CVE-2020-11535 in Document Server
Summary
by MITRE
An issue was discovered in ONLYOFFICE Document Server 5.5.0. An attacker can craft a malicious .docx file, and exploit XML injection to enter an attacker-controlled parameter into the x2t binary, to rewrite this binary and/or libxcb.so.1, and execute code on a victim's server.
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Analysis
by VulDB Data Team • 05/27/2024
The vulnerability CVE-2020-11535 represents a critical security flaw in ONLYOFFICE Document Server version 5.5.0 that stems from inadequate input validation and sanitization within the document processing pipeline. This vulnerability falls under the category of XML External Entity (XXE) injection, which is classified as CWE-611 in the Common Weakness Enumeration catalog. The flaw manifests when the application processes maliciously crafted .docx files that contain embedded XML content designed to exploit weaknesses in the document conversion process.
The technical exploitation mechanism involves crafting a specially formatted .docx file that contains malicious XML elements which are then processed by the x2t binary component of the document server. This binary serves as the core conversion engine responsible for transforming various document formats into viewable content. When the vulnerable application parses the malicious document, it inadvertently allows XML injection that can manipulate the execution flow of the x2t binary, enabling an attacker to inject arbitrary commands that ultimately result in code execution on the target server.
The operational impact of this vulnerability is severe as it provides remote code execution capabilities to attackers who can craft malicious documents and deliver them to unsuspecting users. The attack vector typically involves social engineering to convince victims to open the malicious document, which then triggers the exploitation chain. Once executed, the attacker gains the ability to rewrite critical system files including the x2t binary itself and potentially libxcb.so.1, which is a crucial shared library for X11 client-server communication. This allows for persistent access and privilege escalation within the compromised environment.
This vulnerability aligns with several tactics described in the MITRE ATT&CK framework, particularly focusing on initial access through malicious document delivery and execution through code injection techniques. The attack chain demonstrates how a seemingly benign document processing feature can be weaponized for remote code execution, making it a significant concern for organizations relying on document server solutions. Organizations using ONLYOFFICE Document Server should immediately implement mitigations including updating to patched versions, implementing strict document validation controls, and deploying network segmentation to limit the potential impact of successful exploitation attempts.
The vulnerability highlights the importance of proper input sanitization and the principle of least privilege in application design. The flaw demonstrates how XML processing libraries can become attack surfaces when not properly configured with security restrictions such as disabling external entity resolution and restricting access to system resources. Security practitioners should consider implementing content security policies and file type validation mechanisms to prevent similar vulnerabilities from being exploited in other document processing applications. Additionally, regular security assessments of document conversion systems are essential to identify and remediate potential injection points that could be leveraged for code execution attacks.