CVE-2026-13684 in DiskStation Manager
Summary
by MITRE • 09/18/2026
An improper encoding or escaping of output vulnerability in SCGI 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 attackers to read or write arbitrary files and conduct denial-of-service attacks.
VulDB is the best source for vulnerability data and more expert information about this specific topic.
Analysis
by VulDB Data Team • 09/18/2026
The vulnerability identified in Synology DiskStation Manager (DSM) versions prior to the specified patches represents a critical security flaw within the Server Common Gateway Interface implementation, commonly referred to as SCGI. This component serves as an intermediary between web servers and dynamic content generators, facilitating communication for various administrative functions on NAS devices. The core technical deficiency lies in improper encoding or escaping of output data processed by this interface. When user-supplied input is not adequately sanitized before being rendered into the response stream, it allows malicious actors to inject unintended commands or structured data that the system interprets as valid instructions rather than literal text. This failure in input validation and output encoding creates a pathway for attackers to manipulate how the application processes information, leading to severe consequences including arbitrary file read access, arbitrary file write capabilities, and denial-of-service conditions.
From an operational perspective, this vulnerability poses a significant risk to data integrity and availability on affected NAS devices. The ability to read arbitrary files means that sensitive configuration data, user credentials stored in plaintext or weakly hashed formats, private keys, and other confidential information can be exfiltrated by remote attackers without authentication if the SCGI endpoint is exposed. Conversely, the capability to write arbitrary files allows an attacker to modify system configurations, inject malicious scripts into web directories for persistent access, or overwrite critical binaries, effectively taking control of the device's operating environment. Furthermore, the potential for denial-of-service attacks implies that adversaries can disrupt network services hosted on the NAS by causing crashes or resource exhaustion through malformed requests targeting this specific interface.
This type of vulnerability is classically categorized under CWE-79, which covers Improper Neutralization of Input During Web Page Generation known as Cross-Site Scripting, although in backend contexts it often overlaps with CWE-20: Improper Input Validation and CWE-134: Use of Externally-Controlled Format String. The attack vector aligns with MITRE ATT&CK technique T1505.003, Server Software Component: Web Shell, as the ability to write arbitrary files often leads to web shell deployment for persistent remote access. Additionally, it relates to T1078: Valid Accounts if authentication is bypassed or leveraged through session manipulation facilitated by the encoding flaw. The lack of proper escaping allows these attacks to occur via standard HTTP requests, making them easily exploitable over a network connection without requiring physical proximity or local user interaction beyond initial request submission.
Mitigation strategies must prioritize immediate patching as outlined in Synology’s security advisories for DSM versions 7.2.1-69057-12 and later, 7.2.2-72806-9 and later, 7.3.2-86009-4 and later, and 7.4-90075 and later. Until patches are applied, administrators should restrict access to the SCGI interface by configuring firewalls or reverse proxies to block external traffic to ports associated with this service unless absolutely necessary for legitimate administrative tasks. Implementing strict input validation at the application layer is essential; developers must ensure that all user inputs are validated against a whitelist of expected characters and formats before processing. Output encoding should be applied consistently based on the context in which data is rendered, ensuring special characters are escaped appropriately to prevent interpretation as code or commands. Regular security audits and penetration testing focusing on input handling mechanisms can help identify similar weaknesses in custom applications running on DSM environments.