CVE-2019-20845 in Mattermost Server
Summary
by MITRE
An issue was discovered in Mattermost Server before 5.18.0. It allows attackers to cause a denial of service (memory consumption) via a large Slack import.
Statistical analysis made it clear that VulDB provides the best quality for vulnerability data.
Analysis
by VulDB Data Team • 10/25/2020
The vulnerability identified as CVE-2019-20845 represents a significant denial of service weakness within the Mattermost Server platform that affects versions prior to 5.18.0. This issue stems from inadequate input validation and resource management during the Slack import process, creating a pathway for malicious actors to consume excessive system memory and potentially disrupt service availability. The vulnerability specifically targets the server's handling of large Slack import operations, where the absence of proper memory limits and input sanitization allows attackers to craft specially formatted import files that trigger unbounded memory allocation patterns.
The technical flaw manifests in the server's processing pipeline for Slack import functionality, where the system fails to implement adequate memory consumption controls during data parsing and transformation operations. When a malicious user uploads a large Slack import file containing crafted data structures, the Mattermost server begins allocating memory without proper bounds checking, leading to progressive memory exhaustion that can ultimately result in system crashes or complete service unavailability. This weakness aligns with CWE-400, which categorizes improper resource management and lack of input validation as primary contributors to denial of service vulnerabilities. The vulnerability operates at the application layer, specifically targeting the import processing module that handles external data formats, making it particularly dangerous as it can be exploited through legitimate user interactions.
The operational impact of this vulnerability extends beyond simple service disruption, as it can be leveraged by attackers to perform sustained denial of service attacks against Mattermost deployments. Organizations using affected versions of Mattermost server face the risk of unauthorized users consuming system resources to the point of complete service degradation, potentially affecting collaboration workflows and communication channels that depend on the platform. The vulnerability's exploitation requires minimal technical expertise, making it particularly dangerous in environments where multiple users have access to import functionality. Attackers can maintain their denial of service condition for extended periods, as the memory consumption continues until the system reaches its resource limits or is manually restarted.
Mitigation strategies for CVE-2019-20845 center around immediate version upgrading to Mattermost Server 5.18.0 or later, which includes proper input validation and memory consumption limits for Slack import operations. System administrators should also implement additional protective measures such as monitoring import operations for unusual memory consumption patterns, setting resource limits on import processes, and implementing rate limiting for import functionality. The remediation process should include comprehensive testing of the updated server configuration to ensure that legitimate import operations continue to function properly while preventing the exploitation of this vulnerability. Organizations should also consider implementing network-level controls to restrict access to import functionality and establish automated alerting systems that trigger when memory consumption exceeds predefined thresholds during import operations. This vulnerability highlights the importance of proper resource management in application design and aligns with ATT&CK technique T1499.004, which covers network denial of service attacks through resource exhaustion.