What is CVE-2021-26565?

CVE-2021-26565 is a high-severity vulnerability in Synology Diskstation Manager, classified under Cleartext Transmission of Sensitive Information. CVSS score: 8.3/10. Published 2021-02-26.

How severe is CVE-2021-26565?

High severity. CVSS v3 base score is 8.3 out of 10.

Vulnerability in Synology Diskstation Manager

CVE-2021-26565

Cleartext transmission of sensitive information vulnerability in synorelayd in Synology DiskStation Manager (DSM) before 6.2.3-25426-3 allows man-in-the-middle attackers to obtain sensitive information via an HTTP session.

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

CVSS v3 metric

CVSS v3 base score 8.3 (High). Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:C/C:H/I:H/A:H.

EPSS exploit prediction

EPSS: 0.007 (54.2th percentile), scored 2026-10-24.

Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 54th percentile — 54.2% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.

EPSS over last 30 days · oldest: 0.007 · newest: 0.007 · 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)

CWE-319 — Cleartext Transmission of Sensitive Information

References

www.synology.com/security/advisory/Synology_SA_20_26 (Vendor Advisory)
www.talosintelligence.com/vulnerability_reports/TALOS-2020-1160 (Exploit, Third Party Advisory)

Frequently asked questions

What is CVE-2021-26565?: CVE-2021-26565 is a high-severity vulnerability in Synology Diskstation Manager, classified under Cleartext Transmission of Sensitive Information. CVSS score: 8.3/10. Published 2021-02-26.
How severe is CVE-2021-26565?: High severity. CVSS v3 base score is 8.3 out of 10.