CVE-2026-40536

Executive Summary

CVE-2026-40536 is a path traversal vulnerability (CWE-22) in the Audio API component of Synology DiskStation Manager (DSM). It affects DSM versions prior to 7.2.1-69057-10, 7.2.2-72806-7, and 7.3.2-86009-2, and is exploitable by a remote, authenticated attacker over the network without user interaction. Successful exploitation permits access to non-sensitive information outside the intended restricted directory, resulting in a limited confidentiality impact (CVSS 4.3). No public proof-of-concept (PoC), exploit code, or Nuclei template is currently available, and the vulnerability has not been added to the CISA Known Exploited Vulnerabilities (KEV) catalog.

Technical Analysis

The vulnerability is classified as CWE-22: "Improper Limitation of a Pathname to a Restricted Directory ('Path Traversal')." According to the NVD description, the flaw resides in the Audio API of Synology DiskStation Manager (DSM). The mechanism is a path traversal: the Audio API fails to properly neutralize directory traversal sequences within a supplied pathname, allowing an attacker to reference files outside the intended restricted directory. Because the vulnerability is network-reachable and requires only low privileges (an authenticated user), an attacker with any valid DSM account can craft requests to the Audio API that escape the intended base directory.

The CVSS v3.1 vector CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:N/A:N indicates:

• AV:N (Network) — exploitable remotely over the network.
• AC:L (Low) — no special access conditions or elevated complexity.
• PR:L (Low) — the attacker must be authenticated with low privileges.
• UI:N (None) — no user interaction is required.
• S:U (Unchanged) — impact is confined to the vulnerable component's security scope.
• C:L (Low) — limited confidentiality impact.
• I:N / A:N — no integrity or availability impact.

The improper limitation means the application does not sufficiently canonicalize or validate the pathname before using it to resolve a file. Traversal sequences such as ../ would permit the attacker to climb out of the intended directory and reference non-sensitive files readable by the DSM process. The NVD description explicitly states that only non-sensitive information can be obtained, which constrains the severity; however, this data may still include configuration metadata, filenames, or internal path structures useful for lateral reconnaissance. No specific function names, API endpoints, parameter names, or file paths are disclosed in the available description, so the attack is characterized at the vulnerability-class level. Insufficient technical detail is available to identify the exact parameter or request structure used to deliver the traversal sequence.

Because only three specific DSM build suffixes are enumerated as fixed targets, the affected range spans the 7.2.1, 7.2.2, and 7.3.2 branches. All deployments on those branches below the listed build numbers should be presumed vulnerable until the vendor advisory is confirmed.

Exploitation Scenarios

No public PoC or exploit code is available; the following attack chain is inferred from the vulnerability class (CWE-22) and the CVSS vector. No specific function names, endpoints, or parameters are asserted beyond what the NVD description provides.

1. Reconnaissance / Target Identification. An attacker identifies internet- or intranet-exposed Synology DiskStation Manager instances. Shodan dorks such as product:"Synology DiskStation Manager", http.title:"Synology DiskStation", or http.html:"DiskStation Manager" enumerate candidate hosts. The attacker notes the DSM version/build from login pages, banners, or API responses to determine whether it falls below 7.2.1-69057-10, 7.2.2-72806-7, or 7.3.2-86009-2.

2. Authentication. Because the vulnerability requires low privileges (PR:L) and no user interaction (UI:N), the attacker first authenticates to DSM using any valid low-privilege account — for example, a guest, standard user, or credentials obtained through phishing or credential stuffing. No administrative access is required.

3. Triggering the Path Traversal. The attacker sends a request to the Audio API that includes a crafted pathname containing directory traversal sequences (e.g., ../). As the Audio API does not properly restrict the pathname to its intended restricted directory, the application resolves and returns a file outside that directory. The traversal is delivered over the network (AV:N) with low attack complexity (AC:L).

4. Post-Exploitation Impact. Successful exploitation yields non-sensitive information only (C:L). This could include file metadata, directory listings, or benign configuration values. There is no integrity or availability impact (I:N/A:N), and the security scope is unchanged (S:U). The value to a threat actor is primarily reconnaissance — mapping internal file structures or harvesting data that facilitates follow-on attacks — rather than direct data theft or system compromise.

At the time of writing, there is no evidence of exploit-in-the-wild activity, no known threat actor campaign, and no public PoC. The absence of a Nuclei template and exploit code reduces the immediate likelihood of opportunistic mass exploitation, though the low attack complexity may attract targeted actors once technical details emerge.

Impact Assessment

CVE-2026-40536 carries a base CVSS v3.1 score of 4.3 (Medium). The impact is limited to confidentiality (Low); there are no integrity or availability impacts. The attack is network-based and requires only low privileges, meaning any authenticated DSM user can attempt exploitation without user interaction.

Business impact centers on information disclosure of non-sensitive data from Synology NAS appliances. Affected systems are Synology DiskStation Manager deployments on the 7.2.1, 7.2.2, and 7.3.2 branches below the specified fixed builds. These appliances are frequently deployed as centralized storage and collaboration hubs in SMB, enterprise, and home-office environments, and are commonly internet-exposed for remote access — expanding the blast radius.

While the direct confidentiality impact is low, exposed devices broaden the attack surface: the disclosed non-sensitive information may aid reconnaissance for chained attacks (e.g., mapping internal structure before a higher-severity exploit). Because DSM manages backups, shared files, and user data, any traversal flaw in a core service warrants attention even at moderate severity. The lack of integrity and availability impacts, unchanged scope, and the "non-sensitive" qualifier from the vendor description constrain overall risk to Medium.

Detection Guidance

Sigma Rule

title: Potential Path Traversal Against Synology DSM Audio API (CVE-2026-40536)
id: 3f9c1e42-7a5b-4d18-9c2e-8b6f0d1a4c77
status: experimental
description: Detects HTTP requests containing directory traversal sequences targeting the Synology DSM Audio API.
references:
  - https://nvd.nist.gov/vuln/detail/CVE-2026-40536
author: Threat Intelligence Team
date: 2026-01-01
logsource:
  category: webserver
  product: synology
detection:
  selection_method:
    cs-method:
      - GET
      - POST
  selection_traversal:
    cs-uri-query|contains:
      - '../'
      - '..%2f'
      - '..%5c'
      - '%2e%2e%2f'
      - '%2e%2e/'
      - '..\\'
  selection_audio:
    cs-uri-stem|contains:
      - 'audio'
  condition: selection_method and selection_traversal and selection_audio
falsepositives:
  - Legitimate administrative requests containing encoded path segments
  - Security scanners and vulnerability assessment tools
level: medium
tags:
  - attack.initial_access
  - attack.t1190
  - cve.2026.40536

YARA Rule

rule CVE_2026_40536_Synology_DSM_Audio_PathTraversal
{
    meta:
        description = "Detects HTTP request patterns suggesting path traversal against Synology DSM Audio API (CVE-2026-40536)"
        author = "Threat Intelligence Team"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2026-40536"
        date = "2026-01-01"
        severity = "medium"

    strings:
        $audio = "audio" ascii nocase wide
        $trav1 = "../" ascii
        $trav2 = "..%2f" ascii nocase
        $trav3 = "%2e%2e%2f" ascii nocase
        $trav4 = "..\\" ascii
        $httpget = "GET " ascii
        $httppost = "POST " ascii

    condition:
        ($httpget or $httppost) and $audio and any of ($trav*)
}

Suricata IDS Rule

alert http any any -> any any (msg:"CVE-2026-40536 Synology DSM Audio API Path Traversal Attempt"; flow:established,to_server; http.method; content:"GET" or "POST"; http.uri; content:"audio"; nocase; http.uri; content:"../" or "..%2f" or "%2e%2e%2f" or "..\\"; nocase; reference:cve,2026-40536; reference:url,nvd.nist.gov/vuln/detail/CVE-2026-40536; classtype:attempted-recon; sid:202640536; rev:1; metadata:created_at 2026_01_01, affected_product Synology_DSM, attack_target Server, deployment Perimeter, signature_severity Minor, tag CVE_2026_40536;)

Elastic Detection

sequence by source.ip with maxspan=30s
  [ network where network.protocol == "http" and http.request.method in ("GET", "POST")
      and wildcard(http.request.uri.path, "*audio*") ]
  [ network where network.protocol == "http"
      and ( wildcard(url.query, "*../*") or wildcard(url.query, "*..%2f*")
         or wildcard(url.query, "*%2e%2e%2f*") or wildcard(url.query, "*..\\\\*") )
      and wildcard(http.request.uri.path, "*audio*") ]

Splunk SPL Query

index=web sourcetype=access_combined OR sourcetype=nginx OR sourcetype=apache
method IN (GET, POST)
uri_path="*audio*"
(uri_query="*../*" OR uri_query="*..%2f%*" OR uri_query="*%2e%2e%2f*" OR uri_query="*..\\*" OR uri="*..%252f*")
| stats count, values(uri) as uris, dc(uri) as unique_uris by src_ip, user_agent
| where count > 3
| sort - count

Wazuh OSSEC Rule

<group name="web,synology,attack,">
  <rule id="202640536" level="10">
    <if_sid>31100</if_sid>
    <id>^40[0-9]$|^50[0-9]$</id>
    <url>audio</url>
    <url>..%2f|../|%2e%2e%2f|..\\</url>
    <description>Possible CVE-2026-40536 Synology DSM Audio API path traversal attempt.</description>
    <mitre>
      <id>T1190</id>
    </mitre>
    <group>web_scan,pci_dss_6.5.1,</group>
  </rule>
</group>

Mitigation & Patch Guidance

Patch confidence: 0/4 (UNPATCHED — no patch evidence from any tracked source). No vendor fix, workaround, or patch source is currently available in the tracked data.

1) Immediate Mitigations

• Restrict access to the DSM Audio API to trusted or internal networks only; do not expose DSM management interfaces directly to the internet.
• Enforce strong, unique credentials and least-privilege accounts; since exploitation requires authentication (PR:L), eliminating low-privilege read accounts reduces exposure.
• Enable DSM's built-in firewall and geo-IP restrictions to limit which sources may reach the appliance.
• Monitor web/API logs for traversal sequences (../, ..%2f, %2e%2e%2f) directed at Audio API paths, using the detection rules above.
• Consider disabling or restricting the Audio API/Audio Station feature if it is not operationally required.

2) Patch / Upgrade Steps

• No patch is confirmed. Monitor Synology's official security advisory channels and the NVD entry for CVE-2026-40536.
• When vendor updates are released, upgrade DSM to 7.2.1-69057-10 or later, 7.2.2-72806-7 or later, or 7.3.2-86009-2 or later, as applicable to the deployed branch.
• Verify the installed build number after upgrading, and re-enable any temporarily disabled functionality only once the fix is applied.

3) Long-Term Hardening

• Maintain a documented patch cadence for DSM appliances and subscribe to Synology security notifications.
• Apply network segmentation so NAS devices cannot be reached directly from untrusted networks.
• Enable multi-factor authentication for all DSM accounts and audit permissions regularly.
• Centralize logs from DSM appliances into a SIEM with the detection content provided above.
• Perform periodic authenticated vulnerability scans and validate input-handling controls for file/path operations in exposed APIs.

References

• NVD — CVE-2026-40536: https://nvd.nist.gov/vuln/detail/CVE-2026-40536
• Synology Security Advisories: https://www.synology.com/en-global/security/advisory
• CWE-22 — Improper Limitation of a Pathname to a Restricted Directory ('Path Traversal'): https://cwe.mitre.org/data/definitions/22.html
• MITRE ATT&CK T1190 — Exploit Public-Facing Application: https://attack.mitre.org/techniques/T1190/
• CISA Known Exploited Vulnerabilities Catalog (CVE-2026-40536 not listed): https://www.cisa.gov/known-exploited-vulnerabilities-catalog

Threat actor campaigns / exploit-in-the-wild evidence: None identified. No public PoC, exploit code, Nuclei template, or KEV listing exists for CVE-2026-40536 at the time of writing.