06 Mar, 2023

Client-side Validation Bypass

Vulnerability Assessment as a Service (VAaaS)

Tests systems and applications for vulnerabilities to address weaknesses.

Client-side Validation (CSV) Bypass refers to a type of security vulnerability where an attacker is able to bypass the validation checks that are performed on the client-side of a web application. CSV is a technique used by web developers to validate user inputs and ensure that they meet certain criteria before being submitted to the server.

However, if the CSV checks are not properly implemented or if an attacker is able to manipulate the client-side code, they may be able to bypass the validation checks and submit malicious data to the server. This can lead to a variety of security issues, including injection attacks, privilege escalation, and data exfiltration.

Example of vulnerable code on different programming languages:


in JavaScript:

				
					function validateForm() {
  var name = document.forms["myForm"]["name"].value;
  var email = document.forms["myForm"]["email"].value;
  var password = document.forms["myForm"]["password"].value;

  if (name == "") {
    alert("Please enter your name.");
    return false;
  }

  if (email == "") {
    alert("Please enter your email.");
    return false;
  }

  if (password == "") {
    alert("Please enter your password.");
    return false;
  }
}

				
			


In this example, the client-side validation checks are performed using JavaScript. However, an attacker could easily bypass these checks by manipulating the client-side code using a browser extension or other tools.

• in PHP:

				
					<?php
$name = $_POST['name'];
$email = $_POST['email'];
$password = $_POST['password'];

if (empty($name)) {
  echo "Please enter your name.";
  exit;
}

if (empty($email)) {
  echo "Please enter your email.";
  exit;
}

if (empty($password)) {
  echo "Please enter your password.";
  exit;
}
?>

				
			


In this example, the server-side validation checks are performed using PHP. However, if the client-side validation checks are not properly implemented, an attacker could still bypass these checks by submitting malicious data to the server.

• in Python:

				
					name = input("Enter your name: ")
email = input("Enter your email: ")
password = input("Enter your password: ")

if not name:
  print("Please enter your name.")
  exit()

if not email:
  print("Please enter your email.")
  exit()

if not password:
  print("Please enter your password.")
  exit()

				
			


In this example, the validation checks are performed using Python. However, if the client-side validation checks are not properly implemented, an attacker could still bypass these checks by manipulating the data submitted to the server.

Examples of exploitation Client-side Validation Bypass

SQL Injection:

An attacker could use a CSV Bypass vulnerability to submit malicious data to a web application, which could then be used to perform a SQL injection attack. This could allow the attacker to extract sensitive data from the database or modify the contents of the database.

Cross-Site Scripting (XSS):

An attacker could use a CSV Bypass vulnerability to inject malicious code into a web application, which could then be executed on the client-side of other users accessing the web application. This could allow the attacker to steal session cookies, hijack user accounts, or steal other sensitive information.

Privilege Escalation:

An attacker could use a CSV Bypass vulnerability to modify the data submitted to a web application, which could then be used to escalate privileges and gain access to sensitive resources or perform unauthorized actions.

Data Exfiltration:

An attacker could use a CSV Bypass vulnerability to bypass the validation checks that are meant to prevent data exfiltration. This could allow the attacker to extract sensitive data from the web application and exfiltrate it to an external location.

Denial of Service (DoS):

An attacker could use a CSV Bypass vulnerability to submit large amounts of data to a web application, which could cause the application to crash or become unresponsive. This could result in a denial of service attack, preventing legitimate users from accessing the web application.

Privilege escalation techniques for Client-side Validation Bypass

Parameter Tampering:

An attacker could manipulate the values of parameters used in requests sent to the server to bypass validation checks and gain access to restricted areas or resources. For example, if a web application uses CSV to validate a user’s role before granting access to certain pages, an attacker could modify the role value in the request to bypass the validation and gain access to those pages.

Cross-Site Scripting (XSS):

An attacker could use a CSV Bypass vulnerability to inject malicious code into a web application, which could then be executed on the client-side of other users accessing the web application. This could allow the attacker to steal session cookies, hijack user accounts, or gain access to privileged functionality.

Session Hijacking:

An attacker could use a CSV Bypass vulnerability to bypass validation checks meant to prevent session hijacking. This could allow the attacker to steal session cookies and impersonate a legitimate user, gaining access to their privileged functionality.

CSRF (Cross-Site Request Forgery):

An attacker could use a CSV Bypass vulnerability to bypass validation checks meant to prevent CSRF attacks. This could allow the attacker to submit requests to the server on behalf of a legitimate user, performing unauthorized actions and gaining access to privileged functionality.

SQL Injection:

An attacker could use a CSV Bypass vulnerability to submit malicious data to a web application, which could then be used to perform a SQL injection attack. This could allow the attacker to extract sensitive data from the database or modify the contents of the database, potentially gaining access to privileged functionality.

General methodology and checklist for Client-side Validation Bypass

Methodology:

  1. Identify the input fields and parameters that are validated on the client-side.

  2. Attempt to bypass the validation checks by manipulating the input values using browser extensions or other tools.

  3. Submit the modified data to the server and observe the response. If the server accepts the data and performs the intended action, then a CSV Bypass vulnerability may be present.

  4. Repeat the above steps for all input fields and parameters that are validated on the client-side.

  5. Verify the server-side validation checks to ensure that they are present and properly implemented. If the server-side validation checks are not properly implemented, then an attacker may still be able to bypass the validation checks even if they are properly implemented on the client-side.

  6. Test for other common vulnerabilities that could be exploited in combination with CSV Bypass vulnerabilities, such as SQL injection, cross-site scripting (XSS), and privilege escalation.

  7. Document any vulnerabilities that are discovered, including detailed information on the steps taken to exploit them, and report them to the appropriate parties for remediation.

  8. Finally, verify that the vulnerabilities have been fixed and re-test to ensure that they have been properly addressed.

Checklist:

  1. Identify all input fields and parameters that are validated on the client-side.

  2. Use a proxy tool to intercept and modify client-side validation data.

  3. Attempt to bypass validation by modifying the input data using browser extensions or other tools.

  4. Test different data formats and boundary values to see if any CSV Bypass vulnerabilities exist.

  5. Attempt to submit malicious data that could be used to perform other types of attacks, such as SQL injection or Cross-Site Scripting (XSS).

  6. Check for any known CSV Bypass vulnerabilities in the target web application or any third-party libraries it uses.

  7. Verify that any server-side validation checks are implemented and functioning properly.

  8. Check if any input sanitization is done on the client-side and test if this can be bypassed.

  9. Test any multi-step forms to check if client-side validation is enforced between steps and if it can be bypassed.

  10. Review any error messages or warnings that are displayed when validation fails, as these can often provide valuable information for attackers.

  11. Document any vulnerabilities discovered and provide clear steps to reproduce them.

  12. Report any vulnerabilities to the appropriate parties for remediation.

  13. Verify that any identified vulnerabilities have been fixed and re-test to ensure they have been properly addressed.

Tools set for exploiting Client-side Validation Bypass

Manual Tools:

  • Burp Suite – A proxy tool that allows the user to intercept and modify HTTP requests and responses. It includes many useful features for testing web applications, including a repeater tool that allows the user to modify and resend requests multiple times.

  • OWASP ZAP – A popular web application security scanner that includes a proxy tool, active and passive vulnerability scanners, and a variety of other tools for testing web applications.

  • Fiddler – A free web debugging proxy tool that allows the user to intercept and modify HTTP requests and responses. It includes many useful features for testing web applications, including a script editor and a powerful traffic monitoring tool.

  • Chrome Developer Tools – A set of web developer tools built into the Google Chrome browser. It includes a variety of useful features for testing and debugging web applications, including a JavaScript console, a network monitor, and a DOM inspector.

  • Firefox Developer Tools – Similar to the Chrome Developer Tools, Firefox Developer Tools are a set of built-in tools designed for web developers. They include a variety of features for testing and debugging web applications, including a JavaScript console, a network monitor, and a DOM inspector.

  • JavaScript Console – A browser console that allows the user to execute JavaScript code and manipulate the DOM of the current web page. This can be useful for testing and bypassing client-side validation checks.

  • Tamper Data – A Firefox add-on that allows the user to intercept and modify HTTP requests and responses. It includes many useful features for testing web applications, including a request editor and a response editor.

  • EditThisCookie – A browser extension that allows the user to modify and manage cookies for the current web page. This can be useful for bypassing session-based client-side validation checks.

  • HackBar – A Firefox add-on that adds a toolbar with various tools for testing web applications, including a parameter editor and an encoding/decoding tool.

  • Firebug – A Firefox add-on that includes a variety of useful features for testing and debugging web applications, including a JavaScript debugger, a DOM inspector, and a network monitor.

Automated Tools:

  • Netsparker – An automated web application security scanner that includes a variety of features for testing web applications, including a proxy tool, an active vulnerability scanner, and a variety of passive vulnerability checks.

  • Acunetix – Another automated web application security scanner that includes a variety of features for testing web applications, including a proxy tool, an active vulnerability scanner, and a variety of passive vulnerability checks.

  • AppScan – A web application security scanner developed by IBM that includes a variety of features for testing web applications, including a proxy tool, an active vulnerability scanner, and a variety of passive vulnerability checks.

  • Qualys – A cloud-based web application security scanner that includes a variety of features for testing web applications, including a proxy tool, an active vulnerability scanner, and a variety of passive vulnerability checks.

  • WebInspect – A web application security scanner developed by Micro Focus that includes a variety of features for testing web applications, including a proxy tool, an active vulnerability scanner, and a variety of passive vulnerability checks.

  • Skipfish – An automated web application security scanner that uses a combination of black-box and white-box testing techniques to identify vulnerabilities in web applications.

The Common Weakness Enumeration (CWE)

• CWE-602: Client-Side Enforcement of Server-Side Security – This CWE category includes weaknesses where the client-side code is responsible for enforcing security measures that should be implemented on the server-side.

• CWE-710: Improper Adherence to Coding Standards – This CWE category includes weaknesses where developers do not follow coding standards or best practices, which can lead to vulnerabilities in the application.

• CWE-770: Allocation of Resources Without Limits or Throttling – This CWE category includes weaknesses where an application allocates resources without any limits or throttling, which can lead to resource exhaustion and denial of service attacks.

• CWE-795: Improper Handling of Exceptional Conditions – This CWE category includes weaknesses where an application does not properly handle exceptional conditions, which can lead to unexpected behavior and vulnerabilities.

• CWE-798: Use of Hard-coded Credentials – This CWE category includes weaknesses where an application uses hard-coded credentials, which can be easily discovered and exploited by attackers.

• CWE-807: Reliance on Untrusted Inputs in a Security Decision – This CWE category includes weaknesses where an application relies on untrusted inputs in a security decision, which can lead to bypassing of security controls.

• CWE-918: Server-Side Request Forgery (SSRF) – This CWE category includes weaknesses where an attacker can manipulate the server-side code to send requests to internal systems, bypassing client-side controls.

• CWE-919: Use of Password Hash With Insufficient Computational Effort – This CWE category includes weaknesses where an application uses weak password hashing algorithms or does not use sufficient computational effort to hash passwords, which can lead to easy password cracking.

• CWE-943: Improper Neutralization of Special Elements in Data – This CWE category includes weaknesses where an application does not properly neutralize special characters or elements in user input, which can lead to injection attacks and bypassing of client-side controls.

• CWE-1048: Failure to Restrict Excessive Authentication Attempts – This CWE category includes weaknesses where an application does not properly restrict the number of authentication attempts, which can lead to brute force attacks and bypassing of client-side controls.

Top 10 CVES related to Client-side Validation Bypass

• CVE-2023-0581 – The PrivateContent plugin for WordPress is vulnerable to protection mechanism bypass due to the use of client side validation in versions up to, and including, 8.4.3. This is due to the plugin checking if an IP had been blocklist via client-side scripts rather than server-side. This makes it possible for unauthenticated attackers to bypass any login restrictions that may prevent a brute force attack.

• CVE-2021-0269 – The improper handling of client-side parameters in J-Web of Juniper Networks Junos OS allows an attacker to perform a number of different malicious actions against a target device when a user is authenticated to J-Web. An attacker may be able to supersede existing parameters, including hardcoded parameters within the HTTP/S session, access and exploit variables, bypass web application firewall rules or input validation mechanisms, and otherwise alter and modify J-Web’s normal behavior. An attacker may be able to transition victims to malicious web services, or exfiltrate sensitive information from otherwise secure web forms. 

• CVE-2020-24683 – The affected versions of S+ Operations (version 2.1 SP1 and earlier) used an approach for user authentication which relies on validation at the client node (client-side authentication). This is not as secure as having the server validate a client application before allowing a connection. Therefore, if the network communication or endpoints for these applications are not protected, unauthorized actors can bypass authentication and make unauthorized connections to the server application.

• CVE-2019-16327 – D-Link DIR-601 B1 2.00NA devices are vulnerable to authentication bypass. They do not check for authentication at the server side and rely on client-side validation, which is bypassable. NOTE: this is an end-of-life product.

• CVE-2014-0868 – RICOS in IBM Algo Credit Limits (aka ACLM) 4.5.0 through 4.7.0 before 4.7.0.03 FP5 in IBM Algorithmics relies on client-side input validation, which allows remote authenticated users to bypass intended dual-control restrictions and modify data via a crafted XML document, as demonstrated by manipulation of read-only limit data.

• CVE-2014-0865 – RICOS in IBM Algo Credit Limits (aka ACLM) 4.5.0 through 4.7.0 before 4.7.0.03 FP5 in IBM Algorithmics relies on client-side input validation, which allows remote authenticated users to bypass intended dual-control restrictions and modify data via crafted serialized objects, as demonstrated by limit manipulations.

• CVE-2013-1245 – The user-management page in Cisco WebEx Social relies on client-side validation of values in the Screen Name, First Name, Middle Name, Last Name, Email Address, and Job Title fields, which allows remote authenticated users to bypass intended access restrictions via crafted requests, aka Bug ID CSCue67190.

• CVE-2007-1331 – Multiple cross-site scripting (XSS) vulnerabilities in TKS Banking Solutions ePortfolio 1.0 Java allow remote attackers to inject arbitrary web script or HTML via unspecified vectors that bypass the client-side protection scheme, one of which may be the q parameter to the search program. NOTE: some of these details are obtained from third party information.

Client-side Validation Bypass exploits

  • SQL Injection: Attackers can bypass client-side validation and inject malicious SQL code into the server-side application. This can allow attackers to execute unauthorized SQL commands and potentially gain access to sensitive data.

  • Cross-site Scripting (XSS): Attackers can bypass client-side validation and inject malicious code into a web page, which can be executed by unsuspecting users. This can lead to various attacks such as stealing sensitive data, hijacking user sessions, and injecting malware.

  • CSRF (Cross-Site Request Forgery): Attackers can use client-side validation bypass to trick a user into performing a malicious action on a web application, such as transferring money or changing account settings. This can occur when the attacker crafts a malicious link or script that the user clicks on without realizing the consequences.

  • Path Traversal: Attackers can use client-side validation bypass to access files and directories on the server-side application that they are not authorized to access. This can be done by manipulating input fields or crafting malicious requests.

  • Parameter Tampering: Attackers can modify client-side parameters to bypass server-side validation and perform unauthorized actions on a web application. This can be done by changing values in hidden fields or manipulating URLs.

  • Authentication Bypass: Attackers can use client-side validation bypass to bypass the authentication mechanism of a web application, allowing them to gain unauthorized access to the application or sensitive data.

  • Clickjacking: Attackers can use client-side validation bypass to overlay a malicious web page on top of a legitimate web page. This can trick users into clicking on hidden buttons or links that perform unintended actions.

  • DOM-Based XSS: Attackers can use client-side validation bypass to inject malicious code into a web page’s Document Object Model (DOM). This can occur when the attacker is able to control part of the DOM, such as a URL parameter or form input field.

  • HTTP Header Injection: Attackers can use client-side validation bypass to inject malicious headers into an HTTP request, which can lead to various attacks such as injecting malware or performing unauthorized actions.

  • File Upload Vulnerabilities: Attackers can use client-side validation bypass to upload malicious files to a server-side application, potentially allowing them to execute unauthorized code on the server or access sensitive data.

Practicing in test for Client-side Validation Bypass

  1. Identify the input fields in the application where client-side validation is being performed.

  2. Manually test each input field by entering unexpected or invalid values. Try to bypass the validation and see if the application accepts the input.

  3. Use a web proxy tool like Burp Suite to intercept and modify HTTP requests and responses. This can allow you to manipulate input fields and see how the application responds.

  4. Use automated vulnerability scanning tools like OWASP ZAP or Acunetix to scan the application for common vulnerabilities, including Client-side Validation Bypass.

  5. Use browser plugins like Tamper Data or Cookie Editor to manipulate input fields and see how the application responds.

  6. Conduct penetration testing to simulate attacks and attempt to bypass client-side validation. This can help identify potential vulnerabilities and weaknesses in the application.

  7. Keep a detailed log of your testing activities and any vulnerabilities that you discover. This can help you prioritize and address the vulnerabilities that pose the greatest risk to the application.

For study Client-side Validation Bypass

OWASP Top Ten: The OWASP top 10 is a list of the most critical web application security risks, including Client-side Validation Bypass. The OWASP website provides detailed information on each risk and guidance on how to prevent or mitigate it.

Web Application Hacker’s Handbook: This book is a comprehensive guide to web application security testing and covers a wide range of topics, including Client-side Validation Bypass. It provides practical advice and real-world examples to help you better understand the concepts and techniques involved in testing for vulnerabilities.

Online courses: There are many online courses that cover web application security and Client-side Validation Bypass specifically. Platforms like Udemy, Coursera, and Pluralsight offer courses on these topics, and many of them are free or low cost.

Practice labs: Practice labs provide a safe environment for you to practice testing for vulnerabilities like Client-side Validation Bypass. Platforms like Hack the Box, TryHackMe, and PentesterLab offer virtual labs and challenges that allow you to test your skills and learn new techniques.

Security conferences and events: Attending security conferences and events can give you the opportunity to learn from experts in the field and network with other professionals. Events like Black Hat, DEF CON, and BSides often have talks and workshops on web application security and Client-side Validation Bypass.

Online forums and communities: Online forums and communities like Reddit’s r/netsec and Stack Exchange’s Information Security can provide a wealth of knowledge and resources on web application security and Client-side Validation Bypass. You can ask questions, share information, and learn from others in the community.

Books with review of Client-side Validation Bypass

The Web Application Hacker’s Handbook: Finding and Exploiting Security Flaws, by Dafydd Stuttard and Marcus Pinto – This comprehensive guide covers a wide range of web application security topics, including Client-side Validation Bypass. It provides practical guidance and real-world examples to help you better understand the concepts and techniques involved in testing for vulnerabilities.

Web Security Testing Cookbook: Systematic Techniques to Find Problems Fast, by Paco Hope and Ben Walther – This book provides a collection of practical recipes for testing web application security, including Client-side Validation Bypass. It covers a range of techniques and tools to help you identify and remediate vulnerabilities.

The Tangled Web: A Guide to Securing Modern Web Applications, by Michal Zalewski – This book provides an in-depth look at the complexities of modern web application security, including Client-side Validation Bypass. It covers a range of topics, from browser security to cross-site scripting and beyond.

XSS Attacks: Cross Site Scripting Exploits and Defense, by Seth Fogie, Jeremiah Grossman, and Robert Hansen – This book focuses specifically on cross-site scripting (XSS) attacks, which can be used to bypass client-side validation. It provides a detailed analysis of XSS vulnerabilities and strategies for defending against them.

Black Hat Python: Python Programming for Hackers and Pentesters, by Justin Seitz – This book provides a practical guide to using Python for penetration testing, including testing for Client-side Validation Bypass. It covers a range of topics, from network scanning to web application hacking.

Advanced Penetration Testing: Hacking the World’s Most Secure Networks, by Wil Allsopp – This book provides a comprehensive guide to advanced penetration testing techniques, including testing for Client-side Validation Bypass. It covers a range of topics, from network reconnaissance to post-exploitation.

Professional Penetration Testing: Creating and Operating a Formal Hacking Lab, by Thomas Wilhelm – This book provides a practical guide to setting up and running a penetration testing lab, including testing for Client-side Validation Bypass. It covers a range of topics, from reconnaissance to reporting.

Hacking Web Apps: Detecting and Preventing Web Application Security Problems, by Mike Shema – This book provides a practical guide to web application security, including Client-side Validation Bypass. It covers a range of topics, from identifying vulnerabilities to remediation strategies.

The Basics of Web Hacking: Tools and Techniques to Attack the Web, by Josh Pauli – This book provides an introduction to web application security, including Client-side Validation Bypass. It covers a range of topics, from reconnaissance to exploitation.

The Art of Exploitation, by Jon Erickson – This book provides a practical guide to computer security and exploitation techniques, including testing for Client-side Validation Bypass. It covers a range of topics, from buffer overflows to web application hacking.

List of payloads Client-side Validation Bypass

  1. Removing the “required” attribute from input fields to bypass mandatory validation.

  2. Modifying the JavaScript code that performs the validation to bypass specific checks.

  3. Using SQL injection payloads to bypass server-side validation and manipulate the data sent to the client.

  4. Using cross-site scripting (XSS) payloads to manipulate the data on the client-side and bypass validation.

  5. Modifying the HTTP requests sent by the client to bypass validation checks on the server-side.

  6. Manipulating cookies or local storage to bypass client-side validation.

  7. Changing the values of hidden form fields to bypass validation checks.

  8. Using tools like Burp Suite or OWASP ZAP to modify requests and responses to bypass validation checks.

  9. Crafting custom payloads to target specific types of validation checks and exploit their weaknesses.

  10. Using browser extensions or plugins to modify the behavior of the client-side validation checks.

How to be protected from Client-side Validation Bypass

  1. Perform server-side validation: Always perform validation on the server-side to ensure that the data received from the client is valid and secure. Do not rely on client-side validation alone.

  2. Use a Content Security Policy (CSP): A CSP helps to prevent XSS attacks by allowing you to specify which scripts are allowed to run on your website. This can help prevent attackers from injecting malicious code into your website.

  3. Implement input validation: Implement input validation on both the client-side and server-side to ensure that all data is validated before it is processed or stored.

  4. Use encryption: Use SSL or TLS to encrypt data in transit and protect it from interception or modification.

  5. Use anti-CSRF tokens: Anti-CSRF tokens can help prevent attackers from submitting malicious requests by requiring a valid token in each request.

  6. Keep software up to date: Keep your software, frameworks, and libraries up to date with the latest security patches and updates to ensure that known vulnerabilities are addressed.

  7. Perform regular security testing: Regularly perform security testing and penetration testing to identify vulnerabilities in your applications and systems.

  8. Train employees: Train your employees on how to identify and avoid potential security risks, such as clicking on suspicious links or downloading malicious files.

Conclusion 

Client-side Validation Bypass is a type of vulnerability that allows an attacker to bypass client-side validation checks and submit malicious input to a web application. This can lead to various types of attacks, including XSS, CSRF, and SQL injection.

To mitigate against Client-side Validation Bypass attacks, it is important to implement server-side validation, use a Content Security Policy (CSP), implement input validation on both the client-side and server-side, use encryption, use anti-CSRF tokens, use rate limiting, use intrusion detection and prevention systems (IDPS), keep software up to date, and perform regular security testing.

By following these measures, web developers and security professionals can help protect against Client-side Validation Bypass attacks and other types of security threats, and ensure the security and integrity of web applications.

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