CVE-2026-40530: Synology DSM CRLF Injection Vulnerability

CVE-2026-40530 Overview

CVE-2026-40530 is a Carriage Return Line Feed (CRLF) injection vulnerability in the User API of Synology DiskStation Manager (DSM). The flaw stems from improper neutralization of CRLF sequences [CWE-93]. Remote authenticated users can exploit the vulnerability to read or write arbitrary files on the appliance. Attackers can also trigger denial-of-service conditions that manifest after the system is rebooted. Synology addressed the issue in DSM 7.2.1-69057-10, 7.2.2-72806-7, and 7.3.2-86009-2. Successful exploitation requires network access, low privileges, and user interaction.

Critical Impact
Authenticated attackers can read or write arbitrary files on affected DSM appliances and cause denial-of-service conditions triggered after reboot.

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-2026-40530 published to the National Vulnerability Database (NVD)
2026-09-18 - Last updated in NVD database

Technical Details for CVE-2026-40530

Vulnerability Analysis

The vulnerability resides in the User API component of Synology DSM. The API fails to neutralize CRLF (\r\n) sequences supplied through authenticated requests. Attackers can inject control characters into downstream operations that treat newlines as protocol or file delimiters. The result is arbitrary file read, arbitrary file write, and persistent denial-of-service that surfaces after the appliance reboots. Because DSM manages network attached storage, arbitrary file write can enable tampering with configuration data, scheduled tasks, or system binaries.

Root Cause

The root cause is classified as CWE-93, Improper Neutralization of CRLF Sequences. The User API accepts input containing raw carriage return and line feed characters. It then passes that input to functions where newlines change parser or filesystem behavior. Absent input sanitization, injected control characters break out of intended fields and manipulate the request or file operations that follow.

Attack Vector

Exploitation requires network access to the DSM web interface and valid low-privilege credentials. The attack also requires user interaction, consistent with the UI:R element of the CVSS vector. An attacker submits crafted requests to the User API containing CRLF sequences in user-controllable parameters. Injected sequences pivot the request into unintended file paths or trigger service-affecting state that only manifests after the appliance is restarted. See the Synology Security Advisory SA-26-06 for vendor-authoritative technical details.

Detection Methods for CVE-2026-40530

Indicators of Compromise

HTTP request bodies or query parameters to the DSM User API containing encoded CRLF sequences such as %0d%0a, \r\n, or %0A%0D.
Unexpected new or modified files in DSM configuration directories not correlated with administrator activity.
DSM services failing to start or entering degraded state immediately after a scheduled or unplanned reboot.
Authenticated User API calls originating from accounts or IP ranges that do not typically administer the appliance.

Detection Strategies

Inspect DSM web server access logs for User API endpoints receiving requests with URL-encoded CR or LF characters.
Correlate authenticated API calls with subsequent file system changes on the DSM volume to identify anomalous write operations.
Baseline reboot behavior for DSM appliances and alert on new service failures that appear only after restart.
Use file integrity monitoring against DSM configuration paths to identify unauthorized writes.

Monitoring Recommendations

Forward DSM /var/log and web access logs to a centralized log store and retain them long enough to cover reboot cycles.
Monitor authentication events for the User API and flag low-privilege accounts issuing high volumes of parameterized requests.
Track appliance uptime and post-reboot service health to catch delayed denial-of-service payloads.

How to Mitigate CVE-2026-40530

Immediate Actions Required

Upgrade DSM to 7.2.1-69057-10, 7.2.2-72806-7, or 7.3.2-86009-2 or later, whichever aligns with the deployed release train.
Audit DSM user accounts and remove or disable any low-privilege accounts that are no longer required.
Restrict network exposure of the DSM web interface to management networks or VPN-only access.
Rotate credentials for any accounts that had access to the User API prior to patching.

Patch Information

Synology released fixed DSM builds identified as 7.2.1-69057-10, 7.2.2-72806-7, and 7.3.2-86009-2. Administrators should apply the update that matches their current DSM release channel through DSM Control Panel or the Synology download center. Refer to the Synology Security Advisory SA-26-06 for the authoritative fix mapping.

Workarounds

Limit DSM administrative and API access to trusted source IP ranges using the built-in firewall.
Enforce multi-factor authentication for all DSM accounts to raise the bar for the required authenticated foothold.
Disable or restrict accounts that do not require User API access until patching is complete.

bash

# Verify DSM version and confirm patched build is installed
cat /etc.defaults/VERSION | grep -E "productversion|buildnumber|smallfixnumber"

# Example expected values after patching (verify against advisory)
# productversion="7.2.1" buildnumber="69057" smallfixnumber="10"