What is CVE-2021-26561?

CVE-2021-26561 is a critical-severity vulnerability in Synology Diskstation Manager, classified under Stack-based Buffer Overflow. CVSS score: 9.0/10. Published 2021-02-26.

How severe is CVE-2021-26561?

Critical severity. CVSS v3 base score is 9.0 out of 10.

Buffer overflow in Synology Diskstation Manager

CVE-2021-26561

Stack-based buffer overflow vulnerability in synoagentregisterd in Synology DiskStation Manager (DSM) before 6.2.3-25426-3 allows man-in-the-middle attackers to execute arbitrary code via syno_finder_site HTTP header.

Vulnerability class: Buffer Overflow

Published 2021-02-26 · last modified 2026-06-17.

CVSS v3 metric

CVSS v3 base score 9.0 (Critical). Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:H/I:H/A:H.

EPSS exploit prediction

EPSS: 0.021 (81.0th percentile), scored 2026-10-19.

Low probability of exploitation in the next 30 days, but still tracked. 81st percentile — 81.0% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.

EPSS over last 30 days · oldest: 0.021 · newest: 0.021 · change: 0.000

Affected products

Synology Diskstation Manager
Synology Diskstation Manager Unified Controller — versions 3.0
Synology Skynas
Synology Skynas Firmware
Synology Diskstation Manager (Dsm) — versions unspecified
Synology Uc3200
Synology Vs960hd
Synology Vs960hd Firmware

Weakness classification (CWE)

References

www.synology.com/security/advisory/Synology_SA_20_26 (Vendor Advisory)
www.talosintelligence.com/vulnerability_reports/TALOS-2020-1159 (Exploit, Third Party Advisory)

Frequently asked questions

What is CVE-2021-26561?: CVE-2021-26561 is a critical-severity vulnerability in Synology Diskstation Manager, classified under Stack-based Buffer Overflow. CVSS score: 9.0/10. Published 2021-02-26.
How severe is CVE-2021-26561?: Critical severity. CVSS v3 base score is 9.0 out of 10.