CVE-2026-40538: Synology DSM Auth Bypass Vulnerability

CVE-2026-40538 Overview

CVE-2026-40538 is an improper restriction of excessive authentication attempts vulnerability [CWE-307] in the Auto Block feature of Synology DiskStation Manager (DSM). The flaw allows remote attackers to read limited files through brute-force attacks against affected DSM installations. Synology assigned the issue as tracked in Synology Security Advisory SA-26-06. The vulnerability affects DSM versions prior to 7.2.1-69057-10, 7.2.2-72806-7, and 7.3.2-86009-2. Attack complexity is high and no privileges or user interaction are required, but successful exploitation only exposes limited file content.

Critical Impact
Remote attackers can bypass Auto Block protections through brute-force attacks to read a limited set of files on affected Synology DSM appliances.

Affected Products

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

Discovery Timeline

2026-09-18 - CVE-2026-40538 published to NVD
2026-09-18 - Last updated in NVD database

Technical Details for CVE-2026-40538

Vulnerability Analysis

The Auto Block feature in Synology DSM is designed to block source IP addresses after a configurable number of failed authentication attempts. This vulnerability weakens that safeguard, allowing attackers to continue guessing credentials or authentication tokens beyond the intended threshold. Because the throttling logic does not correctly restrict excessive attempts, remote adversaries can carry out sustained brute-force campaigns against DSM authentication surfaces.

Successful exploitation grants read access to a limited set of files. The impact is scoped to confidentiality, with no integrity or availability effects. Attack complexity is high, which reflects preconditions such as network reachability of DSM authentication interfaces and the volume of attempts required.

Root Cause

The root cause is an incomplete or bypassable implementation of the Auto Block rate-limiting control [CWE-307]. Authentication attempts that should trigger source blocking are not consistently counted or enforced, leaving the brute-force protection ineffective under certain request patterns.

Attack Vector

The attack vector is network-based. An unauthenticated remote attacker submits a large volume of authentication attempts to a DSM endpoint. Because Auto Block does not correctly restrict repeated failures, the attacker can iterate through credential guesses until valid material is discovered, then use that access to read limited files. Refer to Synology Security Advisory SA-26-06 for vendor technical details.

Detection Methods for CVE-2026-40538

Indicators of Compromise

High volumes of failed authentication events in DSM logs originating from single or distributed source IPs.
Successful DSM logins that follow long sequences of failed attempts against the same account.
Unexpected file read activity on DSM shares immediately after authentication anomalies.

Detection Strategies

Correlate DSM authentication logs to identify failed-login bursts that should have triggered Auto Block but did not.
Alert on authentication attempts against DSM administrative interfaces from IP addresses outside approved management ranges.
Baseline normal login frequency per account and flag deviations indicating credential guessing.

Monitoring Recommendations

Forward DSM system and connection logs to a centralized logging or SIEM platform for retention and correlation.
Monitor perimeter and firewall logs for scanning or repeated connection attempts to DSM ports (default 5000/5001).
Track Auto Block list changes to confirm the feature is actively adding offending sources.

How to Mitigate CVE-2026-40538

Immediate Actions Required

Upgrade DSM to 7.2.1-69057-10, 7.2.2-72806-7, 7.3.2-86009-2, or later as applicable to the installed release branch.
Restrict network exposure of DSM management interfaces to trusted networks or VPN only.
Enforce strong, unique passwords and enable two-factor authentication for all DSM accounts.

Patch Information

Synology has released fixed DSM builds 7.2.1-69057-10, 7.2.2-72806-7, and 7.3.2-86009-2 that address the Auto Block enforcement flaw. Administrators should apply the update matching their current DSM branch. Full remediation guidance is available in Synology Security Advisory SA-26-06.

Workarounds

Reduce the Auto Block trigger threshold and extend block duration in DSM security settings.
Place DSM behind a reverse proxy or firewall that enforces additional rate limiting on authentication endpoints.
Disable direct internet exposure of DSM and require VPN access for remote administration.

bash

# Configuration example: verify installed DSM version before and after patching
cat /etc.defaults/VERSION | grep -E "productversion|buildnumber|smallfixnumber"