CVE-2021-26564Public exploitNetwork attack vector

Server spoofing via QuickConnect HTTPS-to-HTTP redirect

Published Feb 26, 2021 · Updated Sep 17, 2024

Cleartext transmission in synorelayd in Synology DiskStation Manager before 6.2.3-25426-3 allows man-in-the-middle attackers to spoof servers. QuickConnect redirects HTTPS browser requests to an HTTP endpoint, letting man-in-the-middle attackers impersonate the relay and remote device. QuickConnect must be enabled, a user must follow the link, and the attacker must control the network path; exploitation exposes credentials and permits device impersonation.

CVSS severity8.3
High
EPSS probability0.66%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Cleartext transmission in synorelayd in Synology DiskStation Manager before 6.2.3-25426-3 allows man-in-the-middle attackers to spoof servers. QuickConnect redirects HTTPS browser requests to an HTTP endpoint, letting man-in-the-middle attackers impersonate the relay and remote device. QuickConnect must be enabled, a user must follow the link, and the attacker must control the network path; exploitation exposes credentials and permits device impersonation.

The record

CVE
CVE-2021-26564
Published
Feb 26, 2021
Updated
Sep 17, 2024
Vendor
Synology
Product
Synology DiskStation Manager
Classifications
CWE-757, CWE-319, T1557
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Feb 26, 2021CVE publishedPublication date reported by the CVE source.
  2. Sep 17, 2024Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=unspecified <6.2.3-25426-3

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Synology · Synology DiskStation Managerversion=unspecified <6.2.3-25426-3

Published CVSS scores

8.3Cisco TalosCVSS:3.0/AV:N/AC:H/PR:N/UI:R/S:C/C:H/I:H/A:H
8.3SynologyCVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:C/C:H/I:H/A:H

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitPublished

Weakness, pattern, technique

CWE-757Selection of Less-Secure Algorithm During Negotiation
CWE-319Cleartext Transmission of Sensitive Information
T1557Adversary-in-the-Middle

Public exploit references

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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