CVE-2026-40531: Synology DSM DOS Vulnerability

CVE-2026-40531 Overview

CVE-2026-40531 is an integer overflow or wraparound vulnerability [CWE-190] affecting the File Operation component of Synology DiskStation Manager (DSM). The flaw allows remote authenticated users to trigger limited denial-of-service (DoS) conditions on affected network-attached storage (NAS) appliances. Synology addressed the issue in DSM versions 7.2.1-69057-10, 7.2.2-72806-7, and 7.3.2-86009-2.

Exploitation requires valid credentials on the target DSM instance and does not compromise data confidentiality or integrity. The impact is restricted to availability degradation of the File Operation subsystem.

Critical Impact
Authenticated remote attackers can trigger integer wraparound in DSM File Operation logic, producing a limited denial-of-service condition on the storage appliance.

Affected Products

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

Discovery Timeline

2026-09-18 - CVE-2026-40531 published to the National Vulnerability Database (NVD)
2026-09-18 - Last updated in NVD database

Technical Details for CVE-2026-40531

Vulnerability Analysis

The vulnerability resides in the File Operation component of Synology DSM, the operating system that powers Synology NAS devices. An integer overflow or wraparound occurs when arithmetic operations on file-related values exceed the storage capacity of the underlying integer type. The resulting wrap produces incorrect size, offset, or length values used by downstream file handling logic.

The advisory classifies the impact as a limited denial-of-service. Authenticated attackers can send crafted requests that cause the File Operation subsystem to enter an inconsistent state, disrupting service availability. Confidentiality and integrity remain intact, and the scope does not change across security boundaries.

Synology has published details in Synology Security Advisory SA-26-06. The Exploit Prediction Scoring System (EPSS) currently rates the probability of exploitation activity as low, and no public proof-of-concept has been observed.

Root Cause

The root cause is improper validation of numeric inputs used during file operations. When a value approaches the maximum representable integer, arithmetic operations wrap around to unexpected values [CWE-190]. Subsequent buffer sizing, length checks, or loop counters then operate on invalid data, leading to service disruption.

Attack Vector

Exploitation requires network access to the DSM management or file interface and valid user credentials. The attacker submits requests containing values crafted to induce arithmetic overflow within the File Operation code path. No user interaction is required beyond the authenticated session.

No verified exploitation code is publicly available. Refer to the Synology Security Advisory SA-26-06 for vendor-provided technical context.

Detection Methods for CVE-2026-40531

Indicators of Compromise

Unexpected termination or restart of DSM File Station or related file service daemons.
Authenticated sessions submitting file operation requests with unusually large size, offset, or length parameters.
Repeated failures or hangs in file listing, upload, or copy operations from a single authenticated user.

Detection Strategies

Monitor DSM system logs for crashes, watchdog restarts, or service failures tied to file operations.
Correlate authenticated user activity with spikes in error rates on DSM file service endpoints.
Baseline typical File Station request parameters and alert on outliers that deviate from normal ranges.

Monitoring Recommendations

Forward DSM logs to a centralized logging or SIEM platform for correlation with authentication events.
Track availability metrics for File Station and related SMB, AFP, and WebDAV services on Synology appliances.
Enable alerting on repeated authentication from accounts that subsequently trigger service disruptions.

How to Mitigate CVE-2026-40531

Immediate Actions Required

Upgrade DSM to version 7.2.1-69057-10, 7.2.2-72806-7, or 7.3.2-86009-2 or later as applicable to the deployed release train.
Audit DSM user accounts and remove or disable unused or shared credentials that could be leveraged by an authenticated attacker.
Restrict DSM management and file service access to trusted networks using firewall rules or VPN gating.

Patch Information

Synology has released fixed DSM builds. Administrators should apply DSM 7.2.1-69057-10, DSM 7.2.2-72806-7, or DSM 7.3.2-86009-2 depending on the version currently deployed. Refer to Synology Security Advisory SA-26-06 for release notes and download links.

Workarounds

Limit account privileges so that only trusted users can invoke file operations on the DSM appliance.
Enforce strong authentication, including two-factor authentication, on all DSM accounts to reduce the risk of credential compromise.
Place DSM appliances behind a network segmentation boundary that blocks untrusted inbound access to management and file service ports.

bash

# Verify current DSM version from the command line
cat /etc.defaults/VERSION

# Recommended: upgrade via DSM Control Panel > Update & Restore
# or download the applicable patch from the Synology Download Center