What is CVE-2021-21739?

CVE-2021-21739 is a medium-severity vulnerability in ZTE Zxctn 6120h, classified under Insufficient Verification of Data Authenticity. CVSS score: 4.6/10. Published 2021-08-05.

How severe is CVE-2021-21739?

Medium severity. CVSS v3 base score is 4.6 out of 10.

Vulnerability in ZTE Zxctn 6120h

CVE-2021-21739

A ZTE's product of the transport network access layer has a security vulnerability. Because the system does not sufficiently verify the data reliability, attackers could replace an authenticated optical module on the equipment with an unauthenticated one, bypassing system authentication and detection, thus affecting signal transmission. This affects: <ZXCTN 6120H><V5.10.00B24>

Published 2021-08-05 · last modified 2026-06-17.

CVSS v3 metric

CVSS v3 base score 4.6 (Medium). Vector: CVSS:3.1/AV:P/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N.

EPSS exploit prediction

EPSS: 0.002 (14.5th percentile), scored 2026-09-30.

Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 14th percentile — 14.5% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.

EPSS over last 30 days · oldest: 0.002 · newest: 0.002 · change: 0.000

Affected products

ZTE Zxctn 6120h
ZTE Zxctn 6120h Firmware — versions 5.10.00b24
<zxctn 6120h> — versions <V5.10.00B24>

Weakness classification (CWE)

CWE-345 — Insufficient Verification of Data Authenticity

References

support.zte.com.cn/support/news/LoopholeInfoDetail.aspx (Vendor Advisory)

Frequently asked questions

What is CVE-2021-21739?: CVE-2021-21739 is a medium-severity vulnerability in ZTE Zxctn 6120h, classified under Insufficient Verification of Data Authenticity. CVSS score: 4.6/10. Published 2021-08-05.
How severe is CVE-2021-21739?: Medium severity. CVSS v3 base score is 4.6 out of 10.