How severe is CVE-2021-26561?

CVE-2021-26561 has a CVSS 3.x base score of 9.0, rated critical severity. It is exploitable over network with high attack complexity, requires no privileges and no user interaction. Impact on confidentiality is high, integrity high, and availability high.

Is CVE-2021-26561 being actively exploited?

It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 2% (77th percentile), an estimate of the probability of exploitation in the next 30 days.

What products are affected by CVE-2021-26561?

CVE-2021-26561 primarily affects Synology Diskstation Manager. In total, 4 product configurations (CPEs) are listed as vulnerable; see the affected-products list for the exact versions.

How do I fix CVE-2021-26561?

Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its critical severity, prioritise patching exposed systems.

When was CVE-2021-26561 published?

CVE-2021-26561 was published on 2021-02-26 and last updated on 2026-06-17.

CVE-2021-26561

CVE-2021-26561 is a critical-severity vulnerability in Synology Diskstation Manager with a CVSS 3.x base score of 9.0. It is not currently listed as actively exploited by CISA, and its EPSS exploit-prediction score is low. The underlying weakness is classified as CWE-119.

Key facts

Severity: Critical (CVSS 3.x base score 9.0)
CVSS v2: 6.8
EPSS exploit prediction: 2% (77th percentile)
Actively exploited: Not listed in CISA KEV
Weakness: CWE-119
Affected product: Synology Diskstation Manager
Published: 2021-02-26
Last modified: 2026-06-17

Description

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.

Frequently asked questions

What is 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.
How severe is CVE-2021-26561?: CVE-2021-26561 has a CVSS 3.x base score of 9.0, rated critical severity. It is exploitable over network with high attack complexity, requires no privileges and no user interaction. Impact on confidentiality is high, integrity high, and availability high.
Is CVE-2021-26561 being actively exploited?: It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 2% (77th percentile), an estimate of the probability of exploitation in the next 30 days.
What products are affected by CVE-2021-26561?: CVE-2021-26561 primarily affects Synology Diskstation Manager. In total, 4 product configurations (CPEs) are listed as vulnerable; see the affected-products list for the exact versions.
How do I fix CVE-2021-26561?: Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its critical severity, prioritise patching exposed systems.
When was CVE-2021-26561 published?: CVE-2021-26561 was published on 2021-02-26 and last updated on 2026-06-17.

References

Affected products (4)

cpe:2.3:o:synology:diskstation_manager:*:*:*:*:*:*:*:*
cpe:2.3:o:synology:vs960hd_firmware:-:*:*:*:*:*:*:*
cpe:2.3:o:synology:skynas_firmware:-:*:*:*:*:*:*:*
cpe:2.3:o:synology:diskstation_manager_unified_controller:3.0:*:*:*:*:*:*:*

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