CVE-2021-25833 in DocumentServer
Summary
by MITRE • 03/02/2021
A file extension handling issue was found in [server] module of ONLYOFFICE DocumentServer v4.2.0.71-v5.6.0.21. The file extension is controlled by an attacker through the request data and leads to arbitrary file overwriting. Using this vulnerability, a remote attacker can obtain remote code execution on DocumentServer.
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Analysis
by VulDB Data Team • 03/14/2021
The vulnerability identified as CVE-2021-25833 represents a critical file extension handling flaw within the server module of ONLYOFFICE DocumentServer versions ranging from v4.2.0.71 through v5.6.0.21. This issue stems from insufficient validation of file extensions provided through HTTP request data, creating a pathway for malicious actors to manipulate the system's file handling mechanisms. The vulnerability operates at the intersection of input validation and file system operations, where user-controllable data directly influences the target file path and extension used during document processing operations.
The technical exploitation of this vulnerability occurs when an attacker crafts malicious requests containing specially crafted file extensions within the request parameters. The server module fails to properly sanitize or validate these extensions before processing, allowing the attacker to specify arbitrary file paths and extensions that can overwrite existing files on the server filesystem. This flaw fundamentally violates the principle of least privilege and proper input validation, enabling attackers to manipulate the file system in ways that were not intended by the application's design. The vulnerability is categorized under CWE-22 Improper Limitation of a Pathname to a Restricted Directory, which specifically addresses issues where applications fail to properly restrict file access to authorized directories.
The operational impact of CVE-2021-25833 extends beyond simple file overwriting to encompass full remote code execution capabilities. When an attacker successfully overwrites critical system files or configuration files, they can effectively compromise the entire DocumentServer instance. This vulnerability allows for privilege escalation scenarios where the attacker can gain elevated system privileges through the manipulation of system files, potentially leading to complete system compromise. The attack vector is particularly dangerous because it requires no authentication for exploitation, making it a significant threat to any organization using affected versions of ONLYOFFICE DocumentServer. The vulnerability aligns with ATT&CK technique T1059.007 Command and Scripting Interpreter: JavaScript, as attackers can leverage the compromised system to execute malicious JavaScript code or other scripting languages.
The exploitation chain typically begins with an attacker sending a malicious request containing crafted file extension parameters to the DocumentServer API endpoints. The server processes this request without proper validation, leading to the creation or overwriting of files with attacker-controlled extensions. This vulnerability has been classified under the broader category of path traversal attacks, where the attacker manipulates the file path to access or modify unauthorized files. Organizations using vulnerable versions of ONLYOFFICE DocumentServer face significant risk of data breaches, system compromise, and potential lateral movement within their network infrastructure. The vulnerability demonstrates the critical importance of implementing proper input validation and secure file handling practices in web applications, particularly those handling user-provided content. Security teams should immediately implement mitigations including version upgrades, input validation patches, and network segmentation to prevent exploitation of this vulnerability. The issue highlights the need for comprehensive security testing including dynamic application security testing and static code analysis to identify similar flaws in other applications.