03 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 method of circumventing the validation checks that are performed by web applications on the client-side, such as in the user’s web browser, without triggering any alerts or error messages. This is typically done by manipulating or modifying the data that is sent from the client-side to the server-side, in order to bypass or evade the validation checks that are in place.

Some common techniques used to bypass client-side validation include modifying HTML code, using JavaScript to alter form data, and using intercepting proxies to intercept and modify HTTP requests. These techniques can be used by attackers to bypass security controls and gain unauthorized access to sensitive data or resources, or to carry out other malicious actions.

Example of vulnerable code on different programming languages:


in JavaScript:

				
					var password = document.getElementById('password').value;
if (password.length < 8) {
    alert('Password must be at least 8 characters long');
    return false;
}

				
			

 

In this example, the JavaScript code is checking whether the user’s password is at least 8 characters long. However, an attacker can easily bypass this check by modifying the HTML code or using browser developer tools to change the value of the password field.

To prevent this, the server-side code should perform additional validation checks to ensure that the password meets the required complexity requirements.

• in PHP:

				
					$password = $_POST['password'];
if (strlen($password) < 8) {
    die('Password must be at least 8 characters long');
}

				
			

 

In this PHP code, the password length is checked before it is processed by the server-side application. However, an attacker can bypass this check by modifying the POST request data before it is sent to the server.

To prevent this, the server-side code should perform additional validation checks to ensure that the password meets the required complexity requirements and sanitize user input to prevent injection attacks.

• in Python:

				
					password = request.form.get('password')
if len(password) < 8:
    return 'Password must be at least 8 characters long'

				
			

 

In this Python code, the password length is checked before it is processed by the server-side application. However, an attacker can bypass this check by modifying the form data before it is sent to the server.

To prevent this, the server-side code should perform additional validation checks to ensure that the password meets the required complexity requirements and sanitize user input to prevent injection attacks.

Examples of exploitation Client-side Validation Bypass

Bypassing length checks:

An attacker can bypass length checks on a client-side validation by modifying the HTML or JavaScript code. For example, an attacker can use browser developer tools to modify the length check function to always return true, allowing them to submit shorter passwords or other input than what is required by the server-side application.

Bypassing input validation:

An attacker can bypass input validation checks on a client-side by submitting malicious input data to the server, without triggering any alerts or error messages. For example, an attacker can use intercepting proxies to intercept and modify HTTP requests, changing the input data to contain malicious code or SQL injection attacks.

Bypassing data format checks:

An attacker can bypass data format checks on a client-side by manipulating the input data to conform to the expected format. For example, an attacker can submit a fake email address or phone number that passes the client-side validation checks but is not valid in the real world.

Bypassing security controls:

An attacker can bypass security controls on a client-side by manipulating or modifying the input data or the application behavior in ways that were not intended by the developers. For example, an attacker can modify the client-side code to disable security controls or to enable debug mode, allowing them to gain access to sensitive information or resources.

Privilege escalation techniques for Client-side Validation Bypass

Injection attacks:

One common technique for privilege escalation is injection attacks, which involve injecting malicious code or data into the server-side application to gain unauthorized access or to escalate privileges. For example, an attacker can use SQL injection to modify the database queries or to bypass authentication checks, allowing them to access sensitive data or to gain administrative access.

Session hijacking:

Another technique for privilege escalation is session hijacking, which involves stealing a user’s session ID or token to gain access to the user’s account or to escalate privileges. For example, an attacker can use Cross-site scripting (XSS) attacks to steal a user’s session token or cookie, allowing them to impersonate the user and to perform actions on their behalf.

Code injection:

An attacker can also use code injection techniques to escalate privileges by injecting malicious code into the server-side application. For example, an attacker can use Remote Code Execution (RCE) attacks to execute arbitrary code on the server, allowing them to take control of the application or to gain administrative access.

Parameter tampering:

Parameter tampering involves modifying the input parameters or data that are sent to the server-side application to bypass validation checks or to escalate privileges. For example, an attacker can modify the parameters of an HTTP request to bypass authentication checks or to execute unauthorized actions.

General methodology and checklist for Client-side Validation Bypass

Methodology:

  1. Identify input validation mechanisms: The first step is to identify the input validation mechanisms used by the application, including client-side validation checks, server-side validation checks, and other security controls.

  2. Analyze client-side code: The next step is to analyze the client-side code, including HTML, CSS, and JavaScript, to identify potential vulnerabilities, such as length checks, data format checks, and other types of input validation.

  3. Manipulate client-side input: The next step is to manipulate the client-side input to bypass validation checks and submit invalid or malicious data to the server-side application. This can include modifying HTML and JavaScript code, using browser developer tools, or intercepting HTTP requests.

  4. Monitor server-side responses: The next step is to monitor the server-side responses to identify any unexpected or unauthorized behavior, such as bypassing authentication checks, accessing unauthorized data or resources, or executing unauthorized actions.

  5. Exploit vulnerabilities: Finally, if vulnerabilities are identified, the tester can attempt to exploit them to gain unauthorized access or to escalate privileges within the system or application.

Checklist:

  1. Identify all input validation mechanisms used by the application, including client-side validation checks, server-side validation checks, and other security controls.

  2. Analyze the client-side code, including HTML, CSS, and JavaScript, to identify potential vulnerabilities, such as length checks, data format checks, and other types of input validation.

  3. Manipulate the client-side input to bypass validation checks and submit invalid or malicious data to the server-side application. This can include modifying HTML and JavaScript code, using browser developer tools, or intercepting HTTP requests.

  4. Test for injection attacks, such as SQL injection, Cross-site scripting (XSS), or Remote Code Execution (RCE), by injecting malicious code or data into the server-side application.

  5. Test for parameter tampering by modifying the parameters of an HTTP request to bypass authentication checks or to execute unauthorized actions.

  6. Test for session hijacking by stealing a user’s session ID or token to gain access to the user’s account or to escalate privileges.

  7. Test for security controls, such as rate limiting, CAPTCHAs, or other mechanisms used to prevent automated attacks.

  8. Test for data validation by submitting different types of data, including invalid data, to ensure that the server-side application is properly validating all input data.

  9. Record all findings, including any vulnerabilities identified, potential security risks, and recommendations for remediation.

  10. Retest and verify that all identified vulnerabilities have been properly remediated before the application is deployed in a production environment.

Tools set for exploiting Client-side Validation Bypass

Manual Tools:

  • Burp Suite – Burp Suite is a web application security testing tool that can be used to intercept and modify HTTP requests and responses. It includes a proxy server that allows testers to intercept and manipulate client-side input to bypass validation checks.

  • Postman – Postman is a tool for API development that allows testers to send HTTP requests to web services and inspect the responses. It can be used to test for client-side validation bypass by sending requests with invalid or malicious data.

  • Fiddler – Fiddler is a web debugging proxy that can be used to intercept and modify HTTP traffic. It can be used to test for client-side validation bypass by manipulating client-side input and observing the resulting server-side responses.

  • OWASP ZAP – OWASP ZAP is an open-source web application security testing tool that can be used to scan web applications for vulnerabilities, including client-side validation bypass. It includes a proxy server that allows testers to intercept and modify HTTP traffic.

  • Chrome Developer Tools – Chrome Developer Tools is a set of web development and debugging tools built into the Chrome browser. It includes a network monitor that can be used to intercept and modify HTTP traffic, as well as a JavaScript console that can be used to modify client-side code.

  • Firefox Developer Tools – Firefox Developer Tools is a set of web development and debugging tools built into the Firefox browser. It includes a network monitor that can be used to intercept and modify HTTP traffic, as well as a JavaScript console that can be used to modify client-side code.

  • Wireshark – Wireshark is a network protocol analyzer that can be used to capture and analyze network traffic. It can be used to intercept and analyze HTTP traffic to identify vulnerabilities, including client-side validation bypass.

  • Manual Code Review – Manual code review involves analyzing the application’s source code to identify potential vulnerabilities, including client-side validation bypass. It requires a deep understanding of programming languages and web development best practices.

Automated Tools:

  • Nikto – Nikto is a web application scanner that can be used to scan web applications for vulnerabilities, including client-side validation bypass. It includes a set of pre-defined tests that can be run against the application to identify potential vulnerabilities.

  • Arachni – Arachni is an open-source web application scanner that can be used to scan web applications for vulnerabilities, including client-side validation bypass. It includes a set of pre-defined tests and can be configured to perform custom tests.

  • Wapiti – Wapiti is a web application scanner that can be used to scan web applications for vulnerabilities, including client-side validation bypass. It includes a set of pre-defined tests and can be configured to perform custom tests.

  • Skipfish – Skipfish is a web application scanner that can be used to scan web applications for vulnerabilities, including client-side validation bypass. It includes a set of pre-defined tests and can be configured to perform custom tests.

  • OWASP Dependency-Check – OWASP Dependency-Check is a tool that can be used to scan web applications for known vulnerabilities in third-party dependencies. It can be used to identify vulnerabilities in client-side JavaScript libraries.

  • Retire.js – Retire.js is a tool that can be used to scan web applications for known vulnerabilities in client-side JavaScript libraries. It can be used to identify vulnerabilities in third-party libraries used by the application.

  • SonarQube – SonarQube is a code analysis tool that can be used to scan web applications for vulnerabilities, including client-side validation bypass. It includes

The Common Weakness Enumeration (CWE)

• CWE-20: Improper Input Validation – This CWE is related to the lack of proper input validation on the client-side, which can allow attackers to submit malicious input to bypass validation checks.

• CWE-74: Improper Neutralization of Special Elements in Output Used by a Downstream Component – This CWE is related to the lack of proper output encoding on the client-side, which can allow attackers to inject malicious code into the output.

• CWE-79: Improper Neutralization of Input During Web Page Generation – This CWE is related to the lack of proper input validation on the client-side, which can allow attackers to submit malicious input to bypass validation checks and inject malicious code.

• CWE-94: Improper Control of Generation of Code (‘Code Injection’) – This CWE is related to the lack of proper input validation on the client-side, which can allow attackers to inject malicious code into the application.

• CWE-98: Improper Control of Filename for Include/Require Statement in PHP Program – This CWE is related to the lack of proper input validation on the client-side, which can allow attackers to inject malicious code into the application.

• CWE-116: Improper Encoding or Escaping of Output – This CWE is related to the lack of proper output encoding on the client-side, which can allow attackers to inject malicious code into the output.

• CWE-602: Client-Side Enforcement of Server-Side Security – This CWE is related to the reliance on client-side validation to enforce server-side security, which can allow attackers to bypass validation checks.

• CWE-754: Improper Check for Unusual or Exceptional Conditions – This CWE is related to the lack of proper handling of unusual or exceptional conditions on the client-side, which can allow attackers to bypass validation checks.

• CWE-807: Reliance on Untrusted Inputs in a Security Decision – This CWE is related to the reliance on untrusted inputs on the client-side to make security decisions, which can allow attackers to bypass validation checks and inject malicious code.

• CWE-918: Server-Side Request Forgery (SSRF) – This CWE is related to the ability of attackers to manipulate client-side input to bypass validation checks and send requests to the server that may result in SSRF attacks.

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. This issue affects: Juniper Networks Junos OS: All versions prior to 17.4R3-S3; 18.1 versions prior to 18.1R3-S12; 18.2 versions prior to 18.2R3-S6; 18.3 versions prior to 18.3R3-S4; 18.4 versions prior to 18.4R3-S6; 19.1 versions prior to 19.1R3-S4; 19.2 versions prior to 19.2R3-S1; 19.3 versions prior to 19.3R3-S1; 19.4 versions prior to 19.4R2-S2, 19.4R3; 20.1 versions prior to 20.1R2; 20.2 versions prior to 20.2R2.

• 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: An attacker can manipulate the client-side validation to inject malicious SQL code into the server, which can lead to unauthorized access to sensitive data.

  • Cross-site scripting (XSS): An attacker can bypass client-side validation to inject malicious scripts into the client-side code, which can steal user data or hijack user sessions.

  • Cross-site request forgery (CSRF): An attacker can bypass client-side validation to submit unauthorized requests to the server, which can lead to unauthorized access to sensitive data or actions.

  • Broken authentication and session management: An attacker can bypass client-side validation to hijack user sessions or manipulate authentication tokens, which can allow them to gain unauthorized access to sensitive data or perform unauthorized actions.

  • File inclusion attacks: An attacker can bypass client-side validation to inject malicious code into file inclusion statements, which can lead to unauthorized access to sensitive data or execution of malicious code.

  • XML External Entity (XXE) injection: An attacker can bypass client-side validation to inject malicious XML entities, which can lead to unauthorized access to sensitive data or denial of service attacks.

  • Open redirect attacks: An attacker can bypass client-side validation to manipulate redirect URLs, which can lead to phishing attacks or unauthorized access to sensitive data.

  • Path traversal attacks: An attacker can bypass client-side validation to manipulate file paths, which can lead to unauthorized access to sensitive data or execution of malicious code.

  • Command injection attacks: An attacker can bypass client-side validation to inject malicious commands, which can lead to execution of malicious code on the server or unauthorized access to sensitive data.

  • Server-side request forgery (SSRF) attacks: An attacker can bypass client-side validation to manipulate server-side requests, which can lead to unauthorized access to sensitive data or attacks on internal systems.

Practicing in test for Client-side Validation Bypass

Identify the input fields and validation checks: First, identify all the input fields on the web application and the validation checks that are in place for each field.

Manipulate the input fields: Try manipulating the input fields by entering values that are not allowed by the validation checks. For example, if a text field only allows letters and numbers, try entering special characters or SQL code.

Observe the application’s response: Observe how the application responds to the manipulated input. If the application accepts the input, it might be vulnerable to client-side validation bypass.

Try bypassing the client-side validation: Once you have identified a vulnerable input field, try bypassing the client-side validation by using tools like Burp Suite, OWASP ZAP, or other similar tools.

Submit the manipulated input: Submit the manipulated input to the server and observe the response. If the server accepts the input and the application behaves unexpectedly, it might be vulnerable to a client-side validation bypass attack.

Test for other vulnerabilities: Once you have identified a client-side validation bypass vulnerability, test for other related vulnerabilities such as SQL injection, cross-site scripting (XSS), and others.

Report the findings: If you find any vulnerabilities, report them to the application owners or developers and provide them with details on how to reproduce the issue and steps to mitigate the vulnerability.

Keep practicing: Client-side validation bypass is a common vulnerability, so it’s important to keep practicing and staying up-to-date with new tools and techniques to detect and exploit these vulnerabilities. Attend conferences, participate in CTFs, and collaborate with other security professionals to continue learning and improving your skills.

For study Client-side Validation Bypass

OWASP Top 10: This is a list of the top 10 web application security risks, which includes client-side validation bypass. The OWASP website provides detailed information on each risk, including how to detect and prevent them.

Web Application Hacker’s Handbook: This book provides in-depth coverage of web application security, including client-side validation bypass. It covers a wide range of topics, from basic web application concepts to advanced attacks and techniques.

Online courses: There are several online courses available that cover web application security and client-side validation bypass, such as “Web Application Penetration Testing” on Udemy and “Web Application Security” on Coursera.

Practice labs: Several online platforms provide practice labs for web application security testing, such as TryHackMe, HackTheBox, and PentesterLab. These platforms offer real-world scenarios to practice testing and exploiting vulnerabilities, including client-side validation bypass.

Capture the Flag (CTF) events: CTFs are competitions that test participants’ skills in finding and exploiting vulnerabilities in web applications. There are several CTF events focused on web application security, such as OWASP Global AppSec and Google CTF.

Open-source tools: There are several open-source tools available for testing and exploiting client-side validation bypass vulnerabilities, such as Burp Suite, OWASP ZAP, and SQLMap. These tools can help you learn and practice techniques for detecting and exploiting vulnerabilities.

Books with review of Client-side Validation Bypass

The Web Application Hacker’s Handbook: Finding and Exploiting Security Flaws, 2nd Edition by Dafydd Stuttard and Marcus Pinto – This book provides a comprehensive guide to web application security, including techniques for bypassing client-side validation.

Black Hat Python: Python Programming for Hackers and Pentesters by Justin Seitz – This book covers Python programming techniques for hacking and penetration testing, including client-side validation bypass.

The Tangled Web: A Guide to Securing Modern Web Applications by Michal Zalewski – This book covers a wide range of web application security topics, including client-side validation bypass, cross-site scripting (XSS), and more.

Breaking Into Information Security: Learning the Ropes 101 by Josh More and Anthony Stieber – This book provides an introduction to information security and covers topics such as client-side validation bypass and other web application vulnerabilities.

Real-World Bug Hunting: A Field Guide to Web Hacking by Peter Yaworski – This book provides a practical guide to web application security testing, including client-side validation bypass and other common vulnerabilities.

Hacker’s Playbook 3: Practical Guide to Penetration Testing by Peter Kim – This book provides a comprehensive guide to penetration testing, including techniques for bypassing client-side validation and exploiting other web application vulnerabilities.

The Basics of Hacking and Penetration Testing: Ethical Hacking and Penetration Testing Made Easy by Patrick Engebretson – This book provides an introduction to ethical hacking and penetration testing, including techniques for finding and exploiting vulnerabilities such as client-side validation bypass.

Penetration Testing: A Hands-On Introduction to Hacking by Georgia Weidman – This book provides an introduction to penetration testing, including techniques for finding and exploiting vulnerabilities in web applications.

Hands-On Web Penetration Testing with Metasploit: Build dynamic, automated, and stealthy attacks by Sunil Gupta – This book provides a practical guide to using Metasploit for web application penetration testing, including techniques for bypassing client-side validation.

Mastering Modern Web Penetration Testing: Build advanced skills in detecting and exploiting vulnerabilities in modern web applications by Prakhar Prasad – This book provides a comprehensive guide to modern web application penetration testing, including client-side validation bypass and other advanced techniques.

List of payloads Client-side Validation Bypass

  1. Injection of special characters like semicolons, single or double quotes, backslashes, etc.

  2. Use of Unicode characters, such as Unicode homoglyphs or mixing full-width and half-width characters.

  3. Bypassing input length limits by using long strings, special characters, or null bytes.

  4. Using alternative encoding schemes like base64 or URL encoding to obfuscate payloads.

  5. Modifying JavaScript code to bypass validation or execute arbitrary code.

  6. Disabling client-side validation by manipulating browser developer tools or browser extensions.

  7. Modifying the HTML code of the page to bypass validation or inject malicious scripts.

  8. Using browser plugins or add-ons that modify the behavior of form submissions.

  9. Submitting data directly to the server using custom scripts or command-line tools.

  10. Leveraging automation tools like Burp Suite, OWASP ZAP, or Selenium to manipulate form submissions and bypass validation.

How to be protected from Client-side Validation Bypass

  1. Implement server-side validation: Client-side validation is a necessary measure, but it should be complemented with server-side validation. Server-side validation is more secure since it is not dependent on the client and can provide more comprehensive checks.

  2. Use frameworks and libraries that provide built-in validation: Many modern web frameworks and libraries come with built-in validation that can be used to implement robust validation checks for input data.

  3. Implement strict input sanitization: Ensure that input data is sanitized to prevent the injection of malicious code or special characters that can bypass validation checks.

  4. Use Content Security Policy (CSP): CSP is a security mechanism that allows website owners to specify which types of content are allowed to be loaded on their website. This can help prevent injection of malicious scripts and code.

  5. Stay up-to-date with security patches and updates: Keep your web application and any third-party libraries up-to-date with the latest security patches and updates.

  6. Implement access controls: Access controls can help prevent unauthorized access to sensitive parts of the application and help protect against attacks that bypass validation.

  7. Educate users: Educate users on the importance of not manipulating form submissions, using trusted sources for input, and avoiding suspicious links or attachments.

Conclusion

Client-side validation is an important aspect of web application security, but it should not be relied on as the sole method of preventing malicious input. Client-side validation bypass attacks can occur when attackers manipulate or bypass the validation checks performed by the client-side script.

To prevent client-side validation bypass attacks, organizations should implement server-side validation, use secure coding frameworks, sanitize input, use regular expressions, implement Content Security Policy (CSP), implement access controls, and perform regular security testing.

It’s also important to note that client-side validation bypass is just one of many web application vulnerabilities that organizations need to protect against. Maintaining a robust security posture, staying up-to-date with security patches and updates, and educating users are all essential elements of a comprehensive web application security strategy.

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