dom based cross site scripting prevention23 Apr dom based cross site scripting prevention
The HTML encoded value above is still executable. For XSS attacks to be successful, an attacker needs to insert and execute malicious content in a webpage. Script manipulation: <script src> and setting text content of <script> elements. These frameworks steer developers towards good security practices and help mitigate XSS by using templating, auto-escaping, and more. 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. -->, "javascript:myFunction('<%=ESAPI.encoder().encodeForJavascript(untrustedData)%>', 'test');", "<%=ESAPI.encoder().encodeForHTML(last_name)%>", //when the value is retrieved the encoding is reversed. 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. There are numerous methods which implicitly eval() data passed to it that must be avoided. For DOM XSS, the attack is injected into the application during runtime in the client directly. Before putting untrusted data into a URL query string ensure it's URL encoded. A rendering context is associated with the parsing of HTML tags and their attributes. Untrusted data is any data that may be controlled by an attacker, HTML form inputs, query strings, HTTP headers, even data sourced from a database as an attacker may be able to breach your database even if they cannot breach your application. Other JavaScript methods which take code as a string types will have a similar problem as outline above (setTimeout, setInterval, new Function, etc.). 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. 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. Output encoding is the primary defense against cross-site scripting vulnerabilities. Safe HTML Attributes include: 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. As HTML attribute encoding is a superset of HTML encoding this means you don't have to concern yourself with whether you should use HTML encoding or HTML attribute encoding. You must regularly patch DOMPurify or other HTML Sanitization libraries that you use. Then, as with HTML sinks, you need to refine your input to see if you can deliver a successful XSS attack. If you pollute a river, it'll flow downstream somewhere. Make sure any attributes are fully quoted, same as JS and CSS. The DOM-based cross-site scripting requires the user to open an infected page. From my experience, calling the expression() function from an execution context (JavaScript) has been disabled. Using the wrong encoding method may introduce weaknesses or harm the functionality of your application. Get started with Burp Suite Professional. This brings up an interesting design point. 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). Also, keep in mind that DOM XSS and other types of XSS are not mutually exclusive. These locations are known as dangerous contexts. Already got an account? 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. DOM-based cross-site scripting happens when data from a user controlled, Most of the violations like this can also be detected by running a code linter or, If the sanitization logic in DOMPurify is buggy, your application might still have a DOM XSS vulnerability. Reduce risk. Some examples of DOM-based XSS attacks include: 1. Free, lightweight web application security scanning for CI/CD. 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. If you must, the following examples describe some approaches that do and do not work. This should never be used in combination with untrusted input as this will expose an XSS vulnerability. The rendered output would now become. There are some further things to consider: Security professionals often talk in terms of sources and sinks. The innerText feature was originally introduced by Internet Explorer, and was formally specified in the HTML standard in 2016 after being adopted by all major browser vendors. It uses the Document Object Model (DOM), which is a standard way to represent HTML objects in a hierarchical manner. When this happens, a script on the web page selects the URL variable and executes the code it contains. Many security training curriculums and papers advocate the blind usage of HTML encoding to resolve XSS. 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. This site is our home for content to help you on that journey, written by members of the Chrome team, and external experts. eval 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. 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. If you sanitize content and then send it to a library for use, check that it doesnt mutate that string somehow. How to prevent DOM-based cross-site scripting? For many years DOM XSS has been one of the most prevalentand dangerousweb security vulnerabilities. Output Encoding is recommended when you need to safely display data exactly as a user typed it in. If your data gets URL-encoded before being processed, then an XSS attack is unlikely to work. Output Encoding and HTML Sanitization help address those gaps. 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 (#). Copyright 2021 - CheatSheets Series Team - This work is licensed under a, "<%=ESAPI.encoder().encodeForJavascript(ESAPI.encoder().encodeForHTML(untrustedData))%>", // In the following line of code, companyName represents untrusted user input, // The ESAPI.encoder().encodeForHTMLAttribute() is unnecessary and causes double-encoding, '<%=ESAPI.encoder().encodeForJavascript(ESAPI.encoder().encodeForHTMLAttribute(companyName))%>', '<%=ESAPI.encoder().encodeForJavascript(companyName)%>', // In the line of code below, the encoded data on the right (the second argument to setAttribute). Always JavaScript encode and delimit untrusted data as quoted strings when entering the application as illustrated in the following example. This is a Safe Sink and will automatically CSS encode data in it. The #redir route is executed by another file, redir.html. Summary. Definition DOM Based XSS (or as it is called in some texts, "type-0 XSS") is an XSS attack wherein the attack payload is executed as a result of modifying the DOM "environment" in the victim's browser used by the original client side script, so that the client side code runs in an "unexpected" manner. There will be times where you need to do something outside the protection provided by your framework. Read the entire Acunetix Web Application Vulnerability Report. We will look at eval, href and dangerouslySetHTML vulnerabilities. Otherwise, again, your security efforts are void. The data is subsequently read from the DOM by the web application and outputted to the browser. Common injection vectors include document.url, document.location, and document.referrer objects. XSS Prevention & Mitigation. In the case above, the attribute name is an JavaScript event handler, so the attribute value is implicitly converted to JavaScript code and evaluated. One example of an attribute which is thought to be safe is innerText. The good news is that if user input is handled properly at the foundation level (e.g. DOM-based cross-site scripting is a type of cross-site scripting (XSS) attack executed within the Document Object Model (DOM) of a page loaded into the browser. If this isn't possible, then ensure the data is JavaScript encoded. 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. The name originated from early versions of the attack where stealing data cross-site was the primary focus. 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. It uses the Document Object Model (DOM), which is a standard way to represent HTML objects in a hierarchical manner. It is particularly common when applications leverage common JavaScript function calls such as document.baseURI to build a part of the page without sanitization. With Trusted Types enabled, the browser accepts a TrustedHTML object for sinks that expect HTML snippets. For details, see the Google Developers Site Policies. The attacker can manipulate this data to include XSS content on the webpage, for example, malicious JavaScript code. In practice, different sources and sinks have differing properties and behavior that can affect exploitability, and determine what techniques are necessary. //The following DOES WORK because the encoded value is a valid variable name or function reference. If you sanitize content and then modify it afterwards, you can easily void your security efforts. Get help and advice from our experts on all things Burp. In an XSS attack, an attacker uses web-pages or web applications to send malicious code and compromise users' interactions with a vulnerable application. DOM-based Cross-site Scripting (DOM XSS) is a particular type of a Cross-site Scripting vulnerability. An attacker can execute a DOM-based cross-site scripting attack if the web application writes user-supplied information directly to the Document Object Model (DOM) and there is no sanitization. Now that you know more about cross-site scripting attacks and their impact, let's take a look at how you can prevent cross-site scripting or XSS attacks. The best way to fix DOM based cross-site scripting is to use the right output method (sink). Strict structural validation (rule #4), CSS Hex encoding, Good design of CSS Features. XSS is one of the most common and dangerous web vulnerabilities, and it is . Therefore, the primary recommendation is to avoid including untrusted data in this context. The name originated from early versions of the attack where stealing data cross-site was the primary focus. This logically seems to be prudent advice as the JavaScript parser does not understand HTML encoding. Consider adopting the following controls in addition to the above. In the case above, JavaScript encoding does not mitigate against DOM based XSS. It simplifies security reviews, and allows you to enforce the type-based security checks done when compiling, linting, or bundling your code at runtime, in the browser. Testing JavaScript execution sinks for DOM-based XSS is a little harder. In a few clicks we can analyze your entire application and see what components are vulnerable in your application, and suggest you quick fixes. Acunetix uses its DeepScan technology to attempt DOM XSS against the client-side code and report vulnerabilities. It contains well written, well thought and well explained computer science and programming articles, quizzes and practice/competitive programming/company interview Questions. DOM Based Attacks. See what Acunetix Premium can do for you. CSS is surprisingly powerful and has been used for many types of attacks. 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. 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, you might need to close some existing elements before using your JavaScript payload. 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. Limit access to object properties when using object[x] accessors (Mike Samuel). Now all the violations are reported to //my-csp-endpoint.example, but the website continues to work. Read about other types of cross-site scripting attacks. It is important to note that when setting an HTML attribute which does not execute code, the value is set directly within the object attribute of the HTML element so there is no concerns with injecting up. Validation can be a useful tool in limiting XSS attacks. For the purposes of this article, we refer to the HTML, HTML attribute, URL, and CSS contexts as subcontexts because each of these contexts can be reached and set within a JavaScript execution context. 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. If data is read from a user-controlled source like the URL, then passed to the attr() function, then it may be possible to manipulate the value sent to cause XSS. \u0064\u006f\u0063\u0075\u006d\u0065\u006e\u0074, \u0077\u0072\u0069\u0074\u0065\u006c\u006e, "\u0048\u0065\u006c\u006c\u006f\u0020\u0057\u006f\u0072\u006c\u0064", "\u0061\u006c\u0065\u0072\u0074\u0028\u0031\u0031\u0029", "url(<%=ESAPI.encoder().encodeForJavascript(ESAPI.encoder().encodeForURL(companyName))%>)", '<%=ESAPI.encoder().encodeForJavascript(ESAPI.encoder().encodeForURL(userRelativePath))%>', "<%= Encode.forJavaScript(untrustedData) %>", "<%=ESAPI.encoder().encodeForJavascript(untrustedData)%>", "customFunction('<%=doubleJavaScriptEncodedData%>', y)", //HTML encoding is happening in JavaScript, "javascript:myFunction('<%=untrustedData%>', 'test');", "javascript:myFunction('<%=ESAPI.encoder().encodeForJavascript(ESAPI.encoder().encodeForURL(untrustedData)) %>', 'test');",