CVE-2026-13639: Synology DSM Insufficient Entropy DoS

CVE-2026-13639 Overview

CVE-2026-13639 is an insufficient entropy vulnerability [CWE-331] in the login logic of Synology DiskStation Manager (DSM). The flaw allows unauthenticated remote attackers to read or write arbitrary files on affected devices and to trigger denial-of-service conditions. Synology addressed the issue in DSM 7.2.1-69057-12, 7.2.2-72806-9, 7.3.2-86009-4, and 7.4-90075. Earlier releases across these branches remain exposed until administrators apply the fixed builds.

Critical Impact
Remote, unauthenticated attackers can predict login-related values, achieve arbitrary file read and write on network-attached storage devices, and disrupt DSM availability.

Affected Products

Synology DSM versions prior to 7.2.1-69057-12
Synology DSM versions prior to 7.2.2-72806-9
Synology DSM versions prior to 7.3.2-86009-4 and 7.4-90075

Discovery Timeline

2026-09-18 - CVE-2026-13639 published to the National Vulnerability Database (NVD)
2026-09-18 - Last updated in NVD database

Technical Details for CVE-2026-13639

Vulnerability Analysis

The vulnerability resides in DSM's login logic, which relies on values generated with insufficient entropy. Attackers who can predict or reproduce these values bypass the intended randomness protections built into the authentication flow. Once the login state can be forged or replayed, the attacker gains the ability to invoke privileged file operations remotely. The advisory identifies three concrete impacts: arbitrary file read, arbitrary file write, and denial of service. Because DSM devices frequently host backups, virtual machines, and shared business data, arbitrary file access on the appliance often translates into broader data-integrity and confidentiality exposure across the network.

Root Cause

The root cause is the use of predictable or low-entropy inputs in login-related token or session generation, as classified under CWE-331: Insufficient Entropy. When cryptographic or authentication primitives are seeded with insufficient randomness, their output becomes guessable within a feasible computation window. Synology did not publish the specific primitive at fault in the public advisory.

Attack Vector

Exploitation occurs over the network against the DSM web interface. The attacker requires no prior credentials and no user interaction. After predicting the low-entropy value tied to a login session, the attacker issues DSM API requests that carry out file read, file write, or resource-exhaustion actions. Refer to the Synology Security Advisory SA-26-13 for vendor-supplied technical context.

Detection Methods for CVE-2026-13639

Indicators of Compromise

Unexpected file creation, modification, or deletion in DSM shared folders and system volumes without a corresponding authenticated user session
Bursts of failed or unusually rapid login attempts targeting /webapi/auth.cgi or DSM login endpoints from a single source
New or unrecognized DSM user sessions originating from external IP addresses outside normal administrative ranges
DSM services restarting unexpectedly or becoming unresponsive, consistent with the denial-of-service impact

Detection Strategies

Enable DSM's connection and file transfer logs, then forward them to a central SIEM for correlation against authentication events
Alert on file read or write operations against sensitive DSM paths that do not map to a preceding successful, interactive login
Baseline normal DSM administrative traffic and flag deviations in request rate, source geography, or user-agent strings

Monitoring Recommendations

Monitor inbound traffic to DSM management ports (default TCP 5000/5001) for anomalous session establishment patterns
Track DSM package and system service health for repeated crash or restart events indicative of DoS attempts
Review audit trails for privileged file operations executed outside sanctioned change windows

How to Mitigate CVE-2026-13639

Immediate Actions Required

Upgrade DSM to 7.2.1-69057-12, 7.2.2-72806-9, 7.3.2-86009-4, or 7.4-90075 as appropriate for the installed branch
Restrict DSM administrative interfaces to trusted management networks and terminate direct exposure to the public internet
Rotate DSM administrator credentials and invalidate existing sessions after patching

Patch Information

Synology has released fixed builds in the DSM 7.2.1, 7.2.2, 7.3.2, and 7.4 branches. Administrators should consult the Synology Security Advisory SA-26-13 for the authoritative version matrix and download DSM updates through Control Panel or the Synology Download Center.

Workarounds

Place DSM behind a VPN or zero-trust access broker to eliminate unauthenticated network reachability to the login endpoint
Enforce firewall rules that allow DSM management access only from named administrator source IP addresses
Enable DSM's account protection and auto-block features to slow automated request bursts against the login service

bash

# Example: restrict DSM management ports to a trusted admin subnet using iptables on an upstream gateway
iptables -A FORWARD -p tcp -s 10.0.10.0/24 -d <DSM_IP> --dport 5000:5001 -j ACCEPT
iptables -A FORWARD -p tcp -d <DSM_IP> --dport 5000:5001 -j DROP