Nginx UI: Unauthenticated Backup Download with Encryption Key Disclosure

Vulnerability

Overview

The vulnerability in Nginx UI (prior to version 2.3.3) stems from two critical flaws in the /api/backup endpoint. First, the endpoint is registered without any authentication middleware, making it accessible to any unauthenticated user [1]. Second, the response includes an X-Backup-Security header that contains the AES-256 encryption key and initialization vector (IV) in plaintext, formatted as key:iv [1]. This combination of missing authentication and cleartext key disclosure violates CWE-306 (Missing Authentication for Critical Function) and CWE-311 (Missing Encryption of Sensitive Data) [1].

Exploitation

An attacker can exploit this vulnerability by sending a simple GET request to /api/backup without any prior authentication or network position requirements [3]. The server responds with a backup archive file and the X-Backup-Security header containing the decryption keys. Because the keys are Base64-encoded and directly exposed, the attacker can immediately decrypt the backup using standard AES-256 decryption tools [1]. No additional privileges or user interaction are needed.

Impact

The backup archive contains highly sensitive data, including user credentials, session tokens, SSL private keys, and Nginx configurations [1][3]. With the decryption keys readily available, an attacker can extract all this information, leading to full compromise of the Nginx UI instance and potentially the underlying web server. The CVSS 3.1 score is 9.8 (Critical) due to the network-based, low-complexity attack vector and high impact on confidentiality, integrity, and availability [1][3].

Mitigation

The issue has been patched in Nginx UI version 2.3.3 [1][2]. Users are strongly advised to upgrade immediately. No workarounds are documented; the only effective mitigation is to apply the update. The project maintainers have addressed the root cause by adding authentication to the backup endpoint and removing the key disclosure from the response headers [1].