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CVE-2026-13639 – Unauthenticated File Read/Write and DoS – Synology DSM 7.2.1 through 7.4

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Summary

CVE-2026-13639 is an insufficient entropy vulnerability (CWE-331) in the login logic of Synology DiskStation Manager (DSM). The weak entropy in session/authentication token generation allows a remote, unauthenticated attacker to read and write arbitrary files and to disrupt service availability on affected NAS devices. The flaw has a CVSS v3.1 base score of 9.8 (Critical) and was disclosed by Synology on September 18, 2026 as part of Synology-SA-26:13.

Technical details

  • Root cause: DSM’s login logic generates authentication material (e.g., session identifiers or tokens) using insufficiently random values, making it feasible for an attacker to predict or brute-force valid authentication data.
  • Trigger conditions: No authentication or user interaction is required; the login mechanism is reachable over the network on any exposed DSM instance.
  • Attack vector: Network (AV:N), low attack complexity (AC:L), no privileges required (PR:N), no user interaction (UI:N).
  • Impact: Successful exploitation grants the attacker the ability to read and write arbitrary files on the device and to conduct denial-of-service, resulting in full loss of confidentiality, integrity, and availability.

Affected software

  • Synology DSM 7.4 — versions before 7.4-90075
  • Synology DSM 7.3.2 — versions before 7.3.2-86009-4
  • Synology DSM 7.2.2 — versions before 7.2.2-72806-9
  • Synology DSM 7.2.1 — versions before 7.2.1-69057-12

Severity

  • CVSS v3.1 Base Score: 9.8 (Critical)
  • Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

Mitigation and recommended actions

  • Immediate: Update DSM to the fixed versions specified by Synology:
    • DSM 7.4 → 7.4-90075 or later
    • DSM 7.3.2 → 7.3.2-86009-4 or later
    • DSM 7.2.2 → 7.2.2-72806-9 or later
    • DSM 7.2.1 → 7.2.1-69057-12 or later
  • If immediate patching is not possible: Restrict network exposure of the DSM management interface (avoid exposing DSM login directly to the internet), place the device behind a VPN or firewall, and monitor for anomalous authentication activity until the update can be applied.

References

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