Security readout for executives and security teams
Plain-English summary
This flaw could let an unauthenticated remote attacker run code on vulnerable Synology DSM systems. The issue is in DSM's file transfer protocol component and affects DSM versions before 6.2.3-25426-3. Treat internet-exposed NAS devices as urgent because compromise could affect stored files, system integrity, and service availability.
Executive priority
Prioritize remediation for internet-facing Synology DSM devices immediately. This is a critical unauthenticated remote code execution issue affecting systems that may hold sensitive business data. Internal-only systems should still be updated promptly after exposure is confirmed.
Technical view
CVE-2021-27649 is a CWE-416 use-after-free in Synology DiskStation Manager's file transfer protocol component. The CVSS 3.1 score is 9.8 with network attack vector, low complexity, no privileges, and no user interaction. Sources state arbitrary code execution is possible through unspecified vectors.
Likely exposure
Organizations using Synology DSM before 6.2.3-25426-3 are potentially exposed, especially where NAS file transfer services are reachable from untrusted networks or the internet. The public source bundle does not identify specific models, ports, or exact protocol vectors.
Exploitation context
The bundle does not show CISA KEV listing or cited evidence of active exploitation. Risk remains high because the vulnerability is remotely reachable, unauthenticated, low complexity, and enables arbitrary code execution. Public details are sparse, so do not assume exploitability is limited.
Researcher notes
Available public details identify the bug class, component, affected DSM version boundary, and impact, but vectors are unspecified. Validation should focus on version state, network exposure, and vendor advisory alignment rather than exploit reproduction.
Mitigation direction
- Update Synology DSM to 6.2.3-25426-3 or later where applicable.
- Review Synology advisory Synology_SA_20_26 for product-specific update guidance.
- Restrict DSM file transfer services from untrusted networks.
- Place internet-facing NAS access behind VPN or trusted access controls.
- Monitor Synology logs for unusual file transfer or administrative activity.
Validation and detection
- Inventory Synology DSM devices and record installed DSM versions.
- Confirm no device runs DSM before 6.2.3-25426-3.
- Identify DSM file transfer services exposed to the internet or partner networks.
- Check change records for completed DSM updates on affected systems.
- Review security logs for unexpected remote access around exposed services.
Public sources used
Generated from the cited source records. This long-tail analysis has not been individually reviewed by a named human.
Conservative CVE-to-ATT&CK context
These mappings and lookup hints may be relevant to the vulnerability behavior, CWE, affected product, or exposure path. Glexia-inferred context is not an official MITRE, ATT&CK, CWE, or CVE Program mapping.
ATT&CK lookup starting points
Use these exact CWE pages and searches to review the Glexia ATT&CK library from this CVE's weakness and description context.
CWE-416: Exact CWE lookup
Use the exact CWE identifier as the starting point before reviewing related ATT&CK behavior. Open the exact CWE lookup page first, then review the ATT&CK searches from that MITRE weakness context. This is a Glexia lookup hint, not an official ATT&CK mapping.
Open ATT&CK lookupExecution behavior lookup
The CVE wording references code or command execution, so execution technique review may help defensive triage. This is a Glexia inferred lookup path, not an official MITRE, ATT&CK, or CVE Program mapping.
Open ATT&CK lookupCVE-2021-27649 mapping review
Open the CVE-to-ATT&CK bridge for reviewed, inferred, or future official mappings tied to this CVE.
Open ATT&CK lookup- Severity
- Critical
- CVSS
- 9.8 (3.1)
- Known Exploited
- No
- Published
Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
CNA and ADP enrichment extracted from CVE v5
These fields come from the CVE record and ADP containers, not from Glexia's Take. They preserve time-varying source decisions such as CISA SSVC, KEV status, CVSS metrics, and provider references.
CVSS vector scores
1 official scoreWe collect every scored CVSS vector available in the official CNA and ADP containers. When more than one version is present, the table keeps the source vectors side by side instead of collapsing them into the highest score.
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H3.95.9Primary CVE scoreVulnerability scoring details
Base CVSS 3.1 score
9.8CriticalVector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
Source materials
- CVE List V5 sourceCVE List V5
- https://www.synology.com/security/advisory/Synology_SA_20_26CVE reference · x_refsource_CONFIRM
Products and packages named in the record
CWE details
CWE links open Glexia weakness intelligence pages with official CWE context, developer remediation guidance, and related CVE mappings.
Use After Free
Use After Free represents a recurring weakness pattern that can create exploitable paths when design, validation, or implementation controls are missing.