CVE-2025-6226 in Mattermost
Summary
by MITRE • 07/18/2025
Mattermost versions 10.5.x <= 10.5.6, 10.8.x <= 10.8.1, 10.7.x <= 10.7.3, 9.11.x <= 9.11.16 fail to verify authorization when retrieving cached posts by PendingPostID which allows an authenticated user to read posts in private channels they don't have access to via guessing the PendingPostID of recently created posts.
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Analysis
by VulDB Data Team • 07/25/2025
The vulnerability identified as CVE-2025-6226 represents a critical authorization bypass flaw within the Mattermost collaboration platform that affects multiple version streams including 9.11.x through 9.11.16, 10.7.x through 10.7.3, 10.8.x through 10.8.1, and 10.5.x through 10.5.6. This security weakness stems from insufficient validation of user permissions when processing cached post retrieval requests, specifically when utilizing the PendingPostID parameter. The flaw operates under the principle of insecure direct object reference as classified by CWE-639, where the system fails to properly verify that the requesting user has legitimate access rights to the requested resource.
The technical implementation of this vulnerability exploits the caching mechanism within Mattermost's architecture where posts are temporarily stored and retrieved using PendingPostID identifiers. When an authenticated user attempts to access cached posts through the affected API endpoints, the system does not adequately validate whether the requesting user possesses the necessary permissions to view the content associated with the specific PendingPostID. This authorization gap allows malicious actors to perform post-forgery attacks by systematically guessing or enumerating PendingPostID values of recently created posts within private channels. The vulnerability is particularly dangerous because it leverages the legitimate caching infrastructure of the platform rather than exploiting external attack vectors, making detection more challenging for security monitoring systems.
The operational impact of CVE-2025-6226 extends beyond simple information disclosure as it fundamentally compromises the integrity of Mattermost's access control mechanisms. An authenticated attacker with knowledge of the platform's internal caching patterns can potentially gain access to sensitive communications, private channel discussions, and confidential information shared within private teams. This vulnerability directly violates the principle of least privilege and undermines the core security model of collaborative platforms where access control is paramount. The attack surface is particularly concerning given that the exploitation requires only basic authentication credentials, making it accessible to users who may not have elevated privileges but can still leverage the system's caching infrastructure for unauthorized data access.
From a threat modeling perspective, this vulnerability aligns with tactics described in the MITRE ATT&CK framework under the T1566 phase of credential access and privilege escalation, where attackers exploit application-level flaws to bypass authorization controls. The vulnerability also relates to the T1078 technique of valid accounts for persistence and privilege escalation, as it allows attackers to leverage their legitimate authentication credentials to access data they should not normally be able to view. Organizations relying on Mattermost for secure communications and collaboration may face significant compliance implications, particularly in regulated environments where unauthorized access to private communications violates data protection standards such as GDPR, HIPAA, or SOX requirements.
The recommended mitigations for CVE-2025-6226 primarily involve immediate version upgrades to patched releases where the authorization verification has been strengthened. Organizations should implement comprehensive access control reviews and monitor for unusual patterns in cached post retrieval requests that may indicate exploitation attempts. Additionally, security teams should consider implementing network-level monitoring to detect anomalous API access patterns and establish proper logging of all post retrieval activities for forensic analysis. The vulnerability demonstrates the importance of rigorous input validation and proper authorization checks even within legitimate system caching mechanisms, highlighting the need for security-by-design principles in collaborative platform development. Organizations should also conduct thorough security assessments of their Mattermost installations to identify any other potential authorization bypass vulnerabilities within their deployment environment.