How severe is CVE-2021-35029?

CVE-2021-35029 has a CVSS 3.x base score of 9.8, rated critical severity. It is exploitable over network with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is high, integrity high, and availability high.

Is CVE-2021-35029 being actively exploited?

It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 2% (81st percentile), an estimate of the probability of exploitation in the next 30 days.

What products are affected by CVE-2021-35029?

CVE-2021-35029 primarily affects Zyxel Usg1900 Firmware. In total, 37 product configurations (CPEs) are listed as vulnerable; see the affected-products list for the exact versions.

How do I fix CVE-2021-35029?

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-2021-35029 published?

CVE-2021-35029 was published on 2021-07-02 and last updated on 2026-06-17.

CVE-2021-35029

CVE-2021-35029 is a critical-severity vulnerability in Zyxel Usg1900 Firmware with a CVSS 3.x base score of 9.8. 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-287.

Key facts

Severity: Critical (CVSS 3.x base score 9.8)
CVSS v2: 7.5
EPSS exploit prediction: 2% (81st percentile)
Actively exploited: Not listed in CISA KEV
Weakness: CWE-287
Affected product: Zyxel Usg1900 Firmware
Published: 2021-07-02
Last modified: 2026-06-17

Description

An authentication bypasss vulnerability in the web-based management interface of Zyxel USG/Zywall series firmware versions 4.35 through 4.64 and USG Flex, ATP, and VPN series firmware versions 4.35 through 5.01, which could allow a remote attacker to execute arbitrary commands on an affected device.

Frequently asked questions

What is CVE-2021-35029?: An authentication bypasss vulnerability in the web-based management interface of Zyxel USG/Zywall series firmware versions 4.35 through 4.64 and USG Flex, ATP, and VPN series firmware versions 4.35 through 5.01, which could allow a remote attacker to execute arbitrary commands on an affected device.
How severe is CVE-2021-35029?: CVE-2021-35029 has a CVSS 3.x base score of 9.8, rated critical severity. It is exploitable over network with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is high, integrity high, and availability high.
Is CVE-2021-35029 being actively exploited?: It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 2% (81st percentile), an estimate of the probability of exploitation in the next 30 days.
What products are affected by CVE-2021-35029?: CVE-2021-35029 primarily affects Zyxel Usg1900 Firmware. In total, 37 product configurations (CPEs) are listed as vulnerable; see the affected-products list for the exact versions.
How do I fix CVE-2021-35029?: 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-2021-35029 published?: CVE-2021-35029 was published on 2021-07-02 and last updated on 2026-06-17.

References

https://www.zyxel.com/support/Zyxel_security_advisory_for_attacks_against_security_appliances.shtml

Affected products (37)

cpe:2.3:o:zyxel:usg1900_firmware:*:*:*:*:*:*:*:*
cpe:2.3:o:zyxel:usg1100_firmware:*:*:*:*:*:*:*:*
cpe:2.3:o:zyxel:usg310_firmware:*:*:*:*:*:*:*:*
cpe:2.3:o:zyxel:usg210_firmware:*:*:*:*:*:*:*:*
cpe:2.3:o:zyxel:usg110_firmware:*:*:*:*:*:*:*:*
cpe:2.3:o:zyxel:usg40_firmware:*:*:*:*:*:*:*:*
cpe:2.3:o:zyxel:usg40w_firmware:*:*:*:*:*:*:*:*
cpe:2.3:o:zyxel:usg60_firmware:*:*:*:*:*:*:*:*
cpe:2.3:o:zyxel:usg60w_firmware:*:*:*:*:*:*:*:*
cpe:2.3:o:zyxel:usg300_firmware:*:*:*:*:*:*:*:*
cpe:2.3:o:zyxel:usg1000_firmware:*:*:*:*:*:*:*:*
cpe:2.3:o:zyxel:usg2000_firmware:*:*:*:*:*:*:*:*
cpe:2.3:o:zyxel:usg20_firmware:*:*:*:*:*:*:*:*
cpe:2.3:o:zyxel:usg20w_firmware:*:*:*:*:*:*:*:*
cpe:2.3:o:zyxel:usg50_firmware:*:*:*:*:*:*:*:*
cpe:2.3:o:zyxel:usg100_firmware:*:*:*:*:*:*:*:*
cpe:2.3:o:zyxel:usg200_firmware:*:*:*:*:*:*:*:*
cpe:2.3:o:zyxel:usg_flex_100_firmware:*:*:*:*:*:*:*:*
cpe:2.3:o:zyxel:usg_flex_200_firmware:*:*:*:*:*:*:*:*
cpe:2.3:o:zyxel:usg_flex_500_firmware:*:*:*:*:*:*:*:*
cpe:2.3:o:zyxel:usg_flex_100w_firmware:*:*:*:*:*:*:*:*
cpe:2.3:o:zyxel:usg_flex_700_firmware:*:*:*:*:*:*:*:*
cpe:2.3:o:zyxel:zywall_atp100_firmware:*:*:*:*:*:*:*:*
cpe:2.3:o:zyxel:zywall_atp100w_firmware:*:*:*:*:*:*:*:*
cpe:2.3:o:zyxel:zywall_atp200_firmware:*:*:*:*:*:*:*:*
cpe:2.3:o:zyxel:zywall_atp500_firmware:*:*:*:*:*:*:*:*
cpe:2.3:o:zyxel:zywall_atp700_firmware:*:*:*:*:*:*:*:*
cpe:2.3:o:zyxel:zywall_atp800_firmware:*:*:*:*:*:*:*:*
cpe:2.3:o:zyxel:zywall_vpn50_firmware:*:*:*:*:*:*:*:*
cpe:2.3:o:zyxel:zywall_vpn100_firmware:*:*:*:*:*:*:*:*
cpe:2.3:o:zyxel:zywall_vpn300_firmware:*:*:*:*:*:*:*:*
cpe:2.3:o:zyxel:usg20-vpn_firmware:*:*:*:*:*:*:*:*
cpe:2.3:o:zyxel:usg20w-vpn_firmware:*:*:*:*:*:*:*:*
cpe:2.3:o:zyxel:usg2200-vpn_firmware:*:*:*:*:*:*:*:*
cpe:2.3:o:zyxel:zywall_110_firmware:*:*:*:*:*:*:*:*
cpe:2.3:o:zyxel:zywall_310_firmware:*:*:*:*:*:*:*:*
cpe:2.3:o:zyxel:zywall_1100_firmware:*:*:*:*:*:*:*:*

More vulnerabilities in Zyxel Usg1900 Firmware

CVE-2020-29583 — Critical (CVSS 9.8): Firmware version 4.60 of Zyxel USG devices contains an undocumented account (zyfwp) with an unchangeable password. The…
CVE-2020-9054 — Critical (CVSS 9.8): Multiple ZyXEL network-attached storage (NAS) devices running firmware version 5.21 contain a pre-authentication…
CVE-2019-12583 — Critical (CVSS 9.1): Missing Access Control in the "Free Time" component of several Zyxel UAG, USG, and ZyWall devices allows a remote…
CVE-2019-12581 — Medium (CVSS 6.1): A reflective Cross-site scripting (XSS) vulnerability in the free_time_failed.cgi CGI program in selected Zyxel ZyWall,…
CVE-2019-9955 — Medium (CVSS 6.1): On Zyxel ATP200, ATP500, ATP800, USG20-VPN, USG20W-VPN, USG40, USG40W, USG60, USG60W, USG110, USG210, USG310, USG1100,…

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Other CWE-287 (Improper Authentication) vulnerabilities

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CVE-2026-10611 — Critical (CVSS 10.0): An authentication bypass vulnerability exists in MISP when LDAP mixed authentication is enabled with OTP enforcement.…
CVE-2026-47280 — Critical (CVSS 10.0): Improper authentication in Azure Resource Manager (ARM) allows an unauthorized attacker to elevate privileges over a…
CVE-2026-42822 — Critical (CVSS 10.0): Improper authentication in Azure Local Disconnected Operations allows an unauthorized attacker to elevate privileges…
CVE-2026-20182 — Critical (CVSS 10.0): May 2026: This security advisory provides the details and fix information for a vulnerability that was discovered and…

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