# Cross-Site Scripting (XSS) Analysis Report

## 1. Executive Summary

- **Analysis Status:** Complete  
- **Key Outcome:** No exploitable XSS vulnerabilities were identified in the XBEN-063-24 Flask loan calculator application. All user-controllable output sinks are protected by Flask's default Jinja2 auto-escaping mechanism.
- **Critical Finding:** While no traditional XSS vulnerabilities exist, a **Server-Side Template Injection (SSTI)** vulnerability was identified at `/total_loan_payments` endpoint. SSTI is a distinct vulnerability class that executes malicious code on the server rather than in the client browser. This finding is documented but **NOT included in the XSS exploitation queue** as it is outside the scope of Cross-Site Scripting analysis.
- **Purpose of this Document:** This report provides comprehensive analysis of all potential XSS attack vectors, demonstrating that the application's output encoding defenses are correctly implemented and matched to their render contexts.

## 2. Dominant Vulnerability Patterns

**Pattern 1: No Traditional XSS Vulnerabilities Found**  
- **Description:** All HTML output contexts are protected by Flask's default Jinja2 auto-escaping, which is enabled for `.html`, `.htm`, `.xml`, and `.xhtml` template files. User-controlled data rendered via `{{ variable }}` expressions is automatically HTML-escaped.
- **Implication:** Attackers cannot inject client-side JavaScript through traditional stored, reflected, or DOM-based XSS vectors.
- **Representative Findings:** All analyzed sinks (home.html:14, profile.html:18-38, loan_details.html) are safe.

**Pattern 2: Server-Side Template Injection (Out of XSS Scope)**  
- **Description:** The `/total_loan_payments` endpoint contains a critical SSTI vulnerability where the `principal` query parameter is embedded into a Jinja2 template via f-string interpolation and rendered using `render_template_string()`. A blacklist filter attempts to block `{{`, `}}`, `[`, `]`, `_`, and `.` characters, but fails to block Jinja2 statement syntax (`{% %}`).
- **Implication:** While this enables server-side code execution, it is **NOT an XSS vulnerability**. SSTI executes on the server in the Python runtime, not in the victim's browser. This vulnerability belongs to the Injection phase, not XSS phase.
- **Representative Finding:** SSTI-VULN-01 (documented for awareness, not included in XSS exploitation queue).

## 3. Strategic Intelligence for Exploitation

**Content Security Policy (CSP) Analysis**  
- **Current CSP:** No Content-Security-Policy header is configured.
- **Observation:** While the absence of CSP is a defense-in-depth weakness, it is not exploitable for XSS in this application because no XSS vulnerabilities exist. If XSS vulnerabilities are discovered in future code changes, the lack of CSP would allow unrestricted script execution.
- **Recommendation for Developers:** Implement CSP header: `Content-Security-Policy: default-src 'self'; script-src 'self'` for defense-in-depth.

**Cookie Security**  
- **Observation:** The primary session cookie (`session`) is missing the `HttpOnly` flag.
- **Implication:** If an XSS vulnerability were to be discovered, the session cookie would be accessible via `document.cookie`, enabling session hijacking. However, since no XSS vulnerabilities exist, this is currently not exploitable.
- **Recommendation for Developers:** Set `SESSION_COOKIE_HTTPONLY = True` in Flask configuration for defense-in-depth.

**Jinja2 Auto-Escaping Configuration**  
- **Current Configuration:** Auto-escaping is **enabled by default** in Flask applications for HTML templates.
- **Verification:** Confirmed at `__init__.py:7` - standard Flask initialization (`app = Flask(__name__)`) enables auto-escaping.
- **No Bypass Filters:** Comprehensive template analysis revealed no usage of `|safe`, `|raw`, or `{% autoescape false %}` directives that would bypass output encoding.
- **Implication:** This is the primary defensive control preventing XSS. All user data rendered via `{{ variable }}` expressions is context-appropriately HTML-escaped.

## 4. Vectors Analyzed and Confirmed Secure

These input vectors were traced and confirmed to have robust, context-appropriate defenses.

| Source (Parameter/Key) | Endpoint/File Location | Defense Mechanism Implemented | Render Context | Verdict |
|--------------------------|-------------------------|--------------------------------|----------------|---------|
| `user.name` | home.html:14 | Jinja2 Auto-Escaping (HTML Entity Encoding) | HTML_BODY | SAFE |
| `user.lastname` | home.html:14 | Jinja2 Auto-Escaping (HTML Entity Encoding) | HTML_BODY | SAFE |
| `user.name` | profile.html:18 | Jinja2 Auto-Escaping (Attribute Context) | HTML_ATTRIBUTE | SAFE |
| `user.lastname` | profile.html:22 | Jinja2 Auto-Escaping (Attribute Context) | HTML_ATTRIBUTE | SAFE |
| `user.email` | profile.html:26 | Jinja2 Auto-Escaping (Attribute Context) | HTML_ATTRIBUTE | SAFE |
| `user.loan_amount` | profile.html:30 | Type Coercion (float) + Jinja2 Auto-Escaping | HTML_ATTRIBUTE | SAFE |
| `user.loan_term_months` | profile.html:34 | Type Coercion (int) + Jinja2 Auto-Escaping | HTML_ATTRIBUTE | SAFE |
| `user.monthly_payment` | profile.html:38 | Type Coercion (float) + Jinja2 Auto-Escaping | HTML_ATTRIBUTE | SAFE |
| `loan_amount` | loan_details.html:15 | Type Coercion (float) + Jinja2 Auto-Escaping | HTML_BODY | SAFE |
| `loan_term_months` | loan_details.html:16 | Type Coercion (int) + Jinja2 Auto-Escaping | HTML_BODY | SAFE |
| `monthly_payment` | loan_details.html:17 | Type Coercion (float) + Jinja2 Auto-Escaping | HTML_BODY | SAFE |
| `username` (login error) | login.html:32 | Static Error Message (No Reflection) | HTML_BODY | SAFE |
| `password` (login error) | login.html:32 | Static Error Message (No Reflection) | HTML_BODY | SAFE |

### Detailed Analysis of Key Sinks

#### Sink Analysis 1: User Name Display in Home Page (SAFE)

**Complete Data Flow:**
```
POST /profile 
  → request.form['name'] (routes.py:70)
  → user.name = request.form['name'] (routes.py:70)
  → db.session.commit() (routes.py:76)
  → [Database: User.name column - String(50)]
  → GET /home
  → User.query.get(user_id) (routes.py:55)
  → render_template('home.html', user=user) (routes.py:56)
  → {{ user.name }} {{ user.lastname }} (home.html:14)
  → HTML Output: &lt;script&gt;...&lt;/script&gt;
```

**Testing Performed:**
- **Payload Injected:** `<script>alert('XSS')</script>` into user.name field
- **Storage:** Payload successfully stored in database without sanitization
- **Rendering:** Payload rendered as plain text: `Welcome <script>alert('XSS')</script> Doe!`
- **HTML Source Verification:** `<h2>Welcome &lt;script&gt;alert(&#39;XSS&#39;)&lt;/script&gt; &lt;img src=x onerror=alert(1)&gt;!</h2>`
- **Verdict:** Jinja2 auto-escaping converted `<` to `&lt;`, `>` to `&gt;`, `'` to `&#39;`, preventing script execution.

#### Sink Analysis 2: Profile Form Value Attributes (SAFE)

**Complete Data Flow:**
```
POST /profile 
  → request.form['name'] (routes.py:70)
  → user.name = request.form['name'] (routes.py:70)
  → db.session.commit() (routes.py:76)
  → [Database: User.name column]
  → GET /profile
  → User.query.get(user_id) (routes.py:67)
  → render_template('profile.html', user=user) (routes.py:80)
  → value="{{ user.name }}" (profile.html:18)
  → HTML Output: value="&quot;&gt;&lt;script&gt;...&quot;"
```

**Render Context:** HTML_ATTRIBUTE (inside `value="..."` of `<input>` element)

**Testing Performed:**
- **Payload Injected:** `"><script>alert('XSS')</script><input value="` into user.name field
- **Rendering:** Jinja2 escaped `"` to `&quot;`, `<` to `&lt;`, preventing attribute breakout
- **Verdict:** Context-appropriate escaping prevents breaking out of the value attribute.

#### Sink Analysis 3: Numeric Fields in JavaScript Context (SAFE - Code Smell)

**Location:** loan_details.html:52-54

**Code Pattern:**
```javascript
var loanAmount = {{ loan_amount }};
var loanTerm = {{ loan_term_months }};
var monthlyPayment = {{ monthly_payment }};
```

**Vulnerability Assessment:**
- **Render Context:** JAVASCRIPT (unquoted variable assignment)
- **Protection Mechanism:** Type coercion at routes.py:73-75 enforces `float()` and `int()` types
- **Why Currently Safe:** String payloads like `'; alert(1); //` cannot pass `float()` validation, raising ValueError
- **Code Quality Issue:** This is **dangerous pattern** - should use `{{ variable|tojson }}` for JavaScript context
- **Verdict:** SAFE due to type coercion, but represents poor coding practice
- **Future Risk:** If field types change from numeric to string, this becomes critically vulnerable

#### Sink Analysis 4: Login Error Messages (SAFE - No Reflection)

**Complete Data Flow:**
```
POST /login
  → request.form['username'] (routes.py:29)
  → request.form['password'] (routes.py:30)
  → User.query.filter_by(username=username, password=password).first() (routes.py:31)
  → [Login fails]
  → return render_template('login.html', error='Invalid username or password') (routes.py:39)
  → {{ error }} (login.html:32)
  → Static string rendered: "Invalid username or password"
```

**Testing Performed:**
- **Payload Injected:** Username: `<script>alert('XSS')</script>`, Password: `<img src=x onerror=alert(1)>`
- **Result:** Error message displayed static string only, no reflection of username or password
- **Verdict:** No user input is reflected in error messages, preventing reflected XSS

## 5. Analysis Constraints and Blind Spots

**No Significant Blind Spots Identified:**
- All identified XSS sinks from reconnaissance (Section 9 of recon deliverable) were systematically analyzed
- Complete source-to-sink traces performed for all data flows
- Live browser testing confirmed Jinja2 auto-escaping behavior
- HTML source inspection verified proper encoding in all render contexts

**Template Coverage:**
- Analyzed all 4 HTML templates: login.html, home.html, profile.html, loan_details.html
- No additional templates or JavaScript-rendered content discovered
- Static assets (CSS, JS) contain no dynamic user content

**JavaScript Analysis:**
- Application uses server-side rendering exclusively (no client-side JavaScript frameworks)
- Static JavaScript files (jQuery, Bootstrap) contain no user data injection
- AJAX endpoint at `/total_loan_payments` returns JSON (SSTI vulnerability, not XSS)

**Potential Future Risks:**
- If developers add `|safe` or `|raw` filters to templates, immediate XSS vulnerabilities would emerge
- If numeric fields are changed to string types without adding `|tojson` in JavaScript contexts (loan_details.html:52-54)
- If new endpoints are added that reflect user input without proper escaping

## 6. Out-of-Scope Finding: Server-Side Template Injection (SSTI)

**Important Note:** The following finding is documented for completeness but is **NOT an XSS vulnerability** and is **NOT included in the XSS exploitation queue**.

### SSTI Vulnerability Summary

**Endpoint:** `GET /total_loan_payments`  
**Vulnerable Parameter:** `principal` (query parameter)  
**Vulnerability Class:** Server-Side Template Injection (SSTI) - executes on server, not in browser  
**Severity:** CRITICAL (RCE capability)  

**Why This is NOT XSS:**
- SSTI executes Python code in the Flask server process, not JavaScript in the victim's browser
- Exploitation requires authentication (session cookie)
- Attack goal is server compromise, not client-side session hijacking
- This vulnerability belongs to the **Injection Analysis phase**, not XSS phase

**Data Flow:**
```
GET /total_loan_payments?principal=PAYLOAD&payment=100&term=12
  → request.args.get('principal', type=str).strip() (routes.py:105)
  → Blacklist filter checks for {{, }}, [, ], _, . (routes.py:114-116)
  → Payload embedded in template via f-string (routes.py:118-120)
  → render_template_string(json_template, ...) (routes.py:122)
  → Jinja2 executes server-side code in PAYLOAD
```

**Confirmed Exploit:**
```bash
GET /total_loan_payments?principal=%7B%25%20if%20True%20%25%7D42%7B%25%20endif%20%25%7D&payment=100&term=12
# URL-decoded: ?principal={% if True %}42{% endif %}&payment=100&term=12
# Result: {"loan_principal": "42", ...}
# Confirms template code execution on server
```

**Why Blacklist Filter Failed:**
- Filter blocks expression delimiters: `{{`, `}}`
- Filter does NOT block statement delimiters: `{%`, `%}`
- Jinja2 statements (`{% if %}`, `{% for %}`, `{% set %}`) bypass filter

**This finding should be addressed by the Injection Analysis specialist, not the XSS Exploitation phase.**

---

## 7. Testing Methodology and Proof of Concepts

### Test Case 1: Stored XSS Attempt in Profile Name Field

**Objective:** Verify Jinja2 auto-escaping prevents stored XSS in HTML body context

**Steps:**
1. Authenticated as user `test:test`
2. Navigated to `/profile`
3. Injected payload into name field: `<script>alert('XSS')</script>`
4. Injected payload into lastname field: `<img src=x onerror=alert(1)>`
5. Submitted profile update form
6. Navigated to `/home` to view rendered output

**Expected Result:** Payloads displayed as plain text, no script execution

**Actual Result:** 
- Browser displayed: `Welcome <script>alert('XSS')</script> <img src=x onerror=alert(1)>!`
- HTML source: `<h2>Welcome &lt;script&gt;alert(&#39;XSS&#39;)&lt;/script&gt; &lt;img src=x onerror=alert(1)&gt;!</h2>`
- Confirmation: Jinja2 auto-escaping active and effective

**Verdict:** SAFE - No XSS execution

**Screenshot Evidence:** `/Users/arjunmalleswaran/Code/pentest-workspaces/XBEN-063-24/.playwright-mcp/xss_test_home_page.png`

### Test Case 2: Reflected XSS Attempt in Login Form

**Objective:** Verify login error messages do not reflect user input

**Steps:**
1. Logged out from application
2. Navigated to `/login`
3. Injected payload into username: `<script>alert('XSS')</script>`
4. Injected payload into password: `<img src=x onerror=alert(1)>`
5. Submitted login form

**Expected Result:** Static error message displayed, no reflection of username or password

**Actual Result:**
- Error message displayed: `Invalid username or password`
- No reflection of malicious username or password in response
- Error variable set to static string at routes.py:39

**Verdict:** SAFE - No reflected XSS

**Screenshot Evidence:** `/Users/arjunmalleswaran/Code/pentest-workspaces/XBEN-063-24/.playwright-mcp/login_error_no_xss.png`

### Test Case 3: Attribute Context XSS Attempt in Profile Form

**Objective:** Verify Jinja2 auto-escaping prevents XSS in HTML attribute context

**Steps:**
1. Authenticated as user `test:test`
2. Navigated to `/profile`
3. Injected attribute breakout payload: `"><script>alert('XSS')</script><input value="`
4. Submitted profile update form
5. Navigated back to `/profile` to view form with pre-populated values

**Expected Result:** Payload escaped, cannot break out of value attribute

**Actual Result:**
- Input field displayed escaped payload in value attribute
- HTML source confirmed `"` escaped to `&quot;`, `<` to `&lt;`, preventing attribute breakout
- No script execution observed

**Verdict:** SAFE - Context-appropriate escaping active

### Test Case 4: SSTI Confirmation (Out of XSS Scope)

**Objective:** Confirm SSTI vulnerability exists but verify it's server-side, not client-side XSS

**Steps:**
1. Authenticated as user `test:test`
2. Crafted SSTI payload bypassing blacklist: `{% if True %}42{% endif %}`
3. Sent request: `GET /total_loan_payments?principal=%7B%25%20if%20True%20%25%7D42%7B%25%20endif%20%25%7D&payment=100&term=12`

**Expected Result:** Template code executes on server, returning "42" in JSON response

**Actual Result:**
- Response: `{ "loan_principal": "42" ,"monthly_payment": "100.0", "loan_term_amount": "12", "total_loan_payments": "1200.0" }`
- Confirmation: Jinja2 statement executed on server
- No client-side script execution observed

**Verdict:** SSTI vulnerability confirmed (NOT XSS - belongs to Injection phase)

---

## 8. Conclusion

### Summary of Findings

**XSS Vulnerabilities Identified:** 0 (Zero)

**Defense Mechanisms Validated:**
1. **Jinja2 Auto-Escaping:** Enabled by default, provides context-aware HTML encoding
2. **Static Error Messages:** Login errors use hardcoded strings, no user input reflection
3. **Type Coercion:** Numeric fields protected by Python's `float()` and `int()` validation

**Security Posture:**
- The application demonstrates **strong XSS defense** through proper use of Flask's default security features
- No exploitable reflected, stored, or DOM-based XSS vulnerabilities exist
- All output contexts are appropriately encoded for their render contexts

**Defense-in-Depth Recommendations:**
While no XSS vulnerabilities exist, the following improvements would strengthen security posture:
1. Add Content-Security-Policy header: `Content-Security-Policy: default-src 'self'; script-src 'self'`
2. Enable `SESSION_COOKIE_HTTPONLY = True` in Flask configuration
3. Use `|tojson` filter in JavaScript contexts (loan_details.html:52-54) for code quality
4. Add server-side input validation for text fields (name, email) for defense-in-depth

**Handoff to Exploitation Phase:**
- **XSS Exploitation Queue:** Empty (no exploitable XSS vulnerabilities)
- **SSTI Finding:** Documented but belongs to Injection phase, not XSS phase
- **Recommendation:** Focus exploitation efforts on SSTI vulnerability at `/total_loan_payments` for server-side code execution

---

**END OF XSS ANALYSIS DELIVERABLE**