CVE-2019-20871 in Mattermost Server
Summary
by MITRE
An issue was discovered in Mattermost Server before 5.9.0, 5.8.1, 5.7.3, and 4.10.8. The Markdown library allows catastrophic backtracking.
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Analysis
by VulDB Data Team • 10/25/2020
The vulnerability identified as CVE-2019-20871 represents a critical security flaw in the Mattermost server software that affects multiple version branches including 4.10.8, 5.7.3, 5.8.1, and versions prior to 5.9.0. This issue resides within the Markdown processing library component that handles user-generated content formatting within the messaging platform. The vulnerability stems from improper input validation and processing of specially crafted Markdown syntax that can trigger catastrophic backtracking behavior in the regular expression engine. This type of vulnerability falls under the CWE-1333 category, which specifically addresses issues related to regular expression matching that can lead to denial of service conditions through excessive computational resource consumption.
The technical implementation of this vulnerability occurs when the Markdown parser encounters malformed or maliciously constructed input that contains regular expressions with exponential backtracking patterns. When processing such input, the parser's regular expression engine enters into a state where it must evaluate an exponentially growing number of possible matching paths, leading to dramatic performance degradation and potential system resource exhaustion. This behavior is characteristic of regular expression denial of service vulnerabilities where the computational complexity grows exponentially rather than linearly with input size. The vulnerability specifically affects the server-side Markdown processing functionality that handles user messages, comments, and other content that may contain formatted text elements.
The operational impact of CVE-2019-20871 extends beyond simple performance degradation to potentially enable full denial of service conditions against the Mattermost server infrastructure. Attackers can craft malicious Markdown content that when processed by the server causes significant CPU utilization and memory consumption, effectively rendering the service unavailable to legitimate users. This vulnerability particularly impacts collaborative environments where users can submit content that gets parsed and displayed to other users, creating a potential vector for attackers to disrupt communications within organizations using Mattermost. The vulnerability aligns with ATT&CK technique T1499.004 which covers network denial of service attacks and can be leveraged to create persistent availability issues that may require system restarts or manual intervention to resolve.
Organizations using affected versions of Mattermost should immediately implement mitigations including upgrading to patched versions 5.9.0, 5.8.1, 5.7.3, or 4.10.8 as these releases contain fixes that address the regular expression backtracking issue. Additional defensive measures include implementing input validation and sanitization at multiple layers, limiting the complexity of Markdown processing, and monitoring for unusual CPU consumption patterns that may indicate exploitation attempts. The fix typically involves updating the Markdown library to use more efficient regular expression patterns or implementing timeouts for processing operations to prevent indefinite backtracking scenarios. Security teams should also consider implementing rate limiting for content processing and conducting thorough testing of the patched software to ensure that the vulnerability has been properly resolved without introducing regressions in functionality.