 Valid Accounts for unauthorized access using legitimate credentials but improper permissions. The exploitation path involves initial reconnaissance via the data retention endpoint and subsequent lateral movement into restricted team spaces.
To mitigate this vulnerability, organizations running affected versions must immediately upgrade to a patched release of Mattermost where the API response has been corrected to filter out sensitive fields like invite_ids for teams that the requesting user does not have explicit access rights to view. Until an upgrade is feasible, administrators should review data retention policy permissions and restrict them to only those users who absolutely require such access, thereby reducing the attack surface. Additionally, implementing strict rate limiting on API endpoints can help mitigate automated enumeration attempts, although this is a compensating control rather than a fix for the underlying logic flaw. Regular security audits of custom plugins or integrations that interact with these APIs are also recommended to ensure no secondary vectors exist for similar data leakage scenarios.