CVE-2026-40538

Executive Summary

CVE-2026-40538 is an improper restriction of excessive authentication attempts vulnerability (CWE-307) in the Auto Block component of Synology DiskStation Manager (DSM). Because the auto-blocking logic fails to adequately throttle or block repeated authentication failures, remote attackers can conduct brute-force attacks against exposed DSM instances. Successful credential guessing yields only limited file read access per the NVD description, resulting in a low-severity confidentiality impact (CVSS 3.7). Synology NAS devices running DSM builds prior to 7.2.1-69057-10, 7.2.2-72806-7, or 7.3.2-86009-2 are affected.

Technical Analysis

The vulnerability is classified under CWE-307 (Improper Restriction of Excessive Authentication Attempts). In Synology DSM, "Auto Block" is the security feature responsible for detecting and blocking source IP addresses that generate excessive failed login attempts within a defined window. CVE-2026-40538 indicates that this protection can be bypassed or is insufficiently enforced, allowing a remote unauthenticated attacker to sustain a high-volume authentication guessing campaign beyond the intended rate or lockout limits.

Key technical observations, strictly limited to what the NVD entry states:

• Attack surface: The authentication interface of DSM exposed over the network. The CVSS vector (AV:N) confirms the attack is network-reachable.
• Attack complexity: Rated High (AC:H), indicating that successful exploitation depends on conditions beyond the attacker's direct control — consistent with a brute-force scenario where success is probabilistic and depends on credential strength, account lockout tuning, or timing.
• Privileges required: None (PR:N), meaning no prior authentication is needed to attempt the attack.
• User interaction: None (UI:N).
• Confidentiality impact: Low (C:L) — the description states attackers may "read limited files."
• Integrity/availability impact: None (I:N, A:N).

The core deficiency is that the Auto Block mechanism does not effectively restrict excessive authentication attempts. This maps to a failure of rate-limiting/lockout controls rather than a memory-safety or injection flaw. No code-level detail (function names, endpoints, or parameters) is available in the NVD description, so implementation-specific claims cannot be made.

No public proof-of-concept, Nuclei template, or exploit code is available as of this report.

Exploitation Scenarios

The attack chain is inferred from the vulnerability class (CWE-307) and the NVD description. No public PoC exists; specific implementation details are not provided in available sources.

1. Reconnaissance / target identification: An attacker scans the internet for internet-exposed Synology DSM login interfaces (e.g., via Shodan or similar services using fingerprints such as the DSM web login page). Targets are DSM versions prior to the fixed builds listed above.

2. Triggering the flaw class: The attacker initiates repeated authentication attempts against the DSM login interface. The improper restriction of excessive authentication attempts means the Auto Block protection fails to slow, throttle, or block the campaign as intended, permitting a sustained brute-force effort.

3. Payload / execution method: At the vulnerability-class level, the "payload" is a dictionary or credential-stuffing attack — automated submission of username/password pairs until valid credentials are found. No crafted request, injection payload, or specific endpoint is described in the NVD entry, so no specific request structure can be asserted.

4. Post-exploitation impact: Per the description, a successful brute-force yields the ability to "read limited files." This is consistent with the Low confidentiality subscore — access is constrained and does not, per the data given, extend to remote code execution, data modification, or denial of service.

Because no public PoC or exploit code exists, this scenario is a class-level reconstruction, not a confirmed weaponized chain.

Impact Assessment

• CVSS Base Score: 3.7 (Low). Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:N/A:N.
• Confidentiality: Low — limited file read access following successful credential guessing.
• Integrity / Availability: None.
• Blast radius: Any internet- or network-exposed Synology DSM instance running an affected version. NAS devices frequently store sensitive organizational and personal data, so even "limited" file read access can be meaningful depending on exposure and the privileges of the compromised account.
• Exploitability: EPSS not yet scored by FIRST.org. No CISA KEV listing. No public exploit or Nuclei template. The High attack complexity and Low impact keep overall risk moderate, but the absence of a vendor fix raises the residual risk for exposed deployments.

Detection Guidance

Sigma Rule

title: Potential Brute Force Against Synology DSM Login (Auto Block Bypass)
id: 1c2d3e4f-5a6b-7c8d-9e0f-1a2b3c4d5e6f
status: experimental
description: Detects excessive failed authentication attempts against Synology DSM that may indicate exploitation of CVE-2026-40538 (improper restriction of excessive authentication attempts).
references:
  - https://nvd.nist.gov/vuln/detail/CVE-2026-40538
author: valtersit.com
date: 2026-01-01
tags:
  - attack.credential_access
  - attack.t1110
logsource:
  product: synology
  service: dsm
detection:
  selection:
    message|contains:
      - 'authentication failure'
      - 'login failed'
      - 'User login failed'
  timeframe: 5m
  condition: selection | count() by source_ip > 20
fields:
  - source_ip
  - user
falsepositives:
  - Legitimate users repeatedly mistyping passwords
  - Misconfigured service accounts
level: medium

YARA Rule

rule CVE_2026_40538_Synology_AutoBlock_BruteForce
{
    meta:
        description = "Detects brute-force / credential stuffing tooling targeting Synology DSM Auto Block (CVE-2026-40538) in HTTP traffic captures"
        author = "valtersit.com"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2026-40538"
        date = "2026-01-01"
        severity = "medium"

    strings:
        $dsm1 = "DiskStation" nocase
        $dsm2 = "webman/modules/LoginMgr" nocase
        $login1 = "/webapi/entry.cgi" nocase
        $login2 = "login" nocase
        $auth1 = "SYNO.API.Auth" nocase
        $pass1 = "passwd=" nocase
        $pass2 = "password=" nocase
        $tool1 = "hydra" nocase
        $tool2 = "patator" nocase
        $tool3 = "medusa" nocase

    condition:
        uint16(0) == 0x4854 and
        (
            ($dsm1 or $dsm2) and
            ($login1 or $login2 or $auth1) and
            ($pass1 or $pass2 or $tool1 or $tool2 or $tool3)
        )
}

Suricata IDS Rule

alert http any any -> any any (msg:"CVE-2026-40538 - Excessive Synology DSM Login Attempts (Auto Block Bypass Brute Force)"; flow:established,to_server; http.method; content:"POST"; http.uri; content:"/webapi/entry.cgi"; http_uri; content:"SYNO.API.Auth"; http_client_body; content:"passwd"; threshold:type both, track by_src, count 20, seconds 60; classtype:attempted-user; reference:cve,2026-40538; reference:url,nvd.nist.gov/vuln/detail/CVE-2026-40538; sid:9000001; rev:1;)

Note: The webapi/entry.cgi and SYNO.API.Auth tokens are the widely documented DSM web API login path used in production rule sets. Confirm against your DSM version before enabling.

Elastic Detection

sequence by source.ip with maxspan=5m
  [ authentication where event.outcome == "failure" and
      event.module == "synology" and user.name != null ] with runs=20

Splunk SPL Query

index=synology sourcetype="synology:dsm"
(event=login_failure OR message="login failed" OR "authentication failure")
| bin _time span=5m
| stats count AS failed_logins, dc(user) AS unique_users values(src_ip) AS src_ip by src_ip
| where failed_logins > 20
| sort - failed_logins

Simplify to by src_ip in the final stats where required by your Splunk indexer.

Wazuh OSSEC Rule

<group name="synology,brute_force,">
  <rule id="100401" level="0">
    <decoded_as>json</decoded_as>
    <field name="program">synology</field>
    <description>Synology DSM authentication event base rule</description>
  </rule>

  <rule id="100402" level="10" frequency="20" timeframe="300">
    <if_matched_sid>100401</if_matched_sid>
    <match>login failed|authentication failure</match>
    <same_source_ip />
    <description>Possible brute-force against Synology DSM login (CVE-2026-40538 - Auto Block bypass)</description>
    <mitre>
      <id>T1110</id>
    </mitre>
    <group>authentication_failures,</group>
  </rule>
</group>

Mitigation & Patch Guidance

Patch confidence: 0/4 (UNPATCHED — no patch evidence from any tracked source).

1. Immediate mitigations:

• Do not expose DSM management/login interfaces directly to the internet. Restrict access to trusted networks or place behind a VPN.
• Enforce strong, unique credentials and Multi-Factor Authentication on all DSM accounts, especially administrative ones.
• Tune and monitor Auto Block and account lockout settings; augment with network-layer rate limiting such as a reverse proxy or WAF.
• Alert on high-volume authentication failures from a single source.

2. Patch / upgrade steps:

• NVD references fixed builds DSM 7.2.1-69057-10, 7.2.2-72806-7, and 7.3.2-86009-2. Upgrade to these or later versions if/when available.
• Given the 0/4 patch-confidence rating and empty patch-source list, verify availability directly with the vendor advisory before relying on any fix. No vendor fix or workaround is confirmed in tracked sources as of this report.

3. Long-term hardening:

• Segment NAS/management devices away from untrusted networks.
• Centralize authentication logs and apply SIEM correlation for credential access (T1110).
• Conduct periodic exposure reviews to detect accidental internet-facing DSM instances.

References

1. NVD — CVE-2026-40538: https://nvd.nist.gov/vuln/detail/CVE-2026-40538
2. MITRE CWE-307 (Improper Restriction of Excessive Authentication Attempts): https://cwe.mitre.org/data/definitions/307.html
3. MITRE ATT&CK T1110 (Brute Force): https://attack.mitre.org/techniques/T1110/
4. Synology Security Advisories (vendor advisory portal): https://www.synology.com/en-global/security
5. FIRST EPSS: not yet scored for this CVE as of this report.

Threat actor campaigns / exploit-in-the-wild: None identified. CISA KEV status: NO. No public PoC, Nuclei template, or exploit code available.