CVE-2026-42221

nginx-ui: Unauthenticated First-Run Installer Allows Remote Initial Admin Claim

Nginx UI is a web user interface for the Nginx web server. From version 2.0.0 to before version 2.3.8, an unauthenticated network attacker can claim the initial administrator account on a fresh nginx-ui instance during the first-run setup window. The public /api/install endpoint is reachable without authentication, and the request-encryption flow only protects payload confidentiality in transit; it does not authenticate who is allowed to perform installation. A remote attacker who reaches the service before the legitimate operator can set the admin email, username, and password, causing permanent initial-instance takeover. This issue has been patched in version 2.3.8.

Affected

0xJacky · nginx-ui

CVSS v3.1 NVD

9.8 CRITICALCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

CVSS v3.1 GITHUB

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

EPSS

1.60% chance of exploitation in the next 30 days, 75th percentile

Nuclei

high · CWE-306

Published

2026-05-04

Updated

2026-05-05

Proof-of-concept exploits (1)

• advisories/GHSA-h27v-ph7w-m9fp

Nuclei templates (1)

• projectdiscovery/nuclei-templates/blob/main/http/cves/2026/CVE-2026-42221.yaml

Security advisories

• 0xJacky/nginx-ui/security/advisories/GHSA-h27v-ph7w-m9fp
• 0xJacky/nginx-ui/releases/tag/v2.3.8

References

• CVE Record
• NVD entry