dom based cross site scripting prevention

Get the latest content on web security in your inbox each week. In those cases, create a Trusted Type object yourself. This is commonly associated with normal XSS, but it can also lead to reflected DOM XSS vulnerabilities. It is difficult to detect DOM-based cross-site scripting because very often it leaves no mark on the server at all (for example, in server logs) the whole attack happens in the client. It is a simple yet effective way to harvest passwords using only the victims browser. How to prevent DOM-based cross-site scripting? Encoding libraries often have a EncodeForJavaScript or similar to support this function. Because JavaScript is based on an international standard (ECMAScript), JavaScript encoding enables the support of international characters in programming constructs and variables in addition to alternate string representations (string escapes). There are many different output encoding methods because browsers parse HTML, JS, URLs, and CSS differently. document.CreateTextNode () and append it in the appropriate DOM location. It is particularly common when applications leverage common JavaScript function calls such as document.baseURI to build a part of the page without sanitization. Use the default policy sparingly, and prefer refactoring the application to use regular policies instead. It is always a bad idea to use a user-controlled input in dangerous sources such as eval. Information on ordering, pricing, and more. Learn more about types of cross-site scripting attacks The styling will not be rendered. When your application no longer produces violations, you can start enforcing Trusted Types: Voila! This means you will need to use alternative elements like img or iframe. Start with using your frameworks default output encoding protection when you wish to display data as the user typed it in. More recent versions of jQuery have patched this particular vulnerability by preventing you from injecting HTML into a selector when the input begins with a hash character (#). Those are Safe Sinks as long as the attribute name is hardcoded and innocuous, like id or class. The complication is compounded by the differing meanings and treatment of encoded values within each subcontext (HTML, HTML attribute, URL, and CSS) within the execution context. For XSS attacks to be successful, an attacker needs to insert and execute malicious content in a webpage. Get started with Burp Suite Enterprise Edition. *Encoder.Default then the default, Basic Latin only safelist will be used. Examples of safe attributes includes: align, alink, alt, bgcolor, border, cellpadding, cellspacing, class, color, cols, colspan, coords, dir, face, height, hspace, ismap, lang, marginheight, marginwidth, multiple, nohref, noresize, noshade, nowrap, ref, rel, rev, rows, rowspan, scrolling, shape, span, summary, tabindex, title, usemap, valign, value, vlink, vspace, width. XSS vulnerabilities generally occur when an application takes user input and outputs it to a page without validating, encoding or escaping it. Identifying and exploiting DOM XSS in the wild can be a tedious process, often requiring you to manually trawl through complex, minified JavaScript. Its critical to use quotation marks like " or ' to surround your variables. To test for DOM-based cross-site scripting manually, you generally need to use a browser with developer tools, such as Chrome. This is the appropriate step to take when outputting data in a rendering context, however using HTML Attribute encoding in an execution context will break the application display of data. Please refer to the list below for details. Ensuring that all variables go through validation and are then escaped or sanitized is known as perfect injection resistance. To signify that the data was securely processed, create a special object - a Trusted Type.DoanElement.innerHTML = aTrustedHTML; With Trusted Types enabled, the browser accepts a TrustedHTML object for sinks that expect HTML snippets. XSS is one of the most common and dangerous web vulnerabilities, and it is . It is, therefore, the application developers' responsibility to implement code-level protection against DOM-based XSS attacks. To prevent DOM-based cross-site scripting, sanitize all untrusted data, even if it is only used in client-side scripts. The third cross site scripting attack occurs entirely in the browser. The most fundamental safe way to populate the DOM with untrusted data is to use the safe assignment property textContent. RULE #1 - HTML Escape then JavaScript Escape Before Inserting Untrusted Data into HTML Subcontext within the Execution Context, RULE #2 - JavaScript Escape Before Inserting Untrusted Data into HTML Attribute Subcontext within the Execution Context, RULE #3 - Be Careful when Inserting Untrusted Data into the Event Handler and JavaScript code Subcontexts within an Execution Context, RULE #4 - JavaScript Escape Before Inserting Untrusted Data into the CSS Attribute Subcontext within the Execution Context, RULE #5 - URL Escape then JavaScript Escape Before Inserting Untrusted Data into URL Attribute Subcontext within the Execution Context, RULE #6 - Populate the DOM using safe JavaScript functions or properties, RULE #7 - Fixing DOM Cross-site Scripting Vulnerabilities, Guidelines for Developing Secure Applications Utilizing JavaScript, GUIDELINE #1 - Untrusted data should only be treated as displayable text, GUIDELINE #2 - Always JavaScript encode and delimit untrusted data as quoted strings when entering the application when building templated JavaScript, GUIDELINE #3 - Use document.createElement(""), element.setAttribute("","value"), element.appendChild() and similar to build dynamic interfaces, GUIDELINE #4 - Avoid sending untrusted data into HTML rendering methods, GUIDELINE #5 - Avoid the numerous methods which implicitly eval() data passed to it, Utilizing an Enclosure (as suggested by Gaz), GUIDELINE #6 - Use untrusted data on only the right side of an expression, GUIDELINE #7 - When URL encoding in DOM be aware of character set issues, GUIDELINE #8 - Limit access to object properties when using object[x] accessors, GUIDELINE #9 - Run your JavaScript in a ECMAScript 5 canopy or sandbox, GUIDELINE #10 - Don't eval() JSON to convert it to native JavaScript objects, Common Problems Associated with Mitigating DOM Based XSS, Insecure Direct Object Reference Prevention, Creative Commons Attribution 3.0 Unported License. In the above example, untrusted data started in the rendering URL context (href attribute of an a tag) then changed to a JavaScript execution context (javascript: protocol handler) which passed the untrusted data to an execution URL subcontext (window.location of myFunction). If you can, entirely avoid using user input, especially if it affects DOM elements such as the document.url, the document.location, or the document.referrer. Make sure that any untrusted data passed to these methods is: Ensure to follow step 3 above to make sure that the untrusted data is not sent to dangerous methods within the custom function or handle it by adding an extra layer of encoding. This behavior was often implemented using a vulnerable hashchange event handler, similar to the following: As the hash is user controllable, an attacker could use this to inject an XSS vector into the $() selector sink. //The following does NOT work because of the encoded "(" and ")". For example if you want to use user input to write in a div tag element don't use innerHtml, instead use innerText or textContent. For example. If you use Burp's browser, however, you can take advantage of its built-in DOM Invader extension, which does a lot of the hard work for you. It is important to use an encoding library that understands which characters can be used to exploit vulnerabilities in their respective contexts. Otherwise, again, your security efforts are void. Validate all data that flows into your application from the server or a third-party API. The primary rule that you must follow to prevent DOM XSS is: sanitize all untrusted data, even if it is only used in client-side scripts. Acunetix developers and tech agents regularly contribute to the blog. XSS sinks are places where variables are placed into your webpage. DOM-based attack Reflected XSS Attacks The simplest type of XSS attack is where the application immediately processes and returns unsanitized user input in a search result, error message, or other HTTP responses. Cross-Site Scripting, or XSS, is a type of web vulnerability that allows an attacker to inject malicious code into a website or web application. However, depending on the tag which innerText is applied, code can be executed. Examples of some JavaScript sandbox / sanitizers: Don't eval() JSON to convert it to native JavaScript objects. Cross-Site Scripting (XSS) is a security vulnerability that allows an attacker to inject malicious code into a web page viewed by other users. We want to help you build beautiful, accessible, fast, and secure websites that work cross-browser, and for all of your users. One of the simplest ways of doing this is to deliver your exploit via an iframe: In this example, the src attribute points to the vulnerable page with an empty hash value. Many security training curriculums and papers advocate the blind usage of HTML encoding to resolve XSS. The attacker can manipulate this data to include XSS content on the web page, for example, malicious JavaScript code. Testing JavaScript execution sinks for DOM-based XSS is a little harder. Also, XSS attacks always execute in the browser. Semgrep rule to identify above dom xss link. Prepare for Content Security Policy violation reports, Switch to enforcing Content Security Policy. Canonicalize input, URL Validation, Safe URL verification, Allow-list http and HTTPS URLs only (Avoid the JavaScript Protocol to Open a new Window), Attribute encoder. A Computer Science portal for geeks. Sometimes it's not possible to remove the functionality, and there is no library to sanitize the value and create a Trusted Type for you. In certain circumstances, such as when targeting a 404 page or a website running PHP, the payload can also be placed in the path. Please look at the OWASP Java Encoder JavaScript encoding examples for examples of proper JavaScript use that requires minimal encoding. It is possible if the web application's client-side scripts write data provided by the user to the Document Object Model (DOM). For example, here we have some JavaScript that changes an anchor element's href attribute using data from the URL: You can exploit this by modifying the URL so that the location.search source contains a malicious JavaScript URL. A list of safe HTML attributes is provided in the Safe Sinks section. Already got an account? This video shows the lab solution of "DOM-based cross-site scripting" from WebGoat 7. You can deploy a report collector (such as the open-source go-csp-collector), or use one of the commercial equivalents. It is also impossible to protect against such client-side attacks using WAFs. For information on sources and sinks, read the following article: Finding the Source of a DOM-based XSS Vulnerability with Acunetix. The encoder safe lists can be customized to include Unicode ranges appropriate to the app during startup, in Program.cs: For example, using the default configuration using a Razor HtmlHelper similar to the following: The preceding markup is rendered with Chinese text encoded: To widen the characters treated as safe by the encoder, insert the following line into Program.cs. document.createElement(""), element.setAttribute("","value"), element.appendChild() and similar are safe ways to build dynamic interfaces. When looking at XSS (Cross-Site Scripting), there are three generally recognized forms of XSS: The XSS Prevention Cheatsheet does an excellent job of addressing Reflected and Stored XSS. You can also debug the violations in the browser: Add the following HTTP Response header to documents that you want to migrate to Trusted Types. Before putting untrusted data into a URL query string ensure it's URL encoded. All of this code originates on the server, which means it is the application owner's responsibility to make it safe from XSS, regardless of the type of XSS flaw it is. The enterprise-enabled dynamic web vulnerability scanner. When URL encoding in DOM be aware of character set issues as the character set in JavaScript DOM is not clearly defined (Mike Samuel). The best manual tools to start web security testing. JavaScript encoding all untrusted input, as shown in these examples: Enclosed within a closure or JavaScript encoded to N-levels based on usage. The best way to fix DOM based cross-site scripting is to use the right output method (sink). There will be times where you need to do something outside the protection provided by your framework. Other CSS Contexts are unsafe and you should not place variable data in them. The following article describes how to exploit different kinds of XSS Vulnerabilities that this article was created to help you avoid: Discussion on the Types of XSS Vulnerabilities: How to Review Code for Cross-site scripting Vulnerabilities: How to Test for Cross-site scripting Vulnerabilities: Copyright 2021 - CheatSheets Series Team - This work is licensed under a, Output Encoding for HTML Attribute Contexts, Output Encoding for JavaScript Contexts, Insecure Direct Object Reference Prevention, OWASP Java Encoder JavaScript encoding examples, Creative Commons Attribution 3.0 Unported License. The line above could have possibly worked to render a link. Its the same with computer security. innerHTML, outerHTML,insertAdjacentHTML, <iframe> srcdoc, document.write, document.writeln, and DOMParser.parseFromString, Executing plugin content: <embed src>, <object data> and <object codebase>, Runtime JavaScript code compilation: eval, setTimeout, setInterval, new Function(). If you have to use user input on your page, always use it in the text context, never as HTML tags or any other potential code. The application logic returns an unsafe input as part of the response without rendering it safely or storing data generated by users. Except for alphanumeric characters, encode all characters with the HTML Entity, Except for alphanumeric characters, encode all characters with the, Out of date framework plugins or components, Where URLs are handled in code such as this CSS { background-url : javascript:alert(xss); }. Get help and advice from our experts on all things Burp. Limit access to object properties when using object[x] accessors (Mike Samuel). Another option provided by Gaz (Gareth) was to use a specific code construct to limit mutability with anonymous closures. If you must, the following examples describe some approaches that do and do not work. Trusted Types work by locking down the following risky sink functions. In general, HTML encoding serves to castrate HTML tags which are placed in HTML and HTML attribute contexts. All the Acunetix developers come with years of experience in the web security sphere. A script on the page then processes the reflected data in an unsafe way, ultimately writing it to a dangerous sink. To deliver a DOM-based XSS attack, you need to place data into a source so that it is propagated to a sink and causes execution of arbitrary JavaScript. DOM based XSS vulnerabilities therefore have to be prevented on the client side. HTML Sanitization will strip dangerous HTML from a variable and return a safe string of HTML. Use one of the following approaches to prevent code from being exposed to DOM-based XSS: The HTML, JavaScript and URL encoders are available to your code in two ways, you can inject them via dependency injection or you can use the default encoders contained in the System.Text.Encodings.Web namespace. Get your questions answered in the User Forum. It uses the Document Object Model (DOM), which is a standard way to represent HTML objects in a hierarchical manner. The most common one would be adding it to an href or src attribute of an tag. DOM-based Cross-site Scripting (DOM XSS) is a particular type of a Cross-site Scripting vulnerability. An XSS attack can be used to steal sensitive information, perform unauthorized actions on behalf of the user, or even take control of the user's session. This is a Safe Sink and will automatically URL encode data in it. "\u0061\u006c\u0065\u0072\u0074\u0028\u0032\u0032\u0029", "\u0061\u006c\u0065\u0072\u0074\u0028\u0031\u0029". The following snippets of HTML demonstrate how to safely render untrusted data in a variety of different contexts. Input validation. //any code passed into lName is now executable. There are a couple of options for fixing a Trusted Type violation. Misconceptions abound related to the proper encoding that is required. This would be like a DOM Based XSS attack as it is using rendered JavaScript rather than HTML, however, as it passes though the server it is still classed as reflected or stored XSS depending on where the value is initially set. If you use the default encoders then any you applied to character ranges to be treated as safe won't take effect - the default encoders use the safest encoding rules possible. If you directly access an encoder via System.Text.Encodings.Web. Generally, attributes that accept JavaScript, such as onClick, are NOT safe to use with untrusted attribute values. You may want to do this to change a hyperlink, hide an element, add alt-text for an image, or change inline CSS styles. ESAPI is one of the few which works on an allow list and encodes all non-alphanumeric characters. Sometimes you can't change the offending code. Others have a root cause on the client, where the JavaScript code calls dangerous functions with user-controlled content. HTML Context refers to inserting a variable between two basic HTML tags like a