Navigating the Bypass of 2FA Security
~ Vinayak Jituri, Jan 23, 2025
Introduction
In today’s cybersecurity landscape, 2FA (Two-Factor Authentication) adds an essential layer of security for online accounts. However, some 2FA implementations can be vulnerable, especially when OTP (One-Time Password) validation lacks stringent protections. In this post, we’ll explore how an attacker might bypass 2FA using brute force techniques. This demonstration is purely educational, aimed at highlighting vulnerabilities and encouraging more robust security practices.
To start, navigate to the 2FA Bypass Using Brute Force Attack page. Here, you’ll find the main user interface, where users typically enter a one-time password (OTP) for validation. This is where we’ll focus our testing.


By intercepting the OTP request, you can observe how the numbers are being encoded. Using a tool like Burp Suite, capture the request to understand the encoding mechanism. This information is essential for automating the OTP input.

After examining the script, you’ll observe that the OTP handling follows this pattern:
function encryptByDESModeEBC(message,key){
var keyHex = CryptoJS.enc.Utf8.parse(key);
var encrypted = CryptoJS.DES.encrypt(message, keyHex, {
mode: CryptoJS.mode.ECB,
padding: CryptoJS.pad.Pkcs7
});
return encrypted.ciphertext.toString();
}
function encrypt(){
var digit1 = document.getElementById(“digit1”).value;
var digit2 = document.getElementById(“digit2”).value;
var digit3 = document.getElementById(“digit3”).value;
document.getElementById(“digit1”).value = CryptoJS.SHA1(digit1);
document.getElementById(“digit2”).value = btoa(digit2);
document.getElementById(“digit3”).value = encryptByDESModeEBC(digit3, “M^K3y”);
return true;
}
This code illustrates how each OTP digit undergoes a different form of encoding:
- digit1 uses SHA1 hashing.
- digit2 is Base64-encoded.
- digit3 undergoes DES encryption in ECB mode with a predefined key.
- Understanding encoding here is crucial since each method protects or masks the OTP differently. This knowledge is essential for configuring a brute-force attack that can correctly encode or hash potential OTPs.
Using SHA1 hashing, Base64 encoding, and DES encryption for sensitive operations poses significant security risks due to their inherent weaknesses. SHA1 is vulnerable to collision attacks, where two different inputs produce the same hash value, compromising data integrity and making it unsuitable for modern cryptographic applications. Base64 encoding, often mistaken for encryption, provides no actual security as it simply encodes data in a reversible format, leaving the information easily recoverable by anyone who intercepts it. DES (Data Encryption Standard), once a widely used encryption method, is now considered obsolete due to its short key length of 56 bits, which makes it highly susceptible to brute-force attacks. These weaknesses collectively expose systems to tampering, unauthorized access, and data breaches, making it crucial to adopt stronger alternatives like SHA-256, AES encryption, and actual cryptographic solutions instead of encoding methods like Base64.
Send the captured request to Intruder in Burp Suite and configure the positions for the attack. Mark the encoded OTP value so that Intruder can replace it with values from a prepared list.




In Intruder’s Resource Pool, adjust the request delay time. This pacing is crucial to avoid overwhelming the server and triggering anti-bot mechanisms.

With everything configured, start the brute-force attack. After a few attempts, Intruder should identify the correct OTP, allowing access to the session.


The root cause of the vulnerability lies in improper OTP validation and insufficient safeguards against brute-force attempts. Weaknesses such as predictable OTP generation, insecure validation mechanisms, and inadequate expiration policies enable attackers to exploit these systems. Additionally, insufficient rate limiting allows multiple attempts to brute-force the OTP unchecked. To address these issues, organizations should enforce dynamic and unpredictable OTP generation, implement robust rate-limiting mechanisms, and use constant-time comparison methods for validation. Employing secure hashing and encryption algorithms such as HMAC-SHA256 for OTP generation and transport, alongside encoding schemes that prevent leakage or tampering, can further fortify 2FA systems against such attacks.
This walk through demonstrates how improper OTP validation and insufficient rate limiting can expose even 2FA-protected systems to brute-force attacks. Organizations should implement dynamic 2FA, use stronger encryption, and enforce strict access control policies to avoid these vulnerabilities.