CVE-2019-20858 in Mattermost Server
Summary
by MITRE
An issue was discovered in Mattermost Server before 5.15.0. It allows attackers to cause a denial of service (CPU consumption) via crafted characters in a SQL LIKE clause to an APIv4 endpoint.
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Analysis
by VulDB Data Team • 10/25/2020
The vulnerability identified as CVE-2019-20858 represents a critical denial of service flaw within the Mattermost Server platform prior to version 5.15.0. This issue stems from improper handling of specially crafted characters within SQL LIKE clause operations when processed through the APIv4 endpoint. The vulnerability exposes a fundamental weakness in the server's input validation and query processing mechanisms, creating an avenue for malicious actors to exploit the system's resource consumption patterns. Attackers can leverage this flaw by constructing specific character sequences that, when processed through the SQL LIKE operator, trigger inefficient query execution paths that consume excessive CPU resources. The vulnerability specifically targets the database query layer where user-provided input is incorporated into SQL statements without adequate sanitization or optimization measures. This allows for the exploitation of inherent inefficiencies in how the database engine processes certain LIKE pattern matching operations, particularly when wildcards and special characters are involved in malicious combinations.
The technical exploitation of this vulnerability occurs through the APIv4 endpoint which handles various user interactions and data operations within the Mattermost platform. When a crafted payload containing specially constructed characters is submitted through this endpoint, the server processes the input through a SQL query that incorporates a LIKE clause. The malicious character sequences are designed to trigger inefficient execution paths within the database engine, causing the query to consume disproportionate CPU cycles and potentially leading to system resource exhaustion. This type of vulnerability falls under the CWE-129 weakness category, which specifically addresses improper validation of input, and aligns with ATT&CK technique T1499.004 for network denial of service attacks. The vulnerability demonstrates how seemingly benign input handling can be transformed into a powerful attack vector when database query optimization is inadequate and input validation is insufficient. The flaw exploits the fundamental relationship between user input and database query execution, where poorly sanitized input can cause the database engine to perform exponentially more work than expected, creating a resource exhaustion scenario.
The operational impact of CVE-2019-20858 extends beyond simple service disruption to potentially compromise the entire Mattermost server infrastructure. When exploited successfully, the vulnerability can cause sustained high CPU utilization that may lead to system instability, application unresponsiveness, and complete service outage. The attack can be executed with relatively simple payloads, making it accessible to threat actors with minimal technical expertise. Organizations using affected Mattermost Server versions face significant risk of operational disruption, particularly in environments where the platform serves as a critical communication infrastructure. The vulnerability's impact is amplified in multi-tenant environments where a single malicious user could potentially affect the entire system. Additionally, the resource consumption pattern makes detection challenging as the attack may appear as legitimate system load, masking the actual malicious activity. This type of vulnerability also creates opportunities for broader attacks, as the server's resource exhaustion could potentially be leveraged to facilitate other attack vectors or serve as a distraction while more sophisticated attacks are underway.
Mitigation strategies for CVE-2019-20858 require immediate implementation of the vendor-provided patch for Mattermost Server version 5.15.0 and subsequent releases, which address the improper SQL query handling through enhanced input validation and query optimization. Organizations should implement additional defensive measures including API rate limiting to prevent excessive requests from single sources, input sanitization at multiple layers of the application stack, and monitoring of CPU utilization patterns for anomalous behavior. Network-level protections such as intrusion detection systems and web application firewalls should be configured to detect and block suspicious API requests containing potentially malicious character sequences. The implementation of database query monitoring and optimization should include regular review of query execution plans to identify inefficient patterns that could be exploited. Security teams should also consider implementing automated alerting for unusual resource consumption patterns and establish incident response procedures specifically addressing denial of service attacks. Organizations should conduct thorough testing of the patched environment to ensure that legitimate functionality remains intact while the vulnerability is addressed. The fix addresses the underlying CWE-129 weakness by implementing proper input validation and ensures that the ATT&CK technique T1499.004 is mitigated through proper resource management and query optimization.