No exploitation reportedFix available

CVE-2020-13364

Q: Is CVE-2020-13364 actively exploited?

Active exploitation of CVE-2020-13364 has not been confirmed. Its EPSS score was 1.2% on 2026-10-22, the estimated probability of exploitation in the next 30 days.

Q: What is the CVSS score for CVE-2020-13364?

CVE-2020-13364 has a CVSS v3 base score of 8.8 (HIGH severity), with vector string CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H.

No exploitation reported by the sources we read.

Published2020-08-06
Fixed inv5.21\(aazf.9\)c0

Am I affected?

With a domain, we read the version its public headers and pages expose. A distribution build can carry the fix under an older number.

Description

A backdoor in certain Zyxel products allows remote TELNET access via a CGI script. This affects NAS520 V5.21(AASZ.4)C0, V5.21(AASZ.0)C0, V5.11(AASZ.3)C0, and V5.11(AASZ.0)C0; NAS542 V5.11(ABAG.0)C0, V5.20(ABAG.1)C0, and V5.21(ABAG.3)C0; NSA325 v2_V4.81(AALS.0)C0 and V4.81(AAAJ.1)C0; NSA310 4.22(AFK.0)C0 and 4.22(AFK.1)C0; NAS326 V5.21(AAZF.8)C0, V5.11(AAZF.4)C0, V5.11(AAZF.2)C0, and V5.11(AAZF.3)C0; NSA310S V4.75(AALH.2)C0; NSA320S V4.75(AANV.2)C0 and V4.75(AANV.1)C0; NSA221 V4.41(AFM.1)C0; and NAS540 V5.21(AATB.5)C0 and V5.21(AATB.3)C0.

Timeline

dates in UTC

2020-08-0617:15CVE publishedNVD

Observed activity

isMalicious corpus · updated 2026-10-23 18:48 UTC

Indicators tagged with this CVE, per day0 indicators over 7 days

Block these indicators

Firewall list (EDL, text): https://api.ismalicious.com/edl/cve/CVE-2020-13364?format=txt
CSV export: https://api.ismalicious.com/edl/cve/CVE-2020-13364?format=csv
TAXII collection of exploited CVEs
The EDL and CSV take an API key on the Pro plan.

Versions per branch

NVD

Zyxel nas326 firmwareversion to verify

Branch	Affected	Fixed in	Source
5.21	< v5.21\(aazf.9\)c0	v5.21\(aazf.9\)c0	NVD

Zyxel nas520 firmwareversion to verify

Branch	Affected	Fixed in	Source
5.21	< v5.21\(aasz.5\)c0	v5.21\(aasz.5\)c0	NVD

Zyxel nas540 firmwareversion to verify

Branch	Affected	Fixed in	Source
5.21	< v5.21\(aatb.6\)c0	v5.21\(aatb.6\)c0	NVD

1 more product in the sources.

as publishedRanges as NVD and OSV publish them; a distribution’s build can be fixed under an older number.

Fix

Update Zyxel nas326 firmware to v5.21\(aazf.9\)c0

Advisories and patches

www.zyxel.comVendor Advisory
www.zyxel.comVendor Advisory

Detection

Nuclei template: none so far
Exploit-DB: no entry
Proof-of-concept repositories: none listed

Sources

NVDCVSS 3.18.8read 2026-10-23
CNACVSS 3.06.5read 2026-10-23
NVDCVSS 2.09read 2026-10-23
FIRST EPSSEPSS score1.2 %2026-10-22

Technical details

CVSS v3 Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
Published: 2020-08-06
Last Modified: 2026-06-17
NVD status: Modified

Frequently asked questions

What is CVE-2020-13364?

Is CVE-2020-13364 actively exploited?

Active exploitation of CVE-2020-13364 has not been confirmed. Its EPSS score was 1.2% on 2026-10-22, the estimated probability of exploitation in the next 30 days.

What is the CVSS score for CVE-2020-13364?

CVE-2020-13364 has a CVSS v3 base score of 8.8 (HIGH severity), with vector string CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H.

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