CVE-2026-13639 in DiskStation Manager
Summary
by MITRE • 09/18/2026
An insufficient entropy vulnerability in login logic 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.
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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 failure in cryptographic implementation, specifically within the authentication subsystem's handling of session tokens and random number generation. This flaw stems from an insufficient entropy source used during the creation of security-critical identifiers such as login sessions or password reset tokens. In modern web applications, particularly those managing sensitive network-attached storage infrastructure like DSM, the unpredictability of these values is paramount to preventing unauthorized access. When the underlying pseudo-random number generator fails to gather sufficient environmental noise or utilizes a weak algorithmic seed, the resulting output becomes predictable or susceptible to brute-force enumeration by an attacker with remote network access. This deficiency directly undermines the core security principle that authentication tokens must be unique and unguessable for each session, thereby creating a foundational weakness in the application's identity verification process.
From a technical perspective, this issue aligns closely with CWE-330: Use of Insufficiently Random Values, which categorizes flaws where random numbers are used in security contexts but lack adequate entropy to prevent prediction. The operational impact is severe because it allows remote attackers to bypass authentication mechanisms entirely or manipulate session states without valid credentials. By predicting the next likely token value based on previous outputs or known patterns, an attacker can forge administrative sessions. This capability effectively grants the adversary full control over the DSM interface, which serves as the central management point for all connected storage volumes and user accounts. Consequently, this is not merely a theoretical risk but a practical pathway to complete system compromise, enabling actions that range from data exfiltration to total service disruption.
The ability to read or write arbitrary files through this vulnerability indicates that session manipulation can be leveraged to escalate privileges beyond the intended scope of the forged user account. If an attacker successfully predicts a token associated with an administrative role, they gain unrestricted access to file system operations managed by DSM. This includes reading sensitive configuration files, logs containing potential secrets, or proprietary data stored on the NAS devices. Furthermore, the capacity to write arbitrary files allows for persistent backdoors, such as placing malicious scripts in web-accessible directories that can be executed later, or modifying critical system binaries and configurations to maintain access even after patches are applied if not properly monitored. This dual capability of reading and writing transforms a simple authentication bypass into a comprehensive remote code execution vector when combined with other potential weaknesses in the file handling logic.
Denial-of-service attacks represent another significant consequence of this vulnerability, stemming from both resource exhaustion and state manipulation. An attacker can flood the system with forged session requests or manipulate internal counters using predictable values, leading to memory leaks or CPU spikes that degrade service availability for legitimate users. In a network-attached storage environment, where uptime is critical for business continuity, such disruptions can halt data access for entire organizations. The lack of proper rate limiting combined with weak entropy makes it computationally inexpensive for an attacker to generate thousands of valid-looking requests per second, overwhelming the authentication server and rendering the DSM interface inaccessible until services are restarted or mitigated manually by administrators.
To mitigate this risk, immediate action is required to update Synology DiskStation Manager to version 7.2.1-69057-12, 7.2.2-72806-9, 7.3.2-86009-4, or 7.4-90075 and later, which contain the necessary fixes for the entropy generation logic. Organizations should also implement network-level controls such as Web Application Firewalls to detect anomalous patterns of session token requests that may indicate brute-force attempts exploiting this weakness. Additionally, enabling multi-factor authentication adds a layer of defense-in-depth that can mitigate the impact even if an attacker successfully predicts or guesses a single-session token. Regular auditing of system logs for unusual login activity and ensuring that administrative interfaces are not exposed directly to the public internet further reduces the attack surface associated with this vulnerability.
This incident underscores the importance of adhering to established security standards regarding cryptographic randomness, as referenced in MITRE ATT&CK technique T1078: Valid Accounts, where attackers leverage legitimate credentials or session tokens obtained through prediction rather than theft. The failure here is not just a coding error but a systemic oversight in applying industry best practices for secure random number generation within high-security applications. By addressing the root cause of insufficient entropy and enforcing strict patch management policies, administrators can restore confidence in the integrity of their storage infrastructure against this specific class of remote exploitation techniques.