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NVD / CVE Program· CNA synology

CVE-2021-26561: Stack-based buffer overflow vulnerability in synoagentregisterd in Synology DiskStation Manager (DSM) before 6.2.3-25426-3 allows…

HighCVSS 8.1 · v3.1—No exploit No fix recorded

Noroxi assessment

Priority: Monitor (action score 33/100)

  • The sources we track (CISA KEV, Exploit-DB, Metasploit, Nuclei, GitHub) show no public exploit code or exploitation in the wild.
  • Probability of exploitation in the next 30 days: 2% (FIRST EPSS); higher than 81.5% of all records.
  • Anyone who can reach it over the internet can trigger it. No account or password is required. No user action is required. Special conditions such as timing or configuration are required.
  • According to the record, no fixed version is available yet.

Low priority for now. Update when a fix is released; follow the vendor's advisory.

Generated automatically from the record's data: CVSS (NVD and CNA), CISA KEV and SSVC, FIRST EPSS, exploit sources, fixed versions. How we calculate it

Primary sources: NVD · CVE.org · FIRST EPSS

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.

Published
Feb 26, 2021
Updated
Jun 16, 2026
EPSS
2.0% · 81th percentile

Report tools

JSON

Action score

33

Monitor

Low priority for now.

CVSS
32 / 40 · 8.1 / 10
CISA KEV
0 / 30 · Not listed
EPSS
1 / 30 · 2.0%

CNA vs NVD score

NVD
8.1
CNA · synology
9.0
0.9 point gap

CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:H/I:H/A:H

The score given by the assigning authority versus NVD’s independent score. A gap means the severity is contested.

Get told when this record changes

  • Not in KEV: no record of exploitation in the wild
  • No public exploit code on record
  • No fixed version yet

If any of these changes, we'll tell you in the daily email digest.

Follow CVE-2021-26561

When you'll hear from us

  • when it enters CISA KEV
  • when exploit code appears or is weaponized
  • when the record is updated (including a fixed version)
  • when EPSS rises 20 points in a week

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Noroxi analysis

What does it mean in your environment?

The assessment above is generated automatically from the record's data; it doesn't know your environment. Our team verifies on your systems whether this vulnerability is actually exploitable and which mitigation is enough.

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Affected systems

Is your version affected?

Pick the product and type the version you run. It's compared with this record's version ranges in your browser; the version isn't sent anywhere.

VendorProduct
synologydiskstation manager
synologyvs960hd firmware
synologyvs960hd
synologyskynas firmware
synologyskynas
synologydiskstation manager unified controller
synologyuc3200

Versions reported by the vendor

Affected version ranges reported by the assigning authority (synology). Independent of NVD's CPE analysis and usually ahead of it.

  • Synology Synology DiskStation Manager (DSM)

    • unspecified and later · before 6.2.3-25426-3affected
Affected versions · NVD (4)

NVD version ranges (for catalog products). Add the product to your stack with a version and matching uses these.

  • synology diskstation managerbefore 6.2.3-25426-3
  • synology diskstation manager unified controller3.0
  • synology skynas firmwareall versions
  • synology vs960hd firmwareall versions

Same product

synology: all records

Other highest-scoring records for the same primary product.

  • CVE-2013-6955webman/imageSelector.cgi in Synology DiskStation Manager (DSM) 4.0 before 4.0-2259, 4.2 before 4.2-3243, and 4.3 before 4.3-3810 Update 1…
    65This week
  • CVE-2017-9554An information exposure vulnerability in forget_passwd.cgi in Synology DiskStation Manager (DSM) before 6.1.3-15152 allows remote attackers…
    60This week
  • CVE-2017-15889Command injection vulnerability in smart.cgi in Synology DiskStation Manager (DSM) before 5.2-5967-5 allows remote authenticated users to…
    57Plan

Published before and after this one in the same product

  • ← EarlierCVE-2020-27656Oct 29, 2020 · Cleartext transmission of sensitive information vulnerability in DDNS in Synology DiskStation Manager (DSM) before 6.2.3-25426-2 allows…
    14Monitor
  • ← EarlierCVE-2021-26560Feb 26, 2021 · Cleartext transmission of sensitive information vulnerability in synoagentregisterd in Synology DiskStation Manager (DSM) before…
    29Monitor
  • Later →CVE-2021-26562Feb 26, 2021 · Out-of-bounds write vulnerability in synoagentregisterd in Synology DiskStation Manager (DSM) before 6.2.3-25426-3 allows man-in-the-middle…
    33Monitor

Remediation

Which version to upgrade to

Fix versions compiled from the vendor, package registries and Microsoft. Verify the vendor's note before upgrading.

No fix version is recorded for this entry. Check the references for vendor advisories.

Exploit status

No known public exploit

No public exploit has been observed yet. That doesn't mean you're safe, only that the bar is a little higher.

Research context

For pentesters and researchers: attack profile, score disagreement, timeline, patch commits, credits, variant and chain candidates, bug bounty scope. All derived from existing data; no exploit code.

Timeline

From publication to today: proof of concept, Metasploit module, CISA KEV and fix record. Dates are as reported by the sources.

No dated events beyond publication.

Not on record for this CVE: EPSS history · patch commit or PR link · researcher credit · variant or chain candidate · public bounty program.

Technical details

Attack conditions

  • Anyone who can reach it over the internet can trigger it.
  • No account or password is required.
  • No user action is required.
  • Special conditions such as timing or configuration are required.

If successful

Confidentiality
high · data can be read
Integrity
high · data or configuration can be modified
Availability
high · the service can be disrupted
Attack vector
Network
Attack complexity
High
Privileges required
None
User interaction
None
Scope
Unchanged
Confidentiality impact
High
Integrity impact
High
Availability impact
High

Weakness class (CWE)

CWE-121 · Stack-based Buffer Overflow

CVSS vector

CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H

nvd-primary

Attack context

MITRE CAPEC attack patterns and ATT&CK techniques for this weakness class (CWE). A starting point for detection rules and threat hunting.

MITRE has no CAPEC/ATT&CK mapping for this CWE.

References

Vendor advisories and official records. Exploit/PoC links are deliberately left out.

All records