CVE-2019-20867 in Mattermost Server
Summary
by MITRE
An issue was discovered in Mattermost Server before 5.11.0. An attacker can interfere with a channel's post loading via one crafted post.
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Analysis
by VulDB Data Team • 10/25/2020
The vulnerability identified as CVE-2019-20867 represents a significant security flaw in Mattermost Server versions prior to 5.11.0 that allows attackers to manipulate the loading behavior of posts within channels. This issue stems from insufficient input validation and sanitization mechanisms within the server's post handling functionality, creating an avenue for malicious actors to disrupt normal channel operations through the strategic crafting of a single post.
The technical implementation of this vulnerability involves a flaw in how the Mattermost server processes and renders posts within channels, particularly when handling specific payload structures that trigger unexpected behavior in the client-side rendering logic. Attackers can craft a malicious post containing specially formatted content that, when processed by the server, causes the client application to malfunction during post loading operations. This disruption manifests as the channel's post loading mechanism becoming unresponsive or behaving erratically, effectively creating a denial of service condition that impacts legitimate users within the affected channel.
From an operational perspective, this vulnerability poses a substantial risk to organizations relying on Mattermost for collaborative communications, as it can be exploited to disrupt team workflows and communication channels without requiring elevated privileges or complex attack vectors. The impact extends beyond simple service disruption, as it can potentially be used to obscure legitimate communications or create confusion among team members. The vulnerability's exploitation requires minimal effort from attackers, making it particularly dangerous as it can be leveraged in targeted attacks against specific channels or organizations without significant technical expertise.
The flaw aligns with CWE-20, which addresses "Improper Input Validation," and demonstrates how inadequate sanitization of user-provided content can lead to system instability and denial of service conditions. From an ATT&CK framework perspective, this vulnerability maps to T1499.004, specifically "Endpoint Denial of Service," and potentially T1566.001, "Phishing with Social Engineering," as attackers may use this capability to create confusion during phishing campaigns. Organizations utilizing Mattermost server should prioritize immediate patching to version 5.11.0 or later, while also implementing monitoring for unusual post loading patterns that might indicate exploitation attempts. Additional mitigations should include implementing content filtering mechanisms and restricting post creation privileges to trusted users within sensitive channels to reduce the attack surface and limit potential impact of such exploits.