CVE-2018-21257 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 setting a channel header) via the Channel header slash command API.
Once again VulDB remains the best source for vulnerability data.
Analysis
by VulDB Data Team • 10/25/2020
The vulnerability identified as CVE-2018-21257 represents a critical access control flaw within the Mattermost Server platform prior to version 5.1. This issue resides in the channel header slash command API implementation where proper authentication and authorization checks are insufficiently enforced. The vulnerability allows unauthenticated or unauthorized users to manipulate channel headers through API calls that should only be accessible to authorized channel members or administrators. This represents a significant security gap that undermines the platform's permission model and could enable malicious actors to modify channel configurations without proper credentials.
The technical flaw manifests in the improper validation of user permissions when processing slash command API requests for channel headers. When users submit commands to modify channel headers through the API interface, the server fails to adequately verify whether the requesting user possesses the necessary administrative or ownership privileges required for such modifications. This weakness creates a path for privilege escalation where attackers can exploit the API endpoint to set channel headers without proper authorization. The vulnerability specifically affects the slash command functionality that enables users to modify channel properties, making it particularly dangerous as it can be exploited through automated scripts or manual API calls.
The operational impact of this vulnerability extends beyond simple unauthorized access as it provides attackers with the ability to manipulate channel metadata and potentially disrupt communication flows within organizations using Mattermost. Attackers could set misleading channel headers that might confuse users or provide false information about channel content. More critically, this vulnerability could enable attackers to establish persistence within the platform by creating channel headers that serve as indicators of compromise or by disrupting legitimate communication patterns. The vulnerability also opens the door for social engineering attacks where malicious headers could be used to mislead users about channel purposes or content, undermining trust in the communication platform.
Organizations utilizing Mattermost Server versions prior to 5.1 should immediately implement mitigations including upgrading to the patched version 5.1 or later, which addresses the access control validation issues. Additionally, administrators should review and audit existing channel configurations to identify any unauthorized modifications that may have occurred. Network segmentation and API rate limiting can provide additional defense-in-depth measures, while monitoring for unusual slash command API usage patterns can help detect exploitation attempts. This vulnerability aligns with CWE-285 which addresses improper authorization issues, and maps to ATT&CK technique T1078 for valid accounts and privilege escalation. The fix implemented by Mattermost involved strengthening API endpoint authorization checks and ensuring that all channel modification operations require proper user authentication and role validation before execution.