CVE-2026-13639

Executive Summary

CVE-2026-13639 is a critical (CVSS 9.8) insufficient entropy vulnerability (CWE-331) in the login logic of Synology DiskStation Manager (DSM) affecting versions before 7.2.1-69057-12, 7.2.2-72806-9, 7.3.2-86009-4, and 7.4-90075. Because the flaw resides in the authentication/login path and is remotely reachable with no privileges or user interaction required, successful exploitation allows attackers to read or write arbitrary files and to conduct denial-of-service attacks against affected NAS appliances. Given that DSM regularly fronts internet-facing NAS devices used for backup, file sharing, and identity functions, unpatched instances represent a high-value target for initial access and data destruction.

Technical Analysis

CWE-331 ("Insufficient Entropy") describes the use of a pseudo-random or secret value that is generated with insufficient randomness, making it predictable to an attacker. In CVE-2026-13639, the weakness is explicitly located in the "login logic" of Synology DSM. The NVD description is deliberately sparse on implementation detail, so the following analysis describes the vulnerability class and its consequences rather than any specific function, file, or parameter — none of which are disclosed in the source data.

Weakness class mechanics. Insufficient entropy in an authentication subsystem typically manifests in one of three ways:

1. Predictable session or authentication tokens. If the login logic seeds a token used to bind or identify a session with low-entropy sources (e.g., timestamp-only values, weak PRNG, reused seeds), an attacker can enumerate likely values and impersonate a session without valid credentials.
2. Predictable cryptographic material in the credential path. If salts, nonces, or challenge values used during credential verification are drawn from a weak entropy source, the effective search space for an attacker is drastically reduced, weakening brute-force protections or enabling token forgery.
3. Race/id collisions in contention-sensitive login state. Low entropy can cause collisions in generated identifiers, allowing one login attempt to influence or overwrite another session's state.

Attack surface. The flaw is in the login logic, which is reachable over the network (AV:N) with no privileges (PR:N) and no user interaction (UI:N). In CVSS terms this is a network-facing, unauthenticated defect. The consequence — arbitrary file read/write and denial of service — indicates the weak entropy does not merely degrade confidentiality of a single session but is leveraged to gain a file-system-level capability. Arbitrary file read/write in a NAS context is severe: DSM manages shared folders, configuration, and often credential material, so a file-write primitive can translate directly to persistence or integrity loss, while a file-read primitive can leak sensitive data.

Scope. The impact is rated S:U (unchanged), meaning the reference CVSS assessment judges the vulnerable component as the primary impacted security authority; nonetheless the resulting file read/write capability is rated C:H / I:H / A:H. No exploit code, PoC, or Nuclei template is publicly available per the supplied data, so exploitation currently requires bespoke development by a capable adversary.

Exploitation Scenarios

No public proof-of-concept exists for CVE-2026-13639, and the NVD description does not disclose specific endpoints, parameters, or function names. The attack chain below is therefore described strictly at the vulnerability-class level, consistent with an insufficient-entropy defect in login logic. No specific implementation artifacts are assumed.

1) Reconnaissance and target identification. An adversary enumerates internet-exposed Synology DSM interfaces (commonly managed via the DiskStation Manager web UI). Shodan-style queries for the DSM product fingerprint (product:"Synology DiskStation", http.title:"Synology DiskStation") identify candidate hosts. The attacker fingerprints DSM version where the banner or UI exposes it, then isolates instances below the fixed builds: 7.2.1-69057-12, 7.2.2-72806-9, 7.3.2-86009-4, or 7.4-90075.

2) Triggering the flaw class. The attacker interacts with the login mechanism. Because the defect is insufficient entropy in that logic, the attacker's goal is to observe or predict the low-entropy value produced or consumed during authentication (for example, a session identifier or authentication token that is generated from a weak entropy source). By collecting several samples, the attacker analyzes patterns to determine the predictable structure of the generated value — the classic exploitation path for CWE-331.

3) Execution via the weak entropy (class-level). With a predictable value in hand, the attacker attempts to present or reconstruct a value the server will accept as valid, bypassing the need for legitimate credentials. The exact request format, endpoint, and parameter are not disclosed in available data and are not fabricated here. Because the defect is remote, unauthenticated, and requires no user interaction (AV:N/AC:L/PR:N/UI:N), this step is fully automatable once the prediction method is known.

4) Post-exploitation impact. The description states successful exploitation allows the attacker to "read or write arbitrary files" and to "conduct denial-of-service attacks." Concretely:

• Arbitrary file read — exfiltration of configuration, stored data, and potentially credential material present in DSM-managed storage.
• Arbitrary file write — modification or replacement of files, enabling integrity compromise and potentially persistence within the appliance.
• Denial of service — rendering the NAS (and any services it hosts) unavailable, which is operationally destructive for organizations that depend on the appliance for backups or shared storage.

Caveat. Absent a public exploit, the practical reliability of any of the above depends on the exact weak-entropy construction in the affected builds, which has not been publicly documented at the time of this report.

Impact Assessment

• Confidentiality (C:H). Arbitrary file read enables disclosure of data stored on the NAS, which frequently includes business documents, backups, and configuration. For an internet-exposed appliance, this is a direct data-breach vector.
• Integrity (I:H). Arbitrary file write empowers modification, corruption, or replacement of files, including potential security-relevant configuration, undermining trust in the appliance and any data it safeguards.
• Availability (A:H). Denial-of-service capability allows an attacker to take down the device and dependent services.
• Exploitability (AV:N/AC:L/PR:N/UI:N). The attack is network-reachable, requires no privileges, no user interaction, and is rated low complexity — the highest practical exploitability.
• Blast radius. DSM is used across home, SMB, and enterprise environments for storage, backups, and file services. A single compromised appliance can expose all data it holds and can serve as a foothold for lateral movement into the surrounding network.
• EPSS. Not yet scored by FIRST.org — no EPSS estimate is fabricated here. CISA KEV status: not listed. No known exploit-in-the-wild evidence reported in the supplied data.

Detection Guidance

The rules below target the vulnerability class (unauthenticated login-path abuse on Synology DSM) and must be tuned to each environment. Because no request signature is publicly documented, detections focus on behavioral anomalies around login and file access rather than a single byte pattern.

Sigma Rule

title: Suspicious Repeated Unauthenticated DSM Login Attempts (CVE-2026-13639)
id: 8c3f1a72-9e44-4b61-b7d0-2a6f5c1e9d10
status: experimental
description: Detects high-volume or anomalous login activity against Synology DSM consistent with exploitation of an insufficient-entropy login flaw.
references:
  - https://nvd.nist.gov/vuln/detail/CVE-2026-13639
author: valtersit.com
date: 2026/01/01
tags:
  - attack.initial-access
  - attack.t1190
logsource:
  product: synology
  service: dsm
detection:
  selection_action:
    event_type|contains:
      - 'login'
      - 'auth'
  selection_failure:
    status|contains:
      - 'fail'
      - 'failure'
      - 'invalid'
  timeframe: 5m
  condition: selection_action and selection_failure | count() by src_ip > 20
fields:
  - src_ip
  - user
  - event_type
level: high
falsepositives:
  - Misconfigured clients with stale credentials
  - Legitimate brute-force protection testing

YARA Rule

rule CVE_2026_13639_DSM_Exploit_Artifacts
{
    meta:
        description = "Detects tooling or scripts referencing Synology DSM login-path exploitation (CVE-2026-13639)"
        author = "valtersit.com"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2026-13639"
        date = "2026-01-01"
        severity = "critical"

    strings:
        $cve = "CVE-2026-13639" ascii nocase
        $dsm = "DiskStation Manager" ascii nocase
        $dsm2 = "Synology DSM" ascii nocase
        $login = "login" ascii nocase
        $entropy = "entropy" ascii nocase

    condition:
        ($cve) or
        (2 of ($dsm, $dsm2) and 1 of ($login, $entropy))
}

Suricata IDS Rule

alert http any any -> any any (msg:"CVE-2026-13639 Possible Synology DSM Login Logic Exploitation Attempt"; flow:established,to_server; http.method; content:"POST"; http.header; content:"Synology"; http.uri; content:"login"; threshold:type both, track by_src, count 20, seconds 300; classtype:attempted-admin; reference:cve,2026-13639; reference:url,nvd.nist.gov/vuln/detail/CVE-2026-13639; sid:2026136390; rev:1;)

Elastic Detection

sequence by source.ip with maxspan=5m
  [ network where event.action == "network" and url.path : "*login*" and
      destination.port in (5000, 5001) ]
  [ network where event.action == "network" and
      http.response.status_code in (401, 403, 500) ]

Splunk SPL Query

index=proxy OR index=firewall
| search (dest_port=5000 OR dest_port=5001) (uri_path="*login*")
| stats count AS attempts, dc(uri_path) AS paths, values(src_ip) AS sources BY src_ip, dest_host
| where attempts > 20
| sort - attempts

Wazuh OSSEC Rule

<group name="synology,dsm,">
  <rule id="2026136" level="12" frequency="15" timeframe="300">
    <if_matched_sid>2026135</if_matched_sid>
    <description>CVE-2026-13639: Excessive failed DSM login attempts (possible insufficient-entropy exploitation)</description>
    <mitre>
      <id>T1190</id>
    </mitre>
  </rule>
</group>

(The referenced lower-severity rule 2026135 should match individual DSM login failures in the authentication log; adjust IDs to fit your Wazuh ruleset.)

Mitigation & Patch Guidance

Patch confidence: 0/4 (UNPATCHED — no patch evidence from any tracked source). No vendor fix or workaround data was returned by the tracked sources; however, the CVE description names fixed build identifiers, which should be treated as the remediation targets pending direct vendor confirmation.

1) Immediate mitigations

• Remove internet exposure. Do not expose DSM management/login interfaces (commonly ports 5000/5001) to the public internet. Restrict access via VPN, firewall allow-listing, or Synology's remote-access hardening guidance.
• Network segmentation. Isolate NAS appliances on management VLANs and block inbound management access from untrusted zones.
• Enable multi-factor authentication on all DSM accounts to reduce the value of any credential or token-bypass primitive.
• Apply rate limiting / lockout on the login path and forward authentication logs to the SIEM for the detections above.
• Restrict outbound egress from the appliance to reduce exfiltration and lateral-movement opportunities.

2) Patch / upgrade steps (pending confirmation)

• Track the Synology Security Advisory for CVE-2026-13639 and upgrade DSM to at least one of the versions stated as fixed: 7.2.1-69057-12, 7.2.2-72806-9, 7.3.2-86009-4, or 7.4-90075 (or the latest available release on your branch).
• Verify the applied build string post-upgrade and stage the update during a maintenance window, confirming backups are restorable beforehand.
• Given the 0/4 patch-confidence rating, verify the fix directly against the vendor advisory rather than relying on this report alone.

3) Long-term hardening

• Maintain a documented DSM update cadence and subscribe to Synology security advisories.
• Enforce least privilege on DSM accounts and remove unused/default accounts.
• Continuously inventory internet-exposed NAS assets and re-validate exposure quarterly.
• Monitor the EPSS and KEV feeds; if this CVE is added to CISA KEV or a public exploit emerges, escalate remediation urgency accordingly.

References

1. NVD — CVE-2026-13639: https://nvd.nist.gov/vuln/detail/CVE-2026-13639
2. MITRE CWE-331 — Insufficient Entropy: https://cwe.mitre.org/data/definitions/331.html
3. Synology Security Advisories (vendor fix channel): https://www.synology.com/en-global/security/advisory
4. CISA Known Exploited Vulnerabilities Catalog (check status): https://www.cisa.gov/known-exploited-vulnerabilities-catalog
5. FIRST EPSS (score not yet assigned at time of writing): https://www.first.org/epss/

Threat actor / exploit-in-the-wild status: No public PoC, exploit code, or Nuclei template is available for CVE-2026-13639 per the supplied data, and the vulnerability is not listed in CISA KEV. No named threat actor campaign has been associated with this CVE at the time of writing. The report is based solely on the provided NVD description and CVSS metadata; no implementation details beyond those sources have been asserted.