CVE-2026-40535: Synology DSM Path Traversal Vulnerability

CVE-2026-40535 Overview

CVE-2026-40535 is a path traversal vulnerability in the Desktop API of Synology DiskStation Manager (DSM). The flaw stems from improper limitation of a pathname to a restricted directory [CWE-22]. Remote attackers can exploit the issue over the network without authentication or user interaction to write limited files and trigger limited denial-of-service conditions. Synology addressed the issue in DSM versions 7.2.1-69057-10, 7.2.2-72806-7, and 7.3.2-86009-2.

Critical Impact
Unauthenticated remote attackers can write restricted files on affected Synology NAS appliances and cause limited denial-of-service, impacting the integrity and availability of storage infrastructure.

Affected Products

Synology DiskStation Manager (DSM) versions before 7.2.1-69057-10
Synology DiskStation Manager (DSM) 7.2.2 versions before 7.2.2-72806-7
Synology DiskStation Manager (DSM) 7.3.2 versions before 7.3.2-86009-2

Discovery Timeline

2026-09-18 - CVE-2026-40535 published to NVD
2026-09-18 - Last updated in NVD database

Technical Details for CVE-2026-40535

Vulnerability Analysis

The vulnerability resides in the Desktop API component of Synology DSM, the web-based management interface used to administer Synology network-attached storage appliances. The Desktop API accepts file path input from remote clients and processes it without adequately validating directory traversal sequences. Attackers can supply crafted pathnames containing traversal characters to escape the intended restricted directory. Successful exploitation allows the attacker to write files to locations outside the API's designated working directory. The vendor characterizes the write capability as limited, meaning attackers cannot arbitrarily overwrite any file, but can still affect files reachable through the traversal boundary. The same primitive enables limited denial-of-service, likely by disrupting files or services that DSM depends on for normal operation.

Root Cause

The root cause is insufficient sanitization of user-supplied path parameters passed to the Desktop API. The affected code path fails to normalize and validate input against the intended base directory, allowing sequences such as ../ to alter the resolved file location. This is a classic instance of [CWE-22: Improper Limitation of a Pathname to a Restricted Directory].

Attack Vector

Exploitation occurs remotely over the network without authentication or user interaction. An attacker sends crafted HTTP requests to the Desktop API endpoint containing manipulated file path parameters. Because Synology DSM appliances are frequently exposed to the internet for remote file access, the attack surface extends beyond internal networks. Refer to the Synology Security Advisory SA-26-06 for vendor-provided technical details.

Detection Methods for CVE-2026-40535

Indicators of Compromise

HTTP requests to DSM Desktop API endpoints containing path traversal sequences such as ../, ..%2f, or encoded variants in file path parameters.
Unexpected files appearing in DSM system directories or configuration paths not typically modified by normal user activity.
Anomalous file modification timestamps on DSM appliance system files.
DSM services entering degraded or failed states without corresponding administrative action.

Detection Strategies

Review DSM web server access logs for requests to Desktop API endpoints containing traversal patterns or unusual encoded characters in path parameters.
Correlate authentication logs with file write events on the appliance to identify unauthenticated file modifications.
Deploy web application firewall rules that inspect requests to DSM management interfaces for directory traversal signatures.

Monitoring Recommendations

Enable verbose logging on DSM appliances and forward logs to a centralized SIEM for retention and correlation.
Monitor DSM system integrity by baselining critical configuration files and alerting on unauthorized modification.
Alert on any external network access to DSM management ports from untrusted source addresses.

How to Mitigate CVE-2026-40535

Immediate Actions Required

Upgrade Synology DSM to version 7.2.1-69057-10, 7.2.2-72806-7, or 7.3.2-86009-2 or later, depending on the deployed branch.
Restrict internet exposure of the DSM management interface by placing it behind a VPN or trusted network segment.
Audit DSM appliances for unexpected file modifications or service disruptions that may indicate prior exploitation.

Patch Information

Synology released fixed versions in DSM 7.2.1-69057-10, 7.2.2-72806-7, and 7.3.2-86009-2. Administrators should apply the update corresponding to their DSM branch through the DSM Control Panel or Synology's download portal. Full remediation details are available in the Synology Security Advisory SA-26-06.

Workarounds

Disable remote access to the DSM Desktop API from untrusted networks until patches are applied.
Enforce firewall rules restricting access to DSM management ports (default 5000 for HTTP and 5001 for HTTPS) to known administrative source addresses.
Enable Synology's built-in Auto Block and Account Protection features to limit repeated abuse attempts against the management interface.

bash

# Example: restrict DSM management ports to a trusted admin subnet using firewall rules
# Replace 10.0.0.0/24 with your administrative network
iptables -A INPUT -p tcp --dport 5000 -s 10.0.0.0/24 -j ACCEPT
iptables -A INPUT -p tcp --dport 5001 -s 10.0.0.0/24 -j ACCEPT
iptables -A INPUT -p tcp --dport 5000 -j DROP
iptables -A INPUT -p tcp --dport 5001 -j DROP