Fewer XSS bugs appear in applications built with modern web frameworks. //The following does NOT work because of the encoded ";". Even newer versions of jQuery can still be vulnerable via the $() selector sink, provided you have full control over its input from a source that doesn't require a # prefix. Reflected and Stored XSS are server side injection issues while DOM based XSS is a client (browser) side injection issue. Based on our research summarized in the Acunetix Web Application Vulnerability Report, DOM-based cross-site scripting is not very common such vulnerabilities exist only in approximately 1.2% of analyzed web applications. Now all the violations are reported to //my-csp-endpoint.example, but the website continues to work. For each potential source, such as location, you first need to find cases within the page's JavaScript code where the source is being referenced. When you are in a DOM execution context you only need to JavaScript encode HTML attributes which do not execute code (attributes other than event handler, CSS, and URL attributes). There are two ways to do this. Based on this context, you need to refine your input to see how it is processed. It is important to use an encoding library that understands which characters can be used to exploit vulnerabilities in their respective contexts. In a stored DOM XSS vulnerability, the server receives data from one request, stores it, and then includes the data in a later response. 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. Trusted Types force you to process a value somehow, but don't yet define what the exact processing rules are, and whether they are safe. 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. Please insert your password to refresh your session. Cross-Site Scripting (XSS) is a security vulnerability that allows an attacker to inject malicious code into a web page viewed by other users. Avoid treating untrusted data as code or markup within JavaScript code. This is commonly associated with normal XSS, but it can also lead to reflected DOM XSS vulnerabilities. The DOM is a programming interface. Download the latest version of Burp Suite. The attacker can manipulate this data to include XSS content on the web page, for example, malicious JavaScript code. The DOM-based cross-site scripting requires the user to open an infected page. DOM-based XSS: In this type of attack, the attacker injects malicious code into a web page that is executed on the client-side within the Document Object Model (DOM) of the web page. When you find a sink that is being assigned data that originated from the source, you can use the debugger to inspect the value by hovering over the variable to show its value before it is sent to the sink. This site is our home for content to help you on that journey, written by members of the Chrome team, and external experts. Cross-Site Scripting (XSS) is a security vulnerability which enables an attacker to place client side scripts (usually JavaScript) into web pages. All other contexts are unsafe and you should not place variable data in them. An attacker can construct a link to send a victim to a vulnerable page with a payload in the query string and fragment portions of the URL. XSS sinks are places where variables are placed into your webpage. HTML Context refers to inserting a variable between two basic HTML tags like a
or . Get started with Burp Suite Enterprise Edition. DOM-based cross-site scripting (DOM XSS) is one of the most common web security vulnerabilities, and it's very easy to introduce it in your application. To prevent DOM-based cross-site scripting, sanitize all untrusted data, even if it is only used in client-side scripts. Reduce the DOM XSS attack surface of your application. Make sure any attributes are fully quoted, same as JS and CSS. It is particularly common when applications leverage common JavaScript function calls such as document.baseURI to build a part of the page without sanitization. In principle, a website is vulnerable to DOM-based cross-site scripting if there is an executable path via which data can propagate from source to sink. For example, if your string appears within a double-quoted attribute then try to inject double quotes in your string to see if you can break out of the attribute. Misconceptions abound related to the proper encoding that is required. DOM-based XSS is an advanced XSS attack. This difference makes JavaScript encoding a less viable weapon in our fight against XSS. Read about other types of cross-site scripting attacks. This is a Safe Sink and will automatically URL encode data in it. It is possible if the web application's client-side scripts write data provided by the user to the Document Object Model (DOM). Document Object Model (DOM) Based XSS. Markdown, coupled with a parser that strips embedded HTML, is a safer option for accepting rich input. Then client-side encode (using a JavaScript encoding library such as node-esapi) for the individual subcontext (DOM methods) which untrusted data is passed to. -->, "javascript:myFunction('<%=ESAPI.encoder().encodeForJavascript(untrustedData)%>', 'test');", "<%=ESAPI.encoder().encodeForHTML(last_name)%>", //when the value is retrieved the encoding is reversed. Output encoding here will prevent XSS, but it will break the intended functionality of the application. Never put untrusted data into your HTML input, unless you follow the rest of the steps below. To actually exploit this classic vulnerability, you'll need to find a way to trigger a hashchange event without user interaction. In a DOM-based attacks, the HTTP response on the server side does not change. 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. 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. The name originated from early versions of the attack where stealing data cross-site was the primary focus. CSS is surprisingly powerful and has been used for many types of attacks. Looking to understand what cross-site scripting (XSS) is and the various techniques used by attackers? To use the configurable encoders via DI your constructors should take an HtmlEncoder, JavaScriptEncoder and UrlEncoder parameter as appropriate. This information should help you narrow down which parts of code may be introducing DOM XSS and need to change.Most of the violations like this can also be detected by running a code linter or static code checkers on your codebase. For example, you can use DOMPurify to sanitize an HTML snippet, removing XSS payloads. Before putting untrusted data inside an HTML element ensure it's HTML encoded. URL Contexts refer to variables placed into a URL. Level up your hacking and earn more bug bounties. Variables should not be interpreted as code instead of text. Your best bet is to use a vulnerability scanner with a DOM-based cross-site scripting detection module. There may be times you want to insert a value into JavaScript to process in your view. Read the entire Acunetix Web Application Vulnerability Report. A rendering context is associated with the parsing of HTML tags and their attributes. Other CSS Contexts are unsafe and you should not place variable data in them. You can remove the offending code, use a library, create a Trusted Type policy or, as a last resort, create a default policy. This can lead to a range of attacks, including stealing sensitive information, hijacking user accounts, and spreading malware. These types of attacks typically occur as a result . See what Acunetix Premium can do for you. We want to help you build beautiful, accessible, fast, and secure websites that work cross-browser, and for all of your users. There are 3 primary types of cross-site scripting: DOM-based XSS. Also, XSS attacks always execute in the browser. To prevent server-side XSS, don't generate HTML by concatenating strings and use safe contextual-autoescaping templating libraries instead. There are other places in JavaScript where JavaScript encoding is accepted as valid executable code. For more information on other types of XSS attacks: reflected XSS and stored XSS, see the following article: Types of XSS: Stored XSS, Reflected XSS, and DOM-based XSS. Normally executing JavaScript from a CSS context required either passing javascript:attackCode() to the CSS url() method or invoking the CSS expression() method passing JavaScript code to be directly executed. Event handlers such as onload and onerror can be used in conjunction with these elements. This is where Output Encoding and HTML Sanitization are critical. A better approach would be to use the following: Run your JavaScript in a ECMAScript 5 canopy or sandbox to make it harder for your JavaScript API to be compromised (Gareth Heyes and John Stevens). You can deploy a report collector (such as the open-source go-csp-collector), or use one of the commercial equivalents. Java Encoder is an active project providing supports for HTML, CSS and JavaScript encoding. For a comprehensive list, check out the DOMPurify allowlist. The difference between Reflected/Stored XSS is where the attack is added or injected into the application. The next section explains how //my-csp-endpoint.example works.CautionTrusted Types are only available in a secure context like HTTPS and localhost. A script within the later response contains a sink which then processes the data in an unsafe way. See how our software enables the world to secure the web. When other users load affected pages the attacker's scripts will run, enabling the attacker to steal cookies and session tokens, change the contents of the web page through DOM manipulation or redirect the browser to another page. DOM-based XSS is a type of cross-site scripting attack that takes advantage of vulnerabilities in the Document Object Model (DOM) of a web page. Each variable used in the user interface should be passed through an output encoding function. If A is double JavaScript encoded then the following if check will return false. This is common when you want users to be able to customize the look and feel of their webpages. DOM-based XSS attacks seek to exploit the DOM in a simple two step process: Create a Source: Inject a malicious script into a property found to be suceptible to DOM-based XSS attacks. Using the wrong encoding method may introduce weaknesses or harm the functionality of your application. Let's look at the sample page and script: Finally there is the problem that certain methods in JavaScript which are usually safe can be unsafe in certain contexts. In that case, use a default policy: The policy with a name default is used wherever a string is used in a sink that only accepts Trusted Type.GotchasUse the default policy sparingly, and prefer refactoring the application to use regular policies instead. You might already recognize some of them, as browsers vendors and web frameworks already steer you away from using these features for security reasons. 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 want to use user input to write in a div tag element don't use innerHtml, instead use innerText or textContent. On the client side, the HTTP response does not change but the script executes in malicious manner. So HTML encoding cannot be used to allow the developer to have alternate representations of the tag for example. Here is an example of the problem using map types: The developer writing the code above was trying to add additional keyed elements to the myMapType object. Before putting untrusted data into JavaScript place the data in an HTML element whose contents you retrieve at runtime. DOM-based XSS Examples. Try to refactor your code to remove references to unsafe sinks like innerHTML, and instead use textContent or value. 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. This is in stark contrast to JavaScript encoding in the event handler attribute of a HTML tag (HTML parser) where JavaScript encoding mitigates against XSS. Its the same with computer security. DOM based XSS is extremely difficult to mitigate against because of its large attack surface and lack of standardization across browsers. eval This section covers each form of output encoding, where to use it, and where to avoid using dynamic variables entirely. If your web site makes heavy use of non-Latin characters, such as Chinese, Cyrillic or others this is probably not the behavior you want. Working example (no HTML encoding): Normally encoded example (Does Not Work DNW): HTML encoded example to highlight a fundamental difference with JavaScript encoded values (DNW): If HTML encoding followed the same semantics as JavaScript encoding. The following snippets of HTML demonstrate how to safely render untrusted data in a variety of different contexts. Also, keep in mind that DOM XSS and other types of XSS are not mutually exclusive. The doubleJavaScriptEncodedData has its first layer of JavaScript encoding reversed (upon execution) in the single quotes. Information on ordering, pricing, and more. If you utilize fully qualified URLs then this will break the links as the colon in the protocol identifier (http: or javascript:) will be URL encoded preventing the http and javascript protocols from being invoked. In Chrome's developer tools, you can use Control+Shift+F (or Command+Alt+F on MacOS) to search all the page's JavaScript code for the source. DOM-based Cross-site Scripting (DOM XSS) is a particular type of a Cross-site Scripting vulnerability. 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. There are three types of XSS attacks: stored, reflected and Document Object Model (DOM) based. For instance, jQuery's attr() function can change the attributes of DOM elements. Any application is vulnerable to DOM-based cross-site scripting if there is an executable path via which data can develop from source to sink. A script on the page then processes the reflected data in an unsafe way, ultimately writing it to a dangerous sink. This video shows the lab solution of "DOM-based cross-site scripting" from WebGoat 7. Web Application Firewalls - These look for known attack strings and block them. The attack functions by manipulating the internal model of the webpage within the browser known as the DOM and are referred to as DOM based attacks . Cross-Site Scripting (XSS) attacks are a type of injection, in which malicious scripts are injected into otherwise benign and trusted websites. Use a trusted and verified library to escape HTML inputs. Cross-site Scripting (XSS) can seriously threaten individual users and companies whose websites may be infected. Safe list ranges are specified as Unicode code charts, not languages. element.SetAttribute () element [attribute]= Here are some examples of how they are used: One option is utilize ECMAScript 5 immutable properties in the JavaScript library. DOM-based XSS is an attack that modifies the domain object model (DOM) on the client side ( the browser). With Trusted Types enabled, the browser accepts a TrustedHTML object for sinks that expect HTML snippets. No single technique will solve XSS. DOM-based XSS vulnerabilities usually arise when JavaScript takes data from an attacker-controllable source, such as the URL, and passes it to a sink that supports dynamic code execution, such as eval() or innerHTML. Most DOM XSS payloads are never sent to the server because they are prepended by the # symbol. Note how the payload is stored in the GET request, making it suitable for social engineering attacks. With these sinks, your input doesn't necessarily appear anywhere within the DOM, so you can't search for it. Aggressive HTML Entity Encoding (rule #2), Only place untrusted data into a list of safe attributes (listed below), Strictly validate unsafe attributes such as background, ID and name. The reasoning behind this is to protect against unknown or future browser bugs (previous browser bugs have tripped up parsing based on the processing of non-English characters). The rendered output would now become. Just using a string will fail, as the browser doesn't know if the data is trustworthy:Don'tanElement.innerHTML = location.href; With Trusted Types enabled, the browser throws a TypeError and prevents use of a DOM XSS sink with a string. If these methods are provided with untrusted input, then an XSS vulnerability could result. Therefore there is little change in the encoding rules for URL attributes in an execution (DOM) context. The application logic returns an unsafe input as part of the response without rendering it safely or storing data generated by users. JavaScript encoding takes dangerous characters for JavaScript and replaces them with their hex, for example < would be encoded as \u003C. However, depending on the tag which innerText is applied, code can be executed. The world's #1 web penetration testing toolkit. From now on, every time Trusted Types detect a violation, a report will be sent to a configured report-uri. 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. Validation becomes more complicated when accepting HTML in user input. XSS is one of the most common and dangerous web vulnerabilities, and it is . One of our Vulnweb test sites features a DOM-based XSS vulnerability that can be exploited using the following payload: The result can be seen in the following image. For example: Modern web applications are typically built using a number of third-party libraries and frameworks, which often provide additional functions and capabilities for developers. Stored XSS is considered the most damaging type of XSS attack. If you sanitize content and then modify it afterwards, you can easily void your security efforts. With Trusted Types enabled, the browser throws a TypeError and prevents use of a DOM XSS sink with a string. You might find that the source gets assigned to other variables. 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 your data gets URL-encoded before being processed, then an XSS attack is unlikely to work. All the Acunetix developers come with years of experience in the web security sphere. Rather, a malicious change in the DOM environment causes client code to run unexpectedly. An important implementation note is that if the JavaScript code tries to utilize the double or triple encoded data in string comparisons, the value may be interpreted as different values based on the number of evals() the data has passed through before being passed to the if comparison and the number of times the value was JavaScript encoded. JavaScript encoding all untrusted input, as shown in these examples: Enclosed within a closure or JavaScript encoded to N-levels based on usage. A list of safe HTML attributes is provided in the Safe Sinks section. This is commonly seen in programs that heavily use custom JavaScript embedded in their web pages. You need to work through each available source in turn, and test each one individually. Each variable in a web application needs to be protected. //The following DOES WORK because the encoded value is a valid variable name or function reference. Catch critical bugs; ship more secure software, more quickly. In order to mitigate against the CSS url() method, ensure that you are URL encoding the data passed to the CSS url() method. Use untrusted data on only the right side of an expression, especially data that looks like code and may be passed to the application (e.g., location and eval()). There will be situations where you use a URL in different contexts. If a framework like AngularJS is used, it may be possible to execute JavaScript without angle brackets or events. Finally, to fix the problem in our initial code, instead of trying to encode the output correctly which is a hassle and can easily go wrong we would simply use element.textContent to write it in a content like this: It does the same thing but this time it is not vulnerable to DOM based cross-site scripting vulnerabilities. Before putting untrusted data into a URL query string ensure it's URL encoded. 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(). Practise exploiting vulnerabilities on realistic targets. This means, that no data will be available in server logs. Cross-Site Scripting (XSS) is a misnomer. It uses the Document Object Model (DOM), which is a standard way to represent HTML objects in a hierarchical manner. Always JavaScript encode and delimit untrusted data as quoted strings when entering the application as illustrated in the following example. That said, developers need to be aware of problems that can occur when using frameworks insecurely such as: Understand how your framework prevents XSS and where it has gaps. Enhance security monitoring to comply with confidence. DOM-based XSS vulnerabilities usually arise when JavaScript takes data from an attacker-controllable source, such as the URL, and passes it to a sink that supports dynamic code execution, such as eval () or innerHTML. For example, Acunetix. Read the entire Acunetix Web Application Vulnerability Report. 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. Semgrep rule to identify above dom xss link. The guidelines below are an attempt to provide guidelines for developers when developing Web based JavaScript applications (Web 2.0) such that they can avoid XSS. Perpetrators can insert malicious code into a page due to modifying the DOM environment (Document Object Model) when it doesn't properly filter user input. DOM-based Cross-site Scripting (DOM XSS) is a particular type of a Cross-site Scripting vulnerability. The following is an example vulnerability which occurs in the JavaScript context and HTML subcontext: Let's look at the individual subcontexts of the execution context in turn. Limit access to object properties when using object[x] accessors (Mike Samuel). DOM-based cross-site scripting attack DOM-based XSS is also sometimes called "type-0 XSS." It occurs when the XSS vector executes as a result of a DOM modification on a website in a user's browser. Doing so encourages designs in which the security rules are close to the data that they process, where you have the most context to correctly sanitize the value. Want to track your progress and have a more personalized learning experience? There are also TrustedScript and TrustedScriptURL objects for other sensitive sinks. Output Encoding and HTML Sanitization help address those gaps. Instead you'll need to use the JavaScript debugger to determine whether and how your input is sent to a sink. How to detect DOM-based cross-site scripting? By default encoders use a safe list limited to the Basic Latin Unicode range and encode all characters outside of that range as their character code equivalents.
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Jocelyn Pua Tulfo Biography, Articles D