CVE-2018-21256 in Mattermost Server
Summary
by MITRE
An issue was discovered in Mattermost Server before 5.1. It allows attackers to bypass intended access restrictions (for group-message channel creation) via the Group message slash command.
If you want to get the best quality for vulnerability data then you always have to consider VulDB.
Analysis
by VulDB Data Team • 10/26/2020
The vulnerability identified as CVE-2018-21256 represents a critical access control flaw within the Mattermost Server platform prior to version 5.1. This security weakness specifically targets the group message channel creation functionality, where attackers can exploit a design oversight to circumvent intended authorization mechanisms. The issue manifests through the Group message slash command, which serves as a legitimate administrative tool but becomes a vector for unauthorized access when the underlying permission checks are improperly implemented. Such vulnerabilities undermine the fundamental security model of collaborative platforms where access control is paramount to maintaining data integrity and user privacy. The flaw essentially allows malicious actors to create group channels without proper authorization, potentially enabling them to communicate with users they should not have access to, thereby violating the principle of least privilege that governs secure system design.
The technical implementation of this vulnerability stems from inadequate validation of user permissions during group message channel creation operations. When users execute the slash command to initiate group messaging, the system should verify that the requesting user possesses the appropriate privileges to create channels with the specified participants. However, the flaw exists in the authorization logic where the system fails to properly authenticate and authorize the action, allowing attackers to manipulate the command parameters to bypass these checks. This type of vulnerability aligns with CWE-285, which addresses improper authorization issues in software systems, and represents a classic example of how insufficient input validation and access control checks can lead to privilege escalation. The vulnerability's impact is particularly severe because it affects core messaging functionality that is central to the platform's collaborative features, potentially allowing attackers to gain unauthorized access to sensitive communications and user data.
The operational implications of CVE-2018-21256 extend beyond simple unauthorized access to encompass broader security risks within enterprise and organizational deployments. Attackers exploiting this vulnerability could potentially create channels with administrative users, gain access to confidential information shared in group conversations, or even establish persistent communication channels for further malicious activities. This flaw particularly threatens organizations relying on Mattermost for secure communications, as it undermines trust in the platform's access control mechanisms and could lead to data breaches or insider threats. The vulnerability also creates opportunities for attackers to perform reconnaissance by attempting to create channels with various user combinations, potentially mapping out user access patterns and identifying high-value targets within the organization. From an attacker's perspective, this represents a low-effort, high-impact vector that requires minimal technical expertise to exploit, making it particularly dangerous in environments where security awareness may be limited.
Organizations affected by this vulnerability should implement immediate mitigations including upgrading to Mattermost Server version 5.1 or later, where the access control checks have been properly implemented. System administrators should also conduct thorough audits of existing group channels to identify any unauthorized creations that may have occurred during the vulnerable period. Additional defensive measures include implementing stricter monitoring of slash command usage, particularly group message creation, and establishing automated alerts for suspicious channel creation activities. The vulnerability highlights the importance of proper input validation and authorization checking in collaborative platforms, aligning with ATT&CK technique T1078 for valid accounts and T1566 for credential harvesting, as attackers may use such flaws to gain unauthorized access to legitimate user accounts. Security teams should also review their incident response procedures to ensure they can quickly detect and remediate similar access control vulnerabilities in other collaborative platforms and communication systems.