CVE-2025-27715 in Mattermost
Summary
by MITRE • 03/21/2025
Mattermost versions 9.11.x <= 9.11.8 fail to prompt for explicit approval before adding a team admin to a private channel, which team admins to joining private channels via crafted permalink links without explicit consent from them.
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Analysis
by VulDB Data Team • 03/28/2025
This vulnerability exists in Mattermost versions 9.11.x up to and including 9.11.8 where the platform fails to enforce proper access controls for private channel administration. The flaw allows team administrators to bypass the standard approval process required for adding new members to private channels through specially crafted permalink links. This represents a significant security gap in the platform's permission model and access control mechanisms. The vulnerability stems from inadequate validation of administrative actions within private channel contexts, specifically failing to require explicit consent from existing team members before granting access through automated link mechanisms. According to CWE-284, this issue involves improper access control where the system does not properly enforce authorization checks for administrative operations. The flaw essentially undermines the fundamental security principle of least privilege by allowing unauthorized expansion of private channel membership without proper administrative oversight.
The technical implementation of this vulnerability exploits the permalink functionality within Mattermost's private channel system. When team administrators generate or share specific permalink links, these links should require explicit approval from channel members before granting access. However, the system fails to validate whether the administrative action was properly authorized, allowing the bypass of consent mechanisms. This creates a scenario where malicious actors or unauthorized administrators can silently add users to private channels, potentially compromising sensitive information shared within those restricted spaces. The vulnerability operates at the application layer and affects the core privilege management system of the platform. It demonstrates a failure in the access control decision-making process where the system should have enforced a mandatory approval step but instead allowed automatic access provisioning. This issue aligns with ATT&CK technique T1078.004 which covers valid accounts used for lateral movement and privilege escalation through legitimate access mechanisms.
The operational impact of this vulnerability extends beyond simple access control bypass to potentially expose sensitive communications and data within private channels. Team administrators who should only have the ability to manage channel membership through proper approval processes can circumvent these controls entirely, leading to unauthorized access to confidential discussions, strategic information, or personal data shared within private channels. This represents a critical risk for organizations that rely on Mattermost for secure communications and collaboration. The vulnerability affects the integrity and confidentiality of private channel data, potentially enabling information leakage or unauthorized surveillance of private communications. Organizations using Mattermost for compliance-sensitive environments face particular risk as this flaw could violate regulatory requirements for access control and audit trails. The impact is particularly severe when considering that private channels are typically used for sensitive discussions, making this vulnerability a potential vector for data exfiltration or insider threats. Security teams may find that their access control policies are effectively bypassed, undermining their ability to maintain secure collaboration environments. This flaw essentially provides a backdoor mechanism for unauthorized access that could be exploited by both external attackers and internal malicious actors.
Organizations should immediately upgrade to Mattermost version 9.11.9 or later where this vulnerability has been addressed through proper access control enforcement. The recommended mitigation includes implementing additional monitoring for unusual administrative activities and channel membership changes, particularly those occurring through permalink mechanisms. Administrators should review existing private channel memberships and remove any unauthorized additions that may have occurred. Network segmentation and additional access controls should be implemented to limit the scope of potential exploitation. Organizations should also conduct comprehensive audits of their Mattermost configurations to ensure that proper approval workflows are enforced for all administrative operations. The fix implemented in the patched version addresses the core access control validation issue by enforcing mandatory approval steps for all channel membership additions, particularly those initiated through permalink links. Regular security assessments should be performed to identify similar access control weaknesses in other collaboration platforms and systems. Security awareness training for administrators should emphasize the importance of proper access control enforcement and the risks associated with automated administrative mechanisms. This vulnerability highlights the critical need for robust access control validation in collaboration platforms where sensitive information is shared and where administrative privileges can have far-reaching impacts on data security and privacy.