CVE-2026-40531

Executive Summary

CVE-2026-40531 is an integer overflow/wraparound vulnerability (CWE-190) affecting a File Operation component within Synology DiskStation Manager (DSM). Remote, authenticated attackers — i.e., any user with valid DSM credentials, regardless of privilege level — can exploit the flaw to trigger limited denial-of-service conditions against the NAS appliance. The impact is bounded (CVSS 4.3, availability-only, Low), but because DSM underpins backup, file sharing, and often identity services in SMB environments, even limited availability loss can disrupt business-critical workflows. No public exploit, PoC, or Nuclei template is currently available, and no patch artifacts have been confirmed by tracked sources (patch confidence 0/4).

Technical Analysis

CVE-2026-40531 is categorized as CWE-190: Integer Overflow or Wraparound. The NVD description states that the vulnerability resides in a "File Operation" subsystem within Synology DSME. DSM's file operation layer handles core NAS functionality — copy, move, rename, delete, and metadata manipulation across shared folders — and is exposed to authenticated users through both the DSM web interface (typically over HTTPS ports 5000/5001) and the underlying file service APIs.

An integer overflow or wraparound occurs when an arithmetic operation on an integer produces a value that exceeds the representable range of the destination type. In file operation contexts, such calculations commonly involve:

• File size or offset arithmetic (e.g., computing buffer sizes, chunk offsets, or remaining bytes).
• Counters or length fields used to pre-allocate memory or iterate over file entries.
• Multiplication of element counts and element sizes.

When the result wraps, the program may allocate an undersized buffer, compute an incorrect offset, or enter an inconsistent state. Depending on the specific code path, this can result in a crash, hang, resource exhaustion, or logic corruption. The CVSS subscores — Availability: Low, Confidentiality: None, Integrity: None, Privileges Required: Low, User Interaction: None, Attack Vector: Network — indicate that the flaw is remotely triggerable by an authenticated user without user interaction and yields only limited availability impact.

Important limitation: The NVD description does not disclose the specific function, parameter, or file operation primitive affected. No CNA technical advisory with code-level detail is referenced. This report therefore describes the vulnerability at the class level (integer overflow in file operations) and does NOT invent function names, API endpoints, or parameter identifiers — none are provided in the source data. Any implementation-specific claim beyond the description would be speculative.

Attack surface: The exploit requires prior authentication to DSM. Attack vector is Network (AV:N), meaning the attacker does not need physical or adjacent-network access — remote access to the DSM management or file service interfaces is sufficient. The attack complexity is Low (AC:L) and no user interaction is needed (UI:N), suggesting the trigger is deterministic once the correct request is issued.

Exploitation Scenarios

No public PoC exists for CVE-2026-40531. The attack chain below is inferred strictly from the vulnerability class (CWE-190, limited DoS), the CVSS vector, and the fact that the flaw resides in "File Operation." No specific endpoints, parameters, or function names are asserted because none are provided in the source data.

1. Reconnaissance / target identification. An attacker enumerates internet- or intranet-exposed Synology NAS devices, typically by fingerprinting DSM login pages on ports 5000/5001 or identifying Synology management interfaces via Shodan/Censys queries. The attacker must possess valid DSM credentials at any privilege level (PR:L).

2. Authentication. The attacker logs into DSM with low-privileged credentials. This could be legitimate access (insider threat) or credentials obtained via prior phishing, credential stuffing, or reuse of leaked credentials.

3. Triggering the flaw (class-level). The attacker issues one or more authenticated file operations crafted to cause an integer wraparound in a size, offset, or count calculation. Because the exact primitive is undisclosed, the specific request format cannot be stated. At the class level, the goal is to pass values that overflow the arithmetic used by the file operation handler, driving the process into an error, hang, or resource-exhaustion state.

4. Post-exploitation impact. The result is a limited denial-of-service condition affecting availability (A:L). Depending on the code path, this may manifest as a crashed or unresponsive file-service process, a stalled operation, or degraded responsiveness of a specific file operation subsystem. Impact is bounded — no confidentiality or integrity loss is indicated, and the scope is Unchanged (S:U), meaning the vulnerable component cannot affect resources beyond its security authority.

The absence of an exploit in the wild, a Nuclei template, and public PoC code means exploitation is currently theoretical and requires custom research by an adversary. Practical exploitation risk is low in the near term but non-zero for targeted insiders or attackers with existing NAS access.

Impact Assessment

• CVSS 3.1 Base Score: 4.3 (Medium). Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L.
  • AV:N — exploit is possible over the network.
  • AC:L — no special conditions; exploit is repeatable.
  • PR:L — requires low-privileged authenticated access.
  • UI:N — no victim interaction needed.
  • S:U — impact confined to the vulnerable component.
  • C:N / I:N — no confidentiality or integrity impact.
  • A:L — limited availability impact.
• Affected systems. Synology DiskStation Manager prior to 7.2.1-69057-10, 7.2.2-72806-7, and 7.3.2-86009-2. This includes a broad installed base of Synology NAS appliances across SMB, enterprise branch offices, and home/edge deployments.
• Blast radius. Because exploit requires authentication, the exposed population is limited to environments where an attacker already holds DSM credentials. Failure modes are limited to availability of file operations — not full device compromise, data theft, or ransomware staging.
• Business impact. NAS devices are frequently used for backup repositories, shared file storage, and collaboration. Even a "limited" DoS against file operations can interrupt backup windows, block file access for teams, and disrupt workflows dependent on SMB/NFS/AFP shares. A repeatable, low-complexity trigger means an authenticated malicious insider or compromised account can cause recurring disruption.
• EPSS. Not yet scored by FIRST.org. No exploit-in-the-wild evidence. CISA KEV: not listed.

Detection Guidance

Because no public exploit code or network signature exists, detection must be behavioral and anomaly-based — monitoring authenticated file operation activity for abnormal patterns (unusually large size/offset values, repeated failures, process crashes).

Sigma Rule

title: Potential Synology DSM Integer Overflow File Operation DoS Attempt
id: 7c9f2a41-3b68-4e0e-9a12-syn40531ab
status: experimental
description: Detects repeated authenticated file operation errors consistent with CVE-2026-40531 integer overflow abuse in Synology DSM.
references:
  - https://nvd.nist.gov/vuln/detail/CVE-2026-40531
author: valtersit.com Threat Intelligence
date: 2026/06/01
logsource:
  product: synology
  service: dsm
detection:
  selection_auth:
    event_type: 'file_operation'
  selection_error:
    status:
      - 'error'
      - 'failed'
      - 'overflow'
      - 'invalid_size'
  timeframe: 5m
  condition: selection_auth and selection_error | count() by user > 20
fields:
  - user
  - source_ip
  - operation
  - status
falsepositives:
  - Legitimate batch file operations with malformed input
  - Backup software retrying failed transfers
level: medium
tags:
  - attack.t1499
  - cve.2026.40531

YARA Rule

rule CVE_2026_40531_Synology_DSM_IntegerOverflow_Indicator
{
    meta:
        description = "Heuristic indicator for artifacts relating to CVE-2026-40531 Synology DSM integer overflow DoS"
        author = "valtersit.com Threat Intelligence"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2026-40531"
        date = "2026-06-01"
        severity = "medium"

    strings:
        $cve = "CVE-2026-40531" ascii nocase
        $dsm = "DiskStation Manager" ascii nocase
        $dsm2 = "Synology" ascii nocase

    condition:
        any of ($cve) or (2 of ($dsm, $dsm2))
}

Note: YARA is intended for file/artifact scanning. Because no malicious binary or specific file artifact is associated with this CVE, this rule is a heuristic helper rather than a high-fidelity detector and should be used only in correlation workflows.

Suricata IDS Rule

alert http any any -> any any (msg:"CVE-2026-40531 Synology DSM File Operation Integer Overflow DoS Attempt"; flow:established,to_server; http.method; content:"POST"; http.header; content:"Cookie"; content:"id="; http.uri; pcre:"/(file|download|upload|copy|move|rename)/Ui"; threshold:type both, track by_src, count 20, seconds 300; classtype:attempted-dos; reference:cve,2026-40531; reference:url,nvd.nist.gov/vuln/detail/CVE-2026-40531; sid:4053101; rev:1;)

Note: This is a broad heuristic based on repeated authenticated file-operation requests to DSM. It may produce false positives and should be tuned to the environment.

Elastic Detection

sequence by source.ip, user.name with maxspan=5m
  [ network where network.transport == "tcp"
      and destination.port in (5000, 5001)
      and http.request.method == "POST"
      and http.request.body.bytes > 5000000 ]
  [ file where event.action in ("error","failed") ]

This EQL correlates an abnormally large authenticated request to the DSM interface with subsequent file-operation errors on the same host/user — a pattern consistent with oversized size/offset parameters being rejected.

Splunk SPL Query

index=synology sourcetype="dsm:file_operation" status IN ("error","failed","overflow","invalid_size")
| stats count AS failures values(operation) AS ops by user, src_ip
| where failures > 20
| sort - failures

Supplement with a broader query for availability anomalies:

index=synology (crash OR "out of memory" OR "service restart")
| stats count by host, process
| where count > 3

Wazuh OSSEC Rule

<group name="synology,dsm,cve-2026-40531,">
  <rule id="100531" level="8">
    <decoded_as>json</decoded_as>
    <field name="status">\.(error|failed|overflow|invalid_size)</field>
    <description>CVE-2026-40531: Synology DSM file operation error possibly indicating integer overflow DoS attempt</description>
    <mitre>
      <id>T1499</id>
    </mitre>
    <group>dos,availability,</group>
  </rule>

  <rule id="100532" level="10" frequency="10" timeframe="300">
    <if_matched_sid>100531</if_matched_sid>
    <same_field>user</same_field>
    <description>CVE-2026-40531: Repeated Synology DSM file operation failures by same user - potential overflow DoS</description>
    <group>dos,availability,</group>
  </rule>
</group>

Mitigation & Patch Guidance

Patch confidence: 0/4 (UNPATCHED — no patch evidence from any tracked source). Vendor Fix: N/A. Workarounds: N/A.

1. Immediate mitigations (no vendor patch confirmed via tracked sources):

   • Restrict DSM management interfaces (ports 5000/5001 and any File Station access) from untrusted networks. Enforce access via VPN or a zero-trust gateway.
   • Enforce least privilege: minimize the number of accounts with file-operation rights; audit and remove dormant accounts.
   • Enable multi-factor authentication for all DSM accounts to reduce the risk of credential-based exploitation (the attack requires authentication).
   • Monitor DSM for unusual file-operation error rates, process crashes, and service restarts; alert on repeated failures by a single user.
   • Segment NAS appliances onto isolated VLANs to limit the blast radius of availability degradation.

2. Patch / upgrade steps:

   • Upgrade DSM to 7.2.1-69057-10, 7.2.2-72806-7, or 7.3.2-86009-2 (or later), per the NVD description's fixed versions.
   • Because patch confidence is 0/4 (no patch confirmation from tracked sources), verify the availability of these fixed builds directly on Synology's official download and security advisory channels before scheduling upgrades.
   • Prioritize upgrades for internet-exposed NAS devices first.

3. Long-term hardening:

   • Establish a routine DSM update cadence with staged rollouts and rollback plans.
   • Continuously inventory NAS appliances and track firmware versions; reconcile against the vulnerability database.
   • Integrate NAS authentication logs and file-operation telemetry into the SIEM with the detections above.
   • Apply network ACLs that deny management access from unauthorized subnets.
   • Subscribe to Synology security advisories and monitor NVD/CISA for EPSS and KEV updates on CVE-2026-40531.

References

• NVD — CVE-2026-40531: https://nvd.nist.gov/vuln/detail/CVE-2026-40531
• MITRE CWE-190 (Integer Overflow or Wraparound): https://cwe.mitre.org/data/definitions/190.html
• MITRE ATT&CK T1499 (Endpoint Denial of Service): https://attack.mitre.org/techniques/T1499/
• Synology Security Advisories (verify fixed DSM builds 7.2.1-69057-10, 7.2.2-72806-7, 7.3.2-86009-2): https://www.synology.com/en-us/security
• CISA Known Exploited Vulnerabilities Catalog (CVE-2026-40531 NOT currently listed): https://www.cisa.gov/known-exploited-vulnerabilities-catalog

Exploit-in-the-wild status: No known threat actor campaigns, PoC, or exploit code have been observed or published for CVE-2026-40531 as of this report. No Nuclei template exists. CISA KEV status: not listed.