# Cross-Site Scripting (XSS) Analysis Report

## 1. Executive Summary
- **Analysis Status:** Complete  
- **Key Outcome:** One high-confidence Stored XSS vulnerability was identified and confirmed via live browser testing. The vulnerability enables arbitrary JavaScript execution in victim browsers through malicious SVG file uploads.
- **Purpose of this Document:** This report provides the strategic context, vulnerability patterns, and environmental intelligence necessary to effectively exploit the confirmed XSS vulnerability.  

## 2. Dominant Vulnerability Patterns

**Pattern 1: Stored XSS via Unsafe Template Rendering with `| safe` Filter**  
- **Description:** The application uses Jinja2's `| safe` filter to render user-uploaded SVG content without sanitization. This disables Jinja2's automatic HTML escaping, allowing malicious SVG files with embedded JavaScript to execute in victim browsers.
- **Root Cause:** The `{{ profile.image | safe }}` template expression at `/app/templates/index.html:32` bypasses all output encoding, treating untrusted user content as safe HTML.
- **Data Flow:** User uploads SVG file → XML parser validates syntax (preserves malicious content) → Content stored in memory → Rendered with `| safe` filter → JavaScript executes.
- **Implication:** Any attacker can upload a weaponized SVG file that executes arbitrary JavaScript in the browser of every user who views the profile page. This is a persistent (Stored) XSS vulnerability with high impact.
- **Representative Finding:** XSS-VULN-01.  

**Pattern 2: Secure Auto-Escaped Attribute Rendering**  
- **Description:** User input rendered in HTML attribute contexts (name, surname, address fields) is protected by Jinja2's default auto-escaping mechanism, which encodes dangerous characters like `"`, `<`, `>`, and `&` to HTML entities.
- **Security Control:** Jinja2 auto-escaping is enabled by default for `.html` templates. The application correctly relies on this framework-level protection for these specific sinks.
- **Implication:** While the application demonstrates inconsistent security practices (using `| safe` for one field but not others), the attribute-context sinks are properly protected and not exploitable.
- **Representative Findings:** Profile name, surname, and address fields (all SAFE).

## 3. Strategic Intelligence for Exploitation

**Content Security Policy (CSP) Analysis**  
- **Current CSP:** None configured.
- **HTTP Headers Observed:** No `Content-Security-Policy` header present in responses.
- **Critical Impact:** The absence of CSP means there are no restrictions on:
  - Inline script execution (`<script>alert(1)</script>`)
  - Event handler execution (`onload`, `onerror`, etc.)
  - External script loading (attacker can load remote scripts)
  - `eval()` and dynamic code execution
- **Recommendation for Exploitation:** All standard XSS payload types will execute without CSP blocking. Exploitation can use the simplest possible payloads (e.g., `<svg onload="alert(1)">`).

**Cookie Security**  
- **Observation:** The application has no authentication system and does not set any session cookies.
- **Current State:** No cookies are used for authentication or session management.
- **Implication:** While cookie theft is not a viable attack vector (no cookies to steal), XSS can still be weaponized for:
  - Defacement of the profile page
  - Keylogging to capture form input
  - Redirection to phishing sites
  - Browser-based exploitation (BeEF framework)
  - Local storage manipulation
  - Cross-site request forgery (CSRF) via XSS

**CORS and Same-Origin Policy**  
- **Observation:** No custom CORS headers configured. Standard browser same-origin policy applies.
- **Implication:** XSS payloads can make authenticated requests to the same origin (`http://localhost:32973`) and read responses, enabling complete takeover of the application's functionality.

**Input Validation**  
- **SVG Upload Validation:** None. The application accepts any file content and attempts to parse it as XML.
- **File Type Check:** Missing. No MIME type validation or magic byte verification.
- **File Size Limit:** None observed. Arbitrarily large files are accepted.
- **Content Sanitization:** None. The XML parser validates syntax but preserves malicious content like `<script>` tags and event handlers.
- **Implication:** There are no defensive layers to bypass. Simple, unobfuscated XSS payloads work immediately.

**XML Parser Behavior**  
- **Parser:** lxml 5.2.2 with `etree.XMLParser(load_dtd=True, resolve_entities=True)`
- **Security Relevance:** The parser configuration enables XXE attacks (separate vulnerability), but for XSS purposes, the key behavior is that it preserves all SVG elements and attributes, including:
  - `<script>` tags within SVG
  - Event handler attributes (`onload`, `onerror`, `onclick`, etc.)
  - Embedded JavaScript in `href` attributes (`javascript:` protocol)
- **Encoding Behavior:** The parser outputs UTF-8 encoded strings. Special characters in JavaScript payloads are preserved correctly.

**Framework Details**  
- **Backend:** FastAPI (Python) with Jinja2 templating
- **Auto-Escaping:** Enabled by default for `.html` templates (protects attribute contexts)
- **Bypass Mechanism:** The `| safe` filter explicitly disables auto-escaping for `profile.image`
- **Template Location:** `/app/templates/index.html:32`

## 4. Vectors Analyzed and Confirmed Secure

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

| Source (Parameter/Key) | Endpoint/File Location | Defense Mechanism Implemented | Render Context | Verdict |
|--------------------------|-------------------------|--------------------------------|----------------|---------|
| `name` (form field) | POST / → `/app/templates/index.html:16` | Jinja2 auto-escaping (HTML entity encoding) | HTML_ATTRIBUTE (value) | SAFE |
| `surname` (form field) | POST / → `/app/templates/index.html:20` | Jinja2 auto-escaping (HTML entity encoding) | HTML_ATTRIBUTE (value) | SAFE |
| `address` (form field) | POST / → `/app/templates/index.html:24` | Jinja2 auto-escaping (HTML entity encoding) | HTML_ATTRIBUTE (value) | SAFE |

**Defense Mechanism Details:**
- **Jinja2 Auto-Escaping:** Converts dangerous characters to HTML entities:
  - `"` → `&#34;` (prevents attribute breakout)
  - `<` → `&lt;` (prevents tag injection)
  - `>` → `&gt;` (prevents tag closure)
  - `&` → `&amp;` (prevents entity injection)
  - `'` → `&#39;` (prevents single-quote attribute attacks)

**Testing Evidence:**
- Attempted payload: `XSSTEST" onload="alert(1)`
- Rendered output: `value="XSSTEST&#34; onload=&#34;alert(1)"`
- Result: Double quotes encoded, attribute breakout prevented, payload neutralized.

## 5. Analysis Constraints and Blind Spots

**Constraints:**
- **Client-Side JavaScript Analysis:** The application uses minimal client-side JavaScript (Bootstrap only). No custom JavaScript files were found that could contain DOM-based XSS vulnerabilities.
- **Template Coverage:** Only one template file exists (`index.html`). Complete template coverage was achieved.
- **Testing Scope:** Testing focused on the publicly accessible interface at `http://localhost:32973`. No internal APIs or admin panels exist in this application.

**Blind Spots:**
- **Mutation XSS (mXSS):** Not extensively tested. The browser's HTML parser might introduce additional XSS vectors through malformed HTML correction. However, the vulnerability found is straightforward and does not rely on mXSS.
- **CSP Bypass Techniques:** Not applicable since no CSP is configured.
- **Browser-Specific Quirks:** Testing was performed with modern browser behavior. Legacy browser vulnerabilities were not assessed.

**Coverage Confidence:** HIGH - The application is extremely simple (3 endpoints, 1 template file, 4 user input fields). All identified sinks were analyzed systematically.

---

## 6. Vulnerability Details - XSS-VULN-01

**Vulnerability ID:** XSS-VULN-01  
**Type:** Stored XSS (Persistent)  
**Severity:** HIGH  
**CVSS 3.1 Score:** 8.1  
**CWE:** CWE-79 (Improper Neutralization of Input During Web Page Generation)

**Location:**
- **Sink:** `/app/templates/index.html:32` - `{{ profile.image | safe }}`
- **Source:** POST / endpoint - `image` file upload parameter (`/app/main.py:29`)

**Complete Data Flow:**
```
1. Attacker uploads malicious SVG file via POST / (multipart/form-data)
   └─> /app/main.py:34 - image_content = await image.read()

2. XML parsing (preserves malicious content)
   └─> /app/main.py:36-37 - parser = etree.XMLParser(load_dtd=True, resolve_entities=True)
   └─> tree = etree.fromstring(image_content, parser)

3. Serialize back to string
   └─> /app/main.py:38 - profile.image = etree.tostring(tree).decode()

4. Store in memory (persistence until server restart)
   └─> /app/main.py:43 - user_profiles[1] = profile

5. Victim visits GET /
   └─> /app/main.py:19 - profile = user_profiles.get(1, ...)

6. Render with unsafe filter (XSS SINK)
   └─> /app/templates/index.html:32 - {{ profile.image | safe }}

7. JavaScript executes in victim's browser
```

**Root Cause Analysis:**
1. **Missing Sanitization:** No SVG sanitization removes dangerous elements (`<script>`) or attributes (`onload`, `onerror`)
2. **Bypass of Auto-Escaping:** The `| safe` filter explicitly disables Jinja2's HTML encoding
3. **XML Parser Preserves Malicious Content:** The lxml parser validates syntax but does not remove JavaScript
4. **No Content Security Policy:** Browser has no restrictions on inline script execution

**Proof-of-Concept Payload:**
```svg
<svg xmlns="http://www.w3.org/2000/svg" onload="alert('XSS-Vulnerability-Confirmed')">
  <circle cx="50" cy="50" r="40" fill="red"/>
</svg>
```

**Live Testing Results:**
- ✅ Payload uploaded successfully via file upload form
- ✅ JavaScript executed immediately upon form submission (POST /)
- ✅ Payload persisted in memory (user_profiles dictionary)
- ✅ JavaScript executed again when page refreshed (GET /)
- ✅ Confirms Stored XSS with persistence across requests
- ✅ Screenshot captured: `/xss_payload_uploaded.png`

**Impact:**
- **Arbitrary JavaScript Execution:** Attacker-controlled scripts run with victim's origin
- **Page Defacement:** Visual appearance can be modified
- **Phishing:** Fake login forms can be injected
- **Keylogging:** User input can be intercepted
- **Form Hijacking:** Form submissions can be redirected to attacker-controlled servers
- **Browser Exploitation:** Integration with frameworks like BeEF for advanced attacks
- **CSRF via XSS:** XSS can bypass anti-CSRF tokens and perform actions on behalf of victims

**Externally Exploitable:** ✅ YES - The endpoint is publicly accessible at `http://localhost:32973/` without authentication.

---

## 7. Remediation Recommendations

**Immediate Fix (Critical Priority):**

1. **Remove the `| safe` Filter:**
   ```html
   <!-- BEFORE (Vulnerable): -->
   <div>{{ profile.image | safe }}</div>
   
   <!-- AFTER (Safe): -->
   <div>{{ profile.image }}</div>
   ```
   This re-enables Jinja2's auto-escaping for the image field.

2. **Implement SVG Sanitization:**
   ```python
   # Install: pip install svg-sanitizer
   from svg_sanitizer import Sanitizer
   
   if image:
       image_content = await image.read()
       sanitizer = Sanitizer()
       safe_svg = sanitizer.sanitize(image_content.decode())
       profile.image = safe_svg
   ```

3. **Add Content Security Policy:**
   ```python
   from fastapi.middleware.cors import CORSMiddleware
   from starlette.middleware.base import BaseHTTPMiddleware
   
   class SecurityHeadersMiddleware(BaseHTTPMiddleware):
       async def dispatch(self, request, call_next):
           response = await call_next(request)
           response.headers["Content-Security-Policy"] = "default-src 'self'; script-src 'self'"
           response.headers["X-Content-Type-Options"] = "nosniff"
           response.headers["X-Frame-Options"] = "DENY"
           return response
   
   app.add_middleware(SecurityHeadersMiddleware)
   ```

**Defense in Depth:**

4. **Input Validation:**
   - Verify file is actually SVG (check MIME type and magic bytes)
   - Enforce maximum file size (e.g., 100KB)
   - Allowlist safe SVG elements and attributes

5. **Output Encoding Awareness:**
   - Audit all uses of `| safe` filter in templates
   - Document when `| safe` is necessary and ensure proper sanitization
   - Consider using `| escape` explicitly for clarity

---