Brian Barton
Prototype pollution is a bug that allows threat actors to inject properties into existing JavaScript construct prototypes, compromising the application. This vulnerability is called prototype pollution because it allows threat actors to inject values that overwrite or pollute the “prototype” of a base object, which can then be inherited by other objects. This can lead to several attacks, such as cross-site scripting (XSS) attacks, SQL injection (SQLi), and authorization and authentication bypass. Prototype pollution vulnerabilities are specific to JavaScript due to its particular management of objects, and it can occur on both the server and client sides. Exploiting prototype pollution vulnerabilities can be complex, and it typically requires a deep analysis of the application logic to determine the attack’s impact.
| Characteristics | Values |
|---|---|
| Type of bug | Prototype pollution is a bug that is not as well-documented as other major ones like SQL Injections (SQLI), Cross-Site Scripting (XSS), and Local File Inclusion (LFI). |
| Vulnerability | Prototype pollution is a vulnerability that enables threat actors to inject properties into existing JavaScript construct prototypes, compromising the application. |
| Impact | Prototype pollution can impact both the server and client sides. On the server side, it can lead to remote code execution (RCE), SQL injection (SQLi), and authorization and authentication bypass. On the client side, it can lead to Cross-Site Scripting (XSS) attacks and DOM XSS attacks. |
| Prevention | To prevent prototype pollution, it is recommended to have a codebase with good programming practices, including user input sanitization. Regularly update libraries with new patches and run npm audits to scan for vulnerabilities. Avoid recursive merges and use Object.freeze() to prevent changes to frozen objects. |
| Detection | To detect prototype pollution, open the browser console and inspect the prototype. For DOM XSS attacks, use tools like DOM Invader to test for vulnerabilities. |
| Exploitation | To exploit prototype pollution, threat actors inject payloads into inputs like URLs that build the client-side logic or application rendering. They can also exploit JavaScript code injections and bypass protections of HTML sanitizers. |
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What You'll Learn
- Prototype pollution is a bug that affects prototype-based languages like JavaScript
- Attackers inject malicious code into text fields, headers, and files
- Prototype pollution can lead to denial of service (DoS) or remote code execution (RCE)
- Exploiting prototype pollution vulnerabilities can allow attackers to steal data or execute malicious code
- Prototype pollution can be mitigated by using Object.freeze() to prevent the addition of new prototypes
Prototype pollution is a bug that affects prototype-based languages like JavaScript
JavaScript is a prototype-based object-oriented programming language. Each object is linked to a “prototype”. When we invoke the toString method on an object, JavaScript will first check to see if we explicitly defined the method for the given object. If not, it will look for its definition in the object’s prototype. In JavaScript, objects inherit properties and behaviours from a prototype. Each object has a prototype, and if a property or method is not found on the object itself, JavaScript looks for it in its prototype, and then recursively up the prototype chain until it reaches the top-level Object.prototype.
The vulnerability of prototype pollution arises from the dynamic nature of JavaScript’s prototype-based inheritance model. If an attacker can modify the prototype of a commonly used object, they can potentially introduce malicious properties or methods that affect all objects derived from that prototype. By polluting the prototype, attackers can manipulate the behaviour of JavaScript programs. Prototype pollution gadgets are the other half of the vulnerability – the ‘Sink’. They are a common artefact of the way JavaScript is written and can be used to exploit any pollution issues identified.
To prevent prototype pollution, it is important to have a codebase with good programming practices. User input sanitization is key as the vulnerability mainly comes from the client side. Updating libraries with new patches while also running npm audits regularly to scan your project for vulnerabilities is a must. Avoiding recursive merges and using the Object.freeze() method are also recommended, as a frozen object can no longer be changed and therefore won’t be able to have properties created or removed.
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Attackers inject malicious code into text fields, headers, and files
Prototype pollution is a bug that is not as well-documented as other major vulnerabilities such as SQL injections, Cross-Site Scripting (XSS), and Local File Inclusion (LFI). It specifically affects prototype-based languages like JavaScript, where objects carry over the properties and methods of the prototype "object".
This vulnerability allows attackers to inject malicious code into text fields, headers, and files. For example, in a URL parser, attackers can assign JavaScript objects properties without verifying if the target property is linked correctly to the Object prototype. This is known as client-side exploitation, which can result in several attacks, including XSS attacks. By targeting a gadget that relies on the property of an object susceptible to pollution, attackers can trigger client-side JavaScript code execution.
Server-side exploitation occurs when attackers modify the Object prototype's properties using gadgets located in the application context. This leads to the execution of modified JavaScript runtime objects on the server, resulting in more severe consequences than client-side attacks. Server-side exploitation can enable remote code execution (RCE), SQL injection (SQLi), and authorization and authentication bypass.
To protect against prototype pollution, it is crucial to have a codebase with good programming practices. User input sanitization is essential as the vulnerability mainly comes from the client side. Regularly updating libraries with new patches and running npm audits to scan for vulnerabilities are also recommended. Additionally, avoiding recursive merges and using the Object.freeze() method can reduce the risk of prototype pollution.
By understanding the principles of prototype pollution vulnerabilities and implementing countermeasures, we can defend against attackers who inject malicious code into text fields, headers, and files.
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Prototype pollution can lead to denial of service (DoS) or remote code execution (RCE)
Prototype pollution is a bug that is not as well-documented as other major vulnerabilities such as SQL injections (SQLi) and Cross-Site Scripting (XSS). It is a dangerous vulnerability that affects prototype-based languages like JavaScript and the Node.js platform. It occurs when threat actors or attackers inject properties into existing JavaScript construct prototypes or an object's root prototype at runtime, compromising the application. This is made possible by the ability to inject values that overwrite or pollute the "prototype" of a base object. This malicious prototype can then be passed on to many other objects that inherit that prototype.
Once the default values of an object's properties are controlled, the application's logic can be tampered with. This can lead to Denial of Service (DoS) attacks, where a targeted user or web server that hosts the application is affected. Prototype pollution can also enable interference with the JavaScript environment, adding or modifying properties to cause unexpected behaviours and prevent some logic from executing as intended.
However, prototype pollution can have even more severe consequences, including Remote Code Execution (RCE). While there is anecdotal evidence of this, current research has not yet fully explored the challenge of gadget detection, only showing the feasibility of DoS attacks. Nevertheless, prototype pollution in combination with powerful universal gadgets has been shown to lead to RCE in Node.js applications. By exploiting prototype pollution vulnerabilities, attackers can escalate their privileges, gain access to sensitive information, and execute malicious code or steal data.
To mitigate prototype pollution vulnerabilities, several measures can be implemented. These include using Object.freeze() to prevent changes to an object's attributes, creating objects without prototype properties (e.g., Object.create(null)), avoiding recursive merges, and regularly updating libraries with patches and audits. Additionally, open-source libraries should be carefully vetted to ensure they are not exposed to these weaknesses.
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Exploiting prototype pollution vulnerabilities can allow attackers to steal data or execute malicious code
Prototype pollution is a bug or vulnerability that affects prototype-based languages, most notably JavaScript. It involves the injection of properties into a JavaScript object's prototypes, allowing attackers to manipulate the behaviour of JavaScript programs and compromise the application. This can lead to several security issues, including data theft, code execution, and denial-of-service (DoS) attacks.
By exploiting prototype pollution vulnerabilities, attackers can gain unauthorised access to sensitive information and perform complex attacks. They can inject malicious payloads into inputs such as URLs, taking advantage of client-side logic or application rendering. For example, a URL parser can assign JavaScript objects properties without verifying if the target property is correctly linked to the Object prototype. This can result in cross-site scripting (XSS) attacks, where attackers trigger client-side JavaScript code execution by exploiting the page's document object model (DOM).
Server-side exploitation of prototype pollution vulnerabilities can have more severe consequences. Attackers can modify the Object prototype's properties using gadgets located in the application context, leading to remote code execution (RCE), SQL injection (SQLi), and authorisation and authentication bypass. It requires a deep analysis of the application logic to determine the impact of the attack and exploit prototype pollution vulnerabilities effectively.
To prevent and mitigate prototype pollution attacks, developers should follow secure coding practices. This includes input validation, escaping user input, sanitising data, and conducting strict code reviews. Additionally, developers should use trusted third-party libraries and ensure proper input sanitisation to avoid vulnerabilities that mainly come from the client side. Avoiding recursive merges and utilising the Object.freeze() method can also reduce the risk of prototype pollution.
Overall, by understanding the mechanisms and impacts of prototype pollution, developers can implement effective defences to protect their applications and prevent data theft, code execution, and other malicious activities.
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Prototype pollution can be mitigated by using Object.freeze() to prevent the addition of new prototypes
Prototype pollution is a bug that is not as well-documented as other major vulnerabilities like SQL Injections (SQLI) and Cross-Site Scripting (XSS). It involves injecting properties into a JavaScript object's prototypes, compromising the application. This vulnerability allows attackers to execute malicious code, tamper with application logic, or steal data.
To mitigate prototype pollution, one effective approach is to use the Object.freeze() method. By invoking Object.freeze() on an object, its properties and values become immutable, preventing any modifications or additions. Since prototypes are objects, applying Object.freeze(Object.prototype) freezes the default prototype, blocking attempts to pollute it. This proactive measure ensures that even if an attacker identifies a prototype pollution vulnerability, it becomes unexploitable.
While Object.freeze() is a robust solution, it may introduce hard-to-identify bugs due to its silent failure behavior when assigning properties. As a compromise, the Object.seal() method allows changes to existing property values while blocking new properties. Additionally, sanitizing property keys before merging them into objects can prevent injecting keys like "__proto__" that reference the object's prototype. An allowlist of permitted keys is ideal, but a blocklist to remove dangerous strings from user input is more commonly used despite its limitations.
Another strategy is to avoid prototypes altogether by using Object.create(null) or creating objects without prototype properties. This ensures that objects do not inherit any properties and are less susceptible to prototype pollution. Using a Map or Set instead of an object provides built-in protection, although they can still inherit malicious properties.
In summary, prototype pollution is a serious vulnerability that can be mitigated using Object.freeze() to prevent changes to prototypes. However, it is important to consider potential trade-offs and explore alternative approaches like object creation without prototypes, sanitizing property keys, and using Maps or Sets for added security.
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Frequently asked questions
Prototype pollution is a bug or vulnerability that affects prototype-based languages, most notably JavaScript. It involves the injection of properties into a JavaScript object's prototypes, allowing threat actors to manipulate the application and potentially gain access to sensitive information or execute malicious code.
Prototype pollution occurs when an attacker modifies the prototype of an object, impacting all other objects that inherit from that prototype. This is often achieved by injecting a payload into an input, such as a URL, that builds the client-side logic or application rendering.
Prototype pollution can lead to several attacks, including cross-site scripting (XSS), SQL injection (SQLi), remote code execution (RCE), and authorization and authentication bypass. It can also be used to escalate privileges on a platform or bypass protections of HTML sanitizers.
To identify prototype pollution vulnerabilities, you can use tools such as DOM Invader, which is a browser-based tool for testing DOM XSS vulnerabilities. You can also scan your project for vulnerabilities using npm audits and ensure that you are using updated libraries with the latest security patches.
To protect against prototype pollution, it is recommended to follow good programming practices, including user input sanitization. Additionally, avoid recursive merges, use Object.freeze() to prevent the addition of new prototypes, and implement security checks to avoid overwriting to "__proto__" or other special properties.
