CVE-2021-26561Network attack vector

Arbitrary code execution via unbounded finder-site parsing

Published Feb 26, 2021 · Updated Sep 16, 2024

Stack-based buffer overflow in synoagentregisterd in Synology DSM before 6.2.3-25426-3 allows man-in-the-middle attackers to execute arbitrary code. The /finder/server response parser uses an unbounded %s conversion in sscanf to copy the syno_finder_site value into a 128-byte stack buffer. Exploitation requires interception and modification of the appliance's plaintext HTTP request to global.quickconnect.to; the root service is confined by AppArmor.

CVSS severity9.0
Critical
EPSS probability2.03%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Stack-based buffer overflow in synoagentregisterd in Synology DSM before 6.2.3-25426-3 allows man-in-the-middle attackers to execute arbitrary code. The /finder/server response parser uses an unbounded %s conversion in sscanf to copy the syno_finder_site value into a 128-byte stack buffer. Exploitation requires interception and modification of the appliance's plaintext HTTP request to global.quickconnect.to; the root service is confined by AppArmor.

The record

CVE
CVE-2021-26561
Published
Feb 26, 2021
Updated
Sep 16, 2024
Vendor
Synology
Product
Synology DiskStation Manager
Classifications
CWE-121, T1557
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Feb 26, 2021CVE publishedPublication date reported by the CVE source.
  2. Sep 16, 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

8.1NIST NVDCVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H
9.6Cisco TalosCVSS:3.0/AV:A/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H
6.8NIST NVDAV:N/AC:M/Au:N/C:P/I:P/A:P
9.0Synology Inc.CVSS:3.1/AV:N/AC:H/PR:N/UI:N/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 exploitUnconfirmed

Weakness, pattern, technique

CWE-121Stack-based Buffer Overflow
T1557Adversary-in-the-Middle

Public exploit references

No public exploit references are available in this record.

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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