# Authorization Analysis Report

## 1. Executive Summary

- **Analysis Status:** Complete  
- **Key Outcome:** One high-confidence vertical privilege escalation vulnerability was identified and recorded. The vulnerability allows an attacker with standard user credentials (or no credentials) to forge an admin session cookie and access the protected flag file. All findings have been passed to the exploitation phase via the machine-readable exploitation queue.
- **Purpose of this Document:** This report provides the strategic context, dominant patterns, and architectural intelligence necessary to effectively exploit the vulnerability listed in the queue. It is intended to be read alongside the JSON deliverable.

**Testing Coverage:**
- ✅ **Horizontal Authorization:** Analyzed all endpoints - NO user-specific resources or object IDs found
- ✅ **Vertical Authorization:** Analyzed admin privilege checks - ONE critical vulnerability identified (padding oracle bypass)
- ✅ **Context-Based Authorization:** Analyzed workflow validation - NO workflow bypass opportunities found

## 2. Dominant Vulnerability Patterns

### Pattern 1: Cryptographic Authorization Bypass (Vertical)

- **Description:** The application relies on encrypted session cookies to establish user identity and privileges, but uses AES-CBC encryption without message authentication (MAC/HMAC). This creates a padding oracle vulnerability that allows attackers to decrypt and forge session cookies containing arbitrary usernames, including "admin".

- **Implication:** An attacker can bypass the admin authorization check (line 63: `if username == 'admin':`) by forging a session cookie that decrypts to the username "admin", granting unauthorized access to the flag file (`/flag.txt`).

- **Technical Root Cause:**
  - **Non-standard padding:** Uses null-byte padding (`b'\x00'`) instead of PKCS#7 (lines 20-21, 35)
  - **No integrity protection:** AES-CBC used without HMAC or authenticated encryption (no MAC verification)
  - **Error oracle disclosure:** Exception messages revealed to client (line 70: `return str(e)`)
  - **Bypassable guard:** Admin check (line 63) relies entirely on the integrity of decrypted session data

- **Representative:** AUTHZ-VULN-01

## 3. Strategic Intelligence for Exploitation

### Session Management Architecture

- **Session Medium:** Client-side encrypted cookie (cookie name: `session`)
- **Encryption Algorithm:** AES-256-CBC (cryptography library 42.0.8)
- **Encryption Key:** 256-bit random key generated at startup with `os.urandom(32)` (line 10)
  - **Key Location:** In-memory only (ephemeral, lost on restart)
  - **Key Persistence:** None (no file storage or environment variable)
- **Cookie Format:** Hex-encoded string containing `[16-byte IV][variable-length ciphertext]`
- **Cookie Retrieval:** Line 55: `request.cookies.get('session')`
- **Cookie Setting:** Line 49: `resp.set_cookie('session', encrypted_data.hex())`

**Critical Finding:** The session cookie is encrypted but not authenticated. No HMAC or signature verification occurs before decryption, allowing ciphertext manipulation attacks.

### Cryptographic Implementation Details

**Encryption Function (lines 18-25):**
```python
def encrypt_data(data):
    iv = os.urandom(16)                                    # Random 16-byte IV
    padding_length = 16 - (len(data) % 16)                 # Calculate padding
    padded_data = data.encode() + b'\x00' * padding_length # NULL-BYTE PADDING (non-standard)
    cipher = Cipher(algorithms.AES(key), modes.CBC(iv), backend=default_backend())
    encryptor = cipher.encryptor()
    ct = encryptor.update(padded_data) + encryptor.finalize()
    return iv + ct  # Returns: IV (16 bytes) || Ciphertext
```

**Decryption Function (lines 27-36):**
```python
def decrypt_data(encrypted_data):
    iv = encrypted_data[:16]               # Extract IV (first 16 bytes)
    ct = encrypted_data[16:]               # Extract ciphertext (remaining bytes)
    cipher = Cipher(algorithms.AES(key), modes.CBC(iv), backend=default_backend())
    decryptor = cipher.decryptor()
    padded_plaintext = decryptor.update(ct) + decryptor.finalize()
    plaintext = padded_plaintext.rstrip(b'\x00')  # STRIP NULL BYTES (vulnerable)
    return plaintext.decode()
```

**Padding Oracle Enablers:**
1. **Non-standard padding removal** (line 35): `rstrip(b'\x00')` removes ALL trailing null bytes, creating ambiguity
2. **No MAC verification:** No integrity check before or after decryption
3. **Error disclosure** (line 70): Different exception types reveal padding/decryption failures

### Role/Permission Model

- **Role Architecture:** Username-based implicit roles (no separate role field)
- **Role Storage:** Embedded in encrypted session cookie as plaintext username
- **Available Roles:**
  - `test1`: Standard user (password: `test1`) - access to welcome page only
  - `admin`: Privileged user (password: `somehardpassword`) - access to flag file
- **Role Check Location:** Line 63: `if username == 'admin':`
- **Role Enforcement:** Single inline string comparison (no RBAC framework, no middleware, no decorators)

**Critical Finding:** The role check assumes the decrypted username is trustworthy. Since the encryption lacks authentication, the role can be forged via padding oracle attack.

### Authorization Decision Points

| Location | File | Line(s) | Check Type | Purpose | Bypassable? |
|----------|------|---------|------------|---------|-------------|
| Session existence | app.py | 58 | `if encrypted_data:` | Verify session cookie present | ❌ No (but weak) |
| Session decryption | app.py | 60 | `decrypt_data()` call | Decrypt and extract username | ✅ Yes (padding oracle) |
| Admin privilege | app.py | 63 | `if username == 'admin':` | Check for admin role | ✅ Yes (via forged session) |

**Attack Path:** An attacker can forge a session cookie that decrypts to `username='admin'` using a padding oracle attack, bypassing the authorization check at line 63.

### Error Oracle Behavior

**Error Disclosure Location:** Line 70 in `/welcome` endpoint
```python
except Exception as e:
    return str(e)
```

**Observable Error Types:**
- `ValueError: non-hexadecimal number found in fromhex()` → Invalid hex format
- `UnicodeDecodeError: 'utf-8' codec can't decode byte...` → Padding/decryption failure
- Different error messages distinguish between decryption success/failure

**Oracle Exploitation:**
1. Attacker modifies ciphertext bytes systematically
2. Observes error responses to determine padding validity
3. Uses responses to decrypt session byte-by-byte
4. Crafts new session encrypting to "admin"

### Resource Access Patterns

- **Public Endpoints:** `GET /` (login page), `POST /login` (authentication)
- **Protected Endpoint:** `GET /welcome` (requires valid session cookie)
- **Privileged Resource:** `/flag.txt` file (requires `username == 'admin'`)
- **Critical Path:** `/welcome` endpoint → decrypt session → check username → read flag file

**No object ID parameters:** The application has no endpoints accepting user IDs, document IDs, or resource identifiers. All access is role-based, not object-based.

## 4. Vectors Analyzed and Confirmed Secure

These authorization checks were traced and confirmed to have robust, properly-placed guards or no authorization vulnerability. They are **low-priority** for further testing.

| **Endpoint** | **Guard Location** | **Defense Mechanism** | **Verdict** |
|--------------|-------------------|----------------------|-------------|
| `GET /` | N/A - Public endpoint | No authentication required (appropriate for login page) | SAFE |
| `POST /login` | app.py:46 | Credential validation via dictionary lookup | SAFE (no authz issue) |
| `GET /static/*` | Flask default handler | Public static files (CSS only) | SAFE (no sensitive data) |

**Horizontal Authorization:** No user-specific resources exist in the application. The application has no endpoints accepting object IDs (user IDs, profile IDs, document IDs) that could enable cross-user data access. All access is role-based (admin vs. user), not resource-based.

**Context-Based Authorization:** The login → welcome flow properly validates session state. The `/welcome` endpoint checks for session existence (line 58), decrypts the session (line 60), and redirects to login if no valid session exists (line 71). Users cannot bypass the authentication step to access protected resources. While the cryptographic implementation is vulnerable, the workflow state validation itself is correct.

**Public Endpoint Access Controls:** 
- `GET /` appropriately public (login page display)
- `POST /login` appropriately public (authentication endpoint - must be accessible to unauthenticated users)
- `GET /static/*` appropriately public (static CSS file serving)

## 5. Analysis Constraints and Blind Spots

### Constraints

1. **Ephemeral Encryption Key:** The AES key is regenerated on every application restart (`key = os.urandom(32)` at line 10). This means:
   - Session cookies become invalid after server restart
   - Cannot decrypt historical sessions captured before current server instance
   - Exploitation requires attacking sessions created with the current key
   - **Impact on exploitation:** Must perform padding oracle attack against a live server, cannot decrypt offline

2. **In-Memory Credential Storage:** User credentials are hardcoded in a Python dictionary (lines 13-16). This means:
   - Only two users exist: `test1:test1` and `admin:somehardpassword`
   - No database queries to analyze for authorization bypass
   - No user registration or account creation functionality
   - **Impact on analysis:** Horizontal authorization issues impossible (no user-to-user data access)

3. **Single-File Application:** The entire application is 76 lines in a single file (`app.py`). This means:
   - No middleware modules to analyze
   - No authorization decorator libraries
   - No permission models or RBAC frameworks
   - **Impact on analysis:** All authorization logic is visible in inline conditionals

### Blind Spots

1. **Static File Path Traversal:** The application uses Flask's default static file handler. While this is generally secure, the analysis did not perform live testing for path traversal vulnerabilities (e.g., `/static/../app.py`, `/static/../../flag.txt`). Recommend manual testing of:
   - `/static/%2e%2e/app.py` (URL encoded traversal)
   - `/static/..%2Fapp.py` (partial encoding)
   - `/static/../templates/index.html`

2. **Timing-Based Username Enumeration:** The login endpoint (line 46) checks `if username in users and users[username] == password`. This may leak timing information revealing valid usernames. Static analysis cannot definitively confirm timing side-channels without live measurement.

3. **Flask Debug Mode RCE:** Line 74 enables Flask debug mode (`debug=True`). Werkzeug's debug console may be accessible and could provide RCE via PIN bypass. This was not tested as it falls outside authorization analysis scope, but represents a potential escalation path.

4. **Docker Layer Secrets:** The flag file is created during Docker build using `ARG FLAG` (Dockerfile lines 12-13). The flag value may be visible in Docker image layers via `docker history`. This was not tested as it requires container inspection privileges.

---

**End of Authorization Analysis Report**