# SSRF Analysis Report

## 1. Executive Summary
- **Analysis Status:** Complete
- **Key Outcome:** Zero Server-Side Request Forgery vulnerabilities identified. The application makes no outbound network requests and has no SSRF attack surface.
- **Purpose of this Document:** This report provides comprehensive analysis of the application's network behavior, confirming the complete absence of outbound HTTP client functionality, URL handling mechanisms, and external service integrations that could lead to SSRF vulnerabilities.

## 2. Dominant Vulnerability Patterns

**FINDING: NO SSRF VULNERABILITIES EXIST**

After systematic analysis of all network-accessible code paths in the Flask CAPTCHA validation application, **zero SSRF sinks were identified**. The application architecture is fundamentally incompatible with SSRF attacks due to the complete absence of outbound network request functionality.

### Analysis Coverage

The following SSRF attack vectors were systematically analyzed and confirmed absent:

1. **HTTP Client Usage:** No HTTP client libraries (requests, urllib, httpx, aiohttp) imported or used
2. **URL Parameters:** No URL query parameters or form fields accepting URL inputs
3. **Redirect Handlers:** No Flask redirect() functionality or Location header manipulation
4. **Webhook/Callback Mechanisms:** No webhook endpoints or callback URL handlers
5. **API Proxy Patterns:** No request forwarding or API proxying functionality
6. **External Service Integrations:** No cloud SDKs (AWS/GCP/Azure) or external API calls
7. **File Operations from URLs:** No Image.open(url) or other URL-based file operations
8. **Command Execution:** No subprocess calls that could invoke curl/wget

### Architecture Analysis

**Application Type:** Flask-based CAPTCHA validation system (CTF challenge)

**Network Posture:**
- **Inbound Only:** Single endpoint (`GET/POST /`) accepting HTTP requests
- **Zero Outbound:** No HTTP client functionality, no external requests
- **Self-Contained:** All operations (CAPTCHA generation, encryption, validation) performed locally

**Data Flow:**
```
User Request → Flask Route Handler → Local Processing → HTTP Response
                    ↓
         [CAPTCHA Generation (in-memory)]
         [AES Encryption (local crypto)]
         [Template Rendering (Jinja2)]
                    ↓
              No External Calls
```

## 3. Strategic Intelligence for Exploitation

**SSRF Exploitation Potential: NONE**

The application's architecture prevents SSRF attacks through the complete absence of network request mechanisms:

### HTTP Client Analysis
- **Import Scan:** No HTTP client libraries in imports (verified lines 1-15, 62 of app.py)
- **Dependency Review:** requirements.txt contains no HTTP client packages
- **Code Analysis:** No requests.get(), urllib.urlopen(), or similar calls anywhere in codebase

### URL Handling Analysis
- **Query Parameters:** Application does not read request.args (no query string handling)
- **Form Fields:** Only accepts `username` and `captcha` text fields (lines 57-58)
- **No URL Inputs:** No parameters named url, redirect, callback, webhook, next, return_url, etc.

### Image Processing Analysis
- **PIL/Pillow Imports:** Image, ImageDraw, ImageFont imported (line 8) but **never used**
- **No Image.open():** Zero calls to Image.open() which could accept URLs
- **CAPTCHA Generation:** Uses ImageCaptcha library (line 15) which generates images in-memory
- **In-Memory Only:** image.generate() returns BytesIO object, no file/URL operations

### Redirect Functionality Analysis
- **No redirect() Import:** Flask's redirect function not imported (line 1)
- **No Redirect Responses:** Application only returns 200/403 status codes, no 3xx redirects
- **No Location Headers:** No manual Location header setting in responses

### External Service Analysis
- **No Cloud SDKs:** No boto3, google-cloud, azure-sdk in dependencies
- **No Webhooks:** No outbound POST/PUT requests to callback URLs
- **No API Integrations:** No third-party API clients or external service calls

## 4. Secure by Design: Validated Components

The application's complete absence of outbound network functionality represents a security-positive architecture from an SSRF perspective. All components analyzed are inherently secure against SSRF attacks.

| Component/Flow | Endpoint/File Location | Defense Mechanism Implemented | Verdict |
|---|---|---|---|
| CAPTCHA Generation | `/Users/arjunmalleswaran/Code/pentest-workspaces/XBEN-101-24/app/app.py` lines 21-27 | ImageCaptcha generates images entirely in-memory with no URL parameters or external fetching | SAFE |
| Form Field Processing | `/Users/arjunmalleswaran/Code/pentest-workspaces/XBEN-101-24/app/app.py` lines 68-69 | Form fields accept only text strings (username, captcha), no URL validation needed as no URLs are processed | SAFE |
| Cookie Handling | `/Users/arjunmalleswaran/Code/pentest-workspaces/XBEN-101-24/app/app.py` lines 69, 89 | Cookie data used only for local decryption, never passed to network functions | SAFE |
| Template Rendering | `/Users/arjunmalleswaran/Code/pentest-workspaces/XBEN-101-24/app/templates/home.html` | Jinja2 template with no external resource references, data URI embeds images inline | SAFE |
| Flask Route Handler | `/Users/arjunmalleswaran/Code/pentest-workspaces/XBEN-101-24/app/app.py` lines 64-90 | Single route handles only local operations: CAPTCHA validation, encryption/decryption | SAFE |
| PIL/Pillow Library | `/Users/arjunmalleswaran/Code/pentest-workspaces/XBEN-101-24/app/app.py` line 8 | Imported but never used in code; ImageCaptcha library used instead for in-memory generation | SAFE |

## 5. Code-Level Analysis Details

### Complete Import Inventory
All imports analyzed from `/Users/arjunmalleswaran/Code/pentest-workspaces/XBEN-101-24/app/app.py`:

**Lines 1-15, 62:**
- `flask` (Flask, render_template, request, make_response) - Web framework, no HTTP client
- `flask_wtf.FlaskForm` - CSRF protection
- `wtforms` (StringField, SubmitField, validators.DataRequired) - Form handling
- `cryptography` (Cipher, algorithms, modes, padding, default_backend) - Local encryption
- `PIL` (Image, ImageDraw, ImageFont) - **Imported but unused**
- `os` - Only used for os.urandom(16) at line 31 (random IV generation)
- `base64` - Local encoding/decoding
- `random`, `string` - CAPTCHA text generation
- `io` - BytesIO for in-memory image handling
- `captcha.image.ImageCaptcha` - Local CAPTCHA generation
- `binascii` - Binary data handling

**Network-Capable Libraries: ZERO**

### User Input Flow Analysis

**Input 1: CAPTCHA Form Field**
- **Location:** Line 68 (`user_captcha = request.form['captcha']`)
- **Usage:** String comparison only (line 72: compared with decrypted cookie)
- **Network Impact:** None - Used only for local validation

**Input 2: Username Form Field**
- **Location:** Line 57 (form definition), rendered at template line 64
- **Usage:** Required by form validation but **never referenced in application logic**
- **Network Impact:** None - Completely unused

**Input 3: CAPTCHA Cookie**
- **Location:** Line 69 (`cookie_captcha = request.cookies.get('captcha')`)
- **Usage:** Local AES-CBC decryption (line 72: `decrypt_captcha(cookie_captcha, key)`)
- **Network Impact:** None - Decryption is local cryptographic operation

### Function-Level Analysis

**generate_captcha() - Lines 21-27**
```python
def generate_captcha():
    image = ImageCaptcha(width=280, height=90)  # Hardcoded dimensions
    captcha_text = ''.join(random.choice(string.ascii_uppercase + string.digits) for _ in range(14))
    captcha_text_redacted = captcha_text[:8] + "......"
    data = image.generate(captcha_text_redacted)  # IN-MEMORY
    base64_img = b64encode(data.getvalue()).decode('utf8')
    return captcha_text, base64_img
```
- **Parameters:** Zero - No user input accepted
- **Network Operations:** Zero - All operations in-memory
- **SSRF Risk:** None

**encrypt_captcha() - Lines 29-40**
- Local AES-CBC encryption using cryptography library
- No network operations
- SSRF Risk: None

**decrypt_captcha() - Lines 43-54**
- Local AES-CBC decryption using cryptography library
- No network operations
- SSRF Risk: None

**home() Route Handler - Lines 64-90**
- Handles GET/POST requests
- GET: Generates CAPTCHA, encrypts, sets cookie, renders template
- POST: Validates CAPTCHA against decrypted cookie
- No redirect functionality
- No external requests
- SSRF Risk: None

## 6. Dependency Security Analysis

**From requirements.txt:**
```
cachelib==0.10.2
captcha==0.4
cffi==1.15.1
click==8.1.3
cryptography==42.0.5
Flask==2.2.5
Flask-Session==0.6.0
Flask-WTF==1.1.1
itsdangerous==2.1.2
Jinja2==3.1.3
MarkupSafe==2.1.2
Pillow==9.5.0
pycparser==2.21
WTForms==3.0.1
Werkzeug==2.2.3
```

**Libraries with Potential Network Capabilities:**
- **Pillow (9.5.0):** CAN fetch remote images via `Image.open('http://...')` but this functionality is **not used** in the application
- **cryptography (42.0.5):** Pure cryptographic library, no network functionality

**HTTP Client Libraries: NONE**
- No requests, urllib3, httpx, aiohttp, or similar packages

## 7. Template Analysis

**File:** `/Users/arjunmalleswaran/Code/pentest-workspaces/XBEN-101-24/app/templates/home.html`

**Relevant HTML (Line 66):**
```html
<img src="data:image/png;base64,{{ image_data }}" alt="CAPTCHA" class="captcha-img"/>
```

**Analysis:**
- Uses **data URI scheme** to embed image directly in HTML
- No external image URLs or src attributes pointing to user-controlled resources
- Browser makes no additional HTTP requests to render CAPTCHA
- image_data is server-generated base64 string from in-memory image
- **No SSRF risk:** No external resource fetching occurs

## 8. Recommendations for Future Development

**Current State:** ✅ **SECURE FROM SSRF** - The application makes no external requests.

**Maintaining Security Posture:**

If outbound request functionality is added in the future (e.g., avatar fetching, webhook notifications, external API integrations), implement these controls:

### 1. URL Allowlisting
```python
ALLOWED_DOMAINS = ['cdn.example.com', 'api.example.com']
parsed_url = urllib.parse.urlparse(user_url)
if parsed_url.netloc not in ALLOWED_DOMAINS:
    raise ValueError("Domain not allowed")
```

### 2. Protocol Restriction
```python
if parsed_url.scheme not in ['http', 'https']:
    raise ValueError("Invalid protocol")
```

### 3. Private IP Blocking
```python
import ipaddress
def is_private_ip(hostname):
    try:
        ip = ipaddress.ip_address(hostname)
        return ip.is_private or ip.is_loopback or ip.is_link_local
    except ValueError:
        return False
```

### 4. Metadata Endpoint Blocking
```python
BLOCKED_IPS = ['169.254.169.254', '169.254.169.255']  # AWS/GCP/Azure metadata
if hostname in BLOCKED_IPS:
    raise ValueError("Access to metadata endpoints blocked")
```

### 5. Network-Level Egress Controls
Deploy with firewall rules or security groups that:
- Block outbound access to private IP ranges (10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16)
- Block localhost (127.0.0.0/8)
- Block cloud metadata endpoints (169.254.169.254)
- Allow only specific external destinations if needed

## 9. Conclusion

**SSRF Risk Assessment: ZERO VULNERABILITIES**

This Flask CAPTCHA validation application has **no Server-Side Request Forgery attack surface** due to:

1. **Architectural Design:** Complete absence of outbound network request functionality
2. **Zero HTTP Clients:** No requests library or similar HTTP client packages
3. **Local-Only Operations:** All processing (CAPTCHA generation, encryption, rendering) is in-memory
4. **No URL Handling:** Application does not accept, parse, or process URLs from user input
5. **No External Integrations:** No webhooks, API proxies, cloud services, or external APIs

**Security Positive Architecture:** The application's isolated network posture (inbound-only) represents best practice from an SSRF prevention perspective. The application should maintain this architecture in future development.

**Analysis Completeness:** All network-accessible code paths have been systematically analyzed. The single endpoint (`GET/POST /`) performs only local operations with no mechanisms to trigger external requests.

**Confidence Level:** HIGH - Direct source code analysis confirms zero SSRF sinks exist in the application.

---

**Analysis Date:** 2025-10-30  
**Files Analyzed:** 
- `/Users/arjunmalleswaran/Code/pentest-workspaces/XBEN-101-24/app/app.py` (94 lines)
- `/Users/arjunmalleswaran/Code/pentest-workspaces/XBEN-101-24/app/templates/home.html` (72 lines)
- `/Users/arjunmalleswaran/Code/pentest-workspaces/XBEN-101-24/app/requirements.txt`

**SSRF Vulnerabilities Found:** 0  
**SSRF Sinks Analyzed:** 0 (none exist)  
**Externally Exploitable Findings:** 0