CVE-2019-20883 in Mattermost Server
Summary
by MITRE
An issue was discovered in Mattermost Server before 5.8.0, when Town Square is set to Read-Only. Users can pin or unpin a post.
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Analysis
by VulDB Data Team • 10/25/2020
The vulnerability identified as CVE-2019-20883 represents a privilege escalation issue within the Mattermost Server platform that undermines the intended access controls for read-only channels. This flaw specifically affects versions prior to 5.8.0 where the Town Square channel has been configured with read-only permissions. The core technical issue stems from insufficient validation of user permissions during post manipulation operations, allowing authenticated users to bypass the intended read-only restrictions through pinning and unpinning actions. The vulnerability demonstrates a clear breakdown in the principle of least privilege enforcement within the messaging platform's access control mechanisms.
The technical implementation of this vulnerability exploits the inconsistency between channel permission settings and the operational capabilities available to users within those channels. When a channel is configured as read-only, standard user expectations dictate that participants should only be able to view content without the ability to modify it. However, the flaw permits users to perform pin operations that effectively alter the channel's content presentation without modifying the underlying post content itself. This creates a scenario where users can manipulate the visibility and prominence of posts while maintaining the illusion of read-only access, thereby undermining the integrity of the channel's access control model.
The operational impact of this vulnerability extends beyond simple permission bypass to potentially enable information manipulation and disruption within collaborative environments. Attackers could use this capability to promote specific posts to the top of the channel, effectively controlling information flow and potentially influencing team discussions or decisions. The ability to unpin posts also allows for content removal from prominent positions, which could be used to obscure important information or disrupt established communication patterns. This vulnerability particularly affects organizations relying on Mattermost for regulated communications or collaborative work environments where channel access controls are essential for maintaining proper information governance and security posture.
Security professionals should recognize this issue as a variant of privilege escalation attacks that exploit weak access control validation mechanisms, aligning with common attack patterns documented in the ATT&CK framework under privilege escalation techniques. The vulnerability also relates to CWE-284, which describes improper access control, and CWE-352, concerning cross-site request forgery, as the flawed permission validation could potentially be exploited in conjunction with other attack vectors. Organizations should prioritize immediate patching of affected systems to prevent exploitation, while implementing monitoring for unusual pinning activities in read-only channels. Additionally, security teams should review their access control policies and conduct regular audits of channel permissions to ensure that the intended security boundaries are maintained. The remediation process should include comprehensive testing of permission models and validation of access control implementations to prevent similar issues from emerging in other components of the messaging platform infrastructure.