CVE-2019-20870 in Mattermost Server
Summary
by MITRE
An issue was discovered in Mattermost Server before 5.10.0. An attacker can bypass the intended appearance of the Edited flag after changing a post's file ID.
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Analysis
by VulDB Data Team • 10/25/2020
The vulnerability identified as CVE-2019-20870 represents a significant security flaw in Mattermost Server versions prior to 5.10.0, specifically targeting the platform's post editing functionality and audit trail mechanisms. This issue exposes a critical weakness in the server's access control and data integrity validation processes, allowing malicious actors to manipulate the visual indicators that users rely upon to identify modified content. The Edited flag serves as a crucial transparency mechanism within collaborative communication platforms, enabling team members to distinguish between original and modified posts. When this flag can be bypassed through file ID manipulation, it undermines the integrity of the communication channel and potentially enables misinformation campaigns or malicious content alteration without proper attribution.
The technical implementation of this vulnerability stems from insufficient validation of post modification operations within the Mattermost server architecture. When users edit posts containing files, the system should maintain proper audit trails and visual indicators to ensure transparency. However, the flaw allows attackers to modify file identifiers in a manner that circumvents the normal editing flag generation process. This bypass occurs at the server-side validation layer where the system fails to properly verify that file ID changes are legitimate modifications rather than malicious manipulations. The vulnerability specifically exploits the gap between the file handling mechanisms and the post editing validation routines, creating an opportunity for privilege escalation through data manipulation. This weakness aligns with CWE-284 Access Control Issues, where improper access control allows unauthorized modification of system data, and represents a failure in the principle of least privilege enforcement within the Mattermost platform.
The operational impact of this vulnerability extends beyond simple visual manipulation to encompass broader security implications for collaborative work environments. Teams relying on Mattermost for critical communications may unknowingly consume altered content that appears authentic due to the bypassed Edited flag. This vulnerability particularly affects organizations that depend on audit trails for compliance purposes, as the integrity of the communication history becomes compromised. Attackers could exploit this weakness to introduce false information, modify evidence of discussions, or manipulate the perceived timeline of events within team communications. The vulnerability's impact is amplified in environments where post editing is frequently used, as the attack surface increases with the volume of editable content. Organizations using Mattermost for regulated industries such as healthcare, finance, or legal services face heightened risk due to potential compliance violations and audit trail corruption. The security implications extend to potential insider threats where authorized users could abuse this functionality to hide malicious modifications from colleagues.
Mitigation strategies for CVE-2019-20870 require immediate implementation of the official Mattermost Server patch version 5.10.0 or subsequent releases that address the validation gap in post editing functionality. Organizations should conduct comprehensive security assessments of their Mattermost deployments to identify any potential exploitation attempts that may have occurred prior to patching. System administrators must ensure that all Mattermost instances are updated to versions containing the fix, which implements proper validation of file ID changes during post editing operations. Additional defensive measures include monitoring for unusual post modification patterns and implementing enhanced logging of file handling operations to detect potential exploitation attempts. Security teams should consider implementing network-based intrusion detection systems to monitor for suspicious file ID manipulation patterns that may indicate exploitation attempts. Organizations should also review their access control policies to ensure that only authorized personnel have the ability to modify existing posts, as this reduces the potential attack surface for this particular vulnerability. The fix addresses the underlying architectural weakness by strengthening the validation process for file ID changes and ensuring that the Edited flag is properly displayed regardless of the modification method used. This aligns with ATT&CK technique T1566 Credential Access through manipulation of system components that can be exploited to bypass security controls, making the patch implementation a critical security measure for maintaining platform integrity.