CVE-2026-6205 in DiskStation Manager
Summary
by MITRE • 09/18/2026
An external control of file name or path vulnerability in Upload API in Synology DiskStation Manager (DSM) before 7.2.1-69057-12, 7.2.2-72806-9, 7.3.2-86009-4 and 7.4-90075 allows remote authenticated users to write arbitrary files and conduct denial-of-service attacks.
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Analysis
by VulDB Data Team • 09/18/2026
The vulnerability identified in Synology DiskStation Manager (DSM) represents a critical security flaw within the file upload functionality, specifically categorized under CWE-22: Improper Limitation of a Pathname to a Restricted Directory, commonly known as path traversal or directory traversal. This weakness exists in the Upload API component across multiple versions of DSM, including those prior to 7.2.1-69057-12, 7.2.2-72806-9, 7.3.2-86009-4, and version 7.4 up to build 90075. The core technical issue stems from insufficient validation of user-supplied input when handling file uploads. Specifically, the application fails to adequately sanitize or canonicalize filenames provided by authenticated users before processing them for storage on the underlying filesystem. This lack of rigorous input validation allows an attacker to manipulate the intended destination path through the use of special characters such as dot-dot-slash sequences (../) or other directory traversal techniques. By exploiting this flaw, a remote authenticated user can bypass the designated upload directories and write files to arbitrary locations within the server's file system that would otherwise be inaccessible due to standard permission models.
The operational impact of this vulnerability is severe because it grants an attacker significant control over the target environment beyond typical application-level permissions. Since the authentication requirement means only valid users with access credentials can exploit this flaw, the attack surface is limited but highly dangerous for compromised accounts or leaked credentials. The ability to write arbitrary files enables a range of malicious activities. Most critically, attackers can overwrite existing system binaries, configuration files, or web scripts located in sensitive directories such as /etc/, /usr/bin/, or web root folders like /web/. This capability facilitates the installation of backdoors, malware, or reverse shells that persist across reboots and are difficult to detect using standard application logs. Furthermore, the vulnerability can be leveraged for denial-of-service attacks by filling up critical disk partitions with large malicious files or overwriting essential system configuration files required for service startup, thereby rendering the NAS device inoperable and disrupting availability for all users relying on the storage infrastructure.
From a threat intelligence perspective, this vulnerability aligns with MITRE ATT&CK techniques related to persistence and privilege escalation. The act of writing arbitrary files corresponds to T1505: Server Software Component, where attackers install web shells or other persistent components. Additionally, if these written files are executed by higher-privileged processes or used to modify system configurations, it may lead to T1068: Exploitation for Privilege Escalation. The lack of proper path canonicalization is a classic example of CWE-22, which remains one of the most prevalent causes of remote code execution and unauthorized access in web applications and network appliances. Organizations managing Synology NAS devices must recognize that authentication alone does not mitigate this risk if input validation on file paths is flawed.
To mitigate this vulnerability, immediate action is required to update the DSM software to a patched version where 7.2.1-69057-12 or later for the 7.2 branch, 7.2.2-72806-9 or later for the 7.2.2 branch, 7.3.2-86009-4 or later for the 7.3 branch, and version 7.4 build 90075 or later are installed. These updates include fixes that enforce strict validation of file paths during upload operations, ensuring that only files within allowed directories can be written. In environments where immediate patching is not feasible due to operational constraints, administrators should implement network-level controls such as firewall rules to restrict access to the DSM web interface and API endpoints to trusted IP addresses only. Additionally, enabling multi-factor authentication for all administrative accounts reduces the likelihood of credential compromise that could lead to exploitation. Regular auditing of uploaded files and monitoring system logs for unusual file creation events in sensitive directories can also aid in early detection of potential exploitation attempts.