CVE-2024-21671

Executive Summary

CVE-2024-21671 is a low-severity information disclosure vulnerability in the vantage6 platform, which is used to manage and deploy privacy-enhancing technologies such as Federated Learning (FL) and Multi-Party Computation (MPC). The vulnerability allows an unauthenticated attacker to enumerate valid usernames by measuring the response time of login requests. This timing-based side-channel attack can aid in credential attacks, such as brute-force or password spraying, by allowing attackers to focus their efforts on known valid accounts. The vulnerability is patched in version 4.2.0.

Supply Chain & Package Security

The vantage6 platform is a Python-based open-source project. The vulnerability exists in the authentication logic of the server-side application. The fix involves implementing constant-time comparison for login responses to eliminate the timing discrepancy. Users should ensure they are using the latest patched version from the official PyPI repository or GitHub source.

CVSS Breakdown & Threat Level

• CVSS Score: 3.7 (Low)
• Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:N
• Attack Vector: Network
• Attack Complexity: Low
• Privileges Required: None
• User Interaction: None
• Scope: Unchanged
• Confidentiality Impact: Low (username enumeration)
• Integrity Impact: None
• Availability Impact: None

The low score reflects the limited direct impact (only username disclosure) and the lack of direct exploitation for data breach or system compromise. However, the information leaked can significantly increase the success rate of subsequent attacks.

Deep Dive & Mechanics

The vulnerability is a classic timing-based side-channel attack. When a login request is processed, the server's response time can vary depending on whether the username exists in the system. If the username is valid, the server may perform additional processing (e.g., password hashing, database lookup) before returning an error, resulting in a slightly longer response time compared to an invalid username. An attacker can send multiple login requests with different usernames and measure the response times to identify which usernames are valid.

The vulnerable code likely performs a database query to check if the username exists before proceeding to password verification. If the username is not found, the server returns an error immediately. If found, it continues to verify the password, which takes additional time. This timing difference, even if small (e.g., a few milliseconds), can be detected over a network with sufficient samples.

Vulnerable vs. Patched Code

Vulnerable Code (Conceptual):

def login(username, password):
    user = db.query(User).filter_by(username=username).first()
    if user is None:
        return "Invalid username or password", 401
    if check_password(user.password_hash, password):
        return "Login successful", 200
    else:
        return "Invalid username or password", 401

Patched Code (Conceptual):

def login(username, password):
    user = db.query(User).filter_by(username=username).first()
    # Always perform password check to avoid timing difference
    if user is None:
        # Use a dummy hash to ensure consistent timing
        dummy_hash = get_dummy_hash()
        check_password(dummy_hash, password)
        return "Invalid username or password", 401
    if check_password(user.password_hash, password):
        return "Login successful", 200
    else:
        return "Invalid username or password", 401

The patch ensures that even if the username does not exist, the server still performs a password verification step (using a dummy hash) to make the response time consistent regardless of whether the username is valid.

Exploitation & Lab Setup

Red Team

Lab Setup:

1. Deploy a vulnerable instance of vantage6 (version < 4.2.0) in a controlled environment.
2. Use a tool like timing_attack.py to measure response times.

Exploitation Steps:

1. Send a series of login requests with different usernames.
2. Measure the response time for each request.
3. Identify usernames with consistently higher response times.
4. Use the enumerated usernames in a password spraying or brute-force attack.

Example Script (Python):

import requests
import time

target_url = "http://vulnerable-vantage6/login"
usernames = ["admin", "user1", "test", "john", "alice"]

for username in usernames:
    times = []
    for _ in range(10):
        start = time.time()
        response = requests.post(target_url, json={"username": username, "password": "wrong"})
        end = time.time()
        times.append(end - start)
    avg_time = sum(times) / len(times)
    print(f"{username}: {avg_time:.4f}s")

Blue Team

Detection:

• Monitor for unusual patterns in login response times.
• Look for multiple login attempts with different usernames from the same IP address.
• Implement rate limiting and account lockout policies.

Mitigation:

• Apply the patch by upgrading to vantage6 version 4.2.0.
• Implement constant-time comparison for all authentication checks.
• Use a Web Application Firewall (WAF) to detect and block timing attacks.

Indicators of Compromise (IOCs)

• Network: Multiple HTTP POST requests to /login endpoint with varying usernames and incorrect passwords.
• Logs: Repeated login failures with different usernames from the same source IP.
• Timing: Unusually consistent response times for valid vs. invalid usernames in logs.

Detection Engineering

YARA Rule

rule vantage6_timing_attack {
    meta:
        description = "Detects potential timing attack on vantage6 login"
        author = "Valters IT"
        date = "2024-01-01"
    strings:
        $login_endpoint = "/login"
        $invalid_cred = "Invalid username or password"
    condition:
        # of ($login_endpoint) > 10 and # of ($invalid_cred) > 10
}

Sigma Rule

title: Vantage6 Timing Attack Detection
id: 12345678-1234-1234-1234-123456789012
status: experimental
description: Detects multiple login attempts with different usernames from same source
logsource:
    category: web
    product: vantage6
detection:
    selection:
        event: login_failure
        username|re: .*
    condition: selection | count() by src_ip > 10 within 5m
falsepositives:
    - Legitimate user testing multiple accounts
level: medium

Suricata Rule

alert http any any -> any any (msg:"Vantage6 Timing Attack - Multiple Login Attempts"; flow:to_server; content:"POST"; http_method; content:"/login"; http_uri; content:"username"; http_client_body; threshold: type both, track by_src, count 10, seconds 300; sid:1000001; rev:1;)

Remediation & Mitigation

1. Immediate: Upgrade to vantage6 version 4.2.0 or later.
2. Workaround: If immediate upgrade is not possible, implement a reverse proxy that adds random delays to login responses to mask timing differences.
3. Long-term: Implement constant-time comparison for all authentication-related operations.
4. Monitoring: Enable detailed logging of login attempts and monitor for unusual patterns.
5. Access Control: Use IP whitelisting or VPN for administrative access to vantage6.

Valters IT