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Finding and Exploiting XSS Vulnerabilities

Srikar NadipallyXSS Vulnerabilities

OutlineFinding and Exploiting XSS VulnerabilitiesStandardReflected XSSStored XSSDOM based XSSPrevention of XSS attackReflectStoredDOM

StandardUse standard proof of concept>alert(document.cookie)Submit the above request to each page and monitor the response. If cases appear attack string appears3Detecting Reflected XSSSystematic approach of finding reflected attackFind all the entry points of the user inputSubmit a benign alphabetical string in each entry pointIdentify all the locations where the string is reflected in the applications responseFor each reflection identify the syntactic context in which string appearsSubmit modified data tailored to the reflection syntactic context, attempting to introduce the arbitrary script in to response.If reflected data is blocked, try to understand and circumvent the application defensive filter

Example 1Tag attribute valueReturn page contains the code

Ways to craft XSS exploit >alert(1) onfoucus=alert(1)

Example 2Java Script StringIf return page code is var a=myxsstest; var b =123; Ways to craft XSS exploit; alert(1); var foo=

Example 3Atrribute Containing URLReturned page codeClick hereWays to craft XSS exploitjavascript:alert(1)#onclick=javascript:alert(1)

Stored XSSStored XSS vulnerability identification is quite similar to that of reflected XSS submitting a unique string in every entry point within the application.Once you identify every instance in which user controllable data is stored by the application and later displayed back to the browser, same process is followed as that of reflected XSS determining what input to be submitted to embed valid JavaScript within the surrounding HTML and then besiege the filters that intervene with your attack payload process.

Techniques to test for stored XSSTesting for XSS in Web Mail applicationsSend all kinds of unusual HTML content within emails to test for bypasses in input filters.Restricting to standard email client will not give you enough control over the raw message content or the client may itself sanitize or clean up your malformed syntax.Using UNIX sendmail command a raw email can be created in a text editor and send it.Sendmail t < email.txtExampleRaw email fileMIME-Version: 1.0From: test@example.orgContent-Type: text/html; charset=us-asciiContent-Transfer-Encoding: 7bitSubject: XSS test

Techniques to Detect stored XSSTesting for XSS in Uploaded FilesHybrid File attacks hybrid files - two different formatsExample: GIFAR (GIF + JAR)Uploaded file attack using GIFARThe attack using GIFAR files can be prevented in current versions of Java browser plug-in by validating whether JAR files being loaded actually content hybrid content.source of stored XSS vulnerabilities arises where an application allows users to upload les that can be downloaded and viewed by other users

Uploaded file attack using GIFAR-- Attacker finds an application function in which GIF les that are uploaded by one user can be downloaded by other users, such as a users prole picture in a social networking application.- The attacker uploads the le as his prole picture. Because the le contains a valid GIF image, the application accepts it.- The attacker identi es a suitable external website from which to deliver an attack leveraging the uploaded le. This may be the attackers own website, or a third-party site that allows authoring of arbitrary HTML, such as a blog.On the external site, the attacker uses the or tag to load the GIFAR le from the social networking site as a Java applet. When a user visits the external site, the attackers Java applet executes in his browser. For Java applets, the same-origin policy is implemented in a different way than for normal script includes. The applet is treated as belonging to the domain from which it was loaded, not the domain that invoked it. Hence, the attackers applet executes in the domain of the social networking application. If the victim user is logged in to the social networking application at the time of the attack, or has logged in recently and selected the stay logged in option, the attackers applet has full access to the users session, and the user is compromised11Detecting DOM-based XSSManually walk through the application with your browser and modify each URL parameter to contain a standard test string.alert(1);alert(1)//-alert(1)-Displaying each returned page in the browser causes all client side scripts to execute referencing the modified URL parameter.If you see a dialog box containing cookies, you will have found a vulnerability.This process can be automated by a tool that implements its own javascript interpreter.12Detecting DOM-based XSSEffective approach: Review all client-side JavaScript for any use of DOM properties that may lead to a vulnerability.DOM-tracer is a tool that helps you to automate this process

Preventing XSS attacksDue to the different root causes different defense mechanisms needs to be applied for reflected and stored XSS on one hand and DOM-based on the other.Reflected and Stored XSSIdentify every instance within the application where user-controllable data is being copied into responses including data that is copied from immediate request and also any stored data that is originated from any user at any prior time, including via out-of-band channels.After identification, follow a threefold approach to prevent any actual vulnerabilities from arising.ContinuedThreefold approach-Validate inputValidate output (Encode the output)Eliminate dangerous insertion pointsValidate input: The application should perform context-dependent validation of data when application receives user-supplied data that may copy into one of its responses at any future point.Potential features to validate data is not too long, contains only a certain permitted character set, matches a particular regular expression. Different Validation rules should also be applied names, email ids, account numbers etc.

15ContinuedValidate OutputData should be HTML-encoded to sanitize potentially malicious characters.HTML encoding involves replacing literal characters with their corresponding HTML entities.HTML encodings of the primary problematic characters are as follows " '& &< >ContinuedEliminate dangerous insertion pointsInserting user-controllable data directly into existing script code should be avoided wherever possible. This applies to the code within tags, and also code within event handlers.Allowing limited HTML

ContinuedPreventing DOM-based XSSApplication should avoid using client-side scripts to process DOM data and insert it into the page.DOM-based XSS flaws can be prevented through two types of defenses-Validate inputValidate outputValidate input:Attack can be prevented by validating the data about to be inserted into the document containing only alphanumeric characters and white space.


var a = document.URL; a = a.substring(a.indexOf(message=) + 8, a.length); a = unescape(a); var regex=/^([A-Za-z0-9+\s])*$/; if (regex.test(a)) document .write(a);

ContinuedServer-side validation can also be employed to detect requests that may contain malicious exploits by verifying the following:The query string contains a single parameter.The parameters name is message (case-sensitive check).The parameters value contains only alphanumeric contentContinuedValidate output:Applications can perform HTML encoding of user-controllable DOM data before it is inserted into the document. HTML encoding can be implemented in client-side Javascript with a function like the following:function sanitize(str){ var d = document.createElement(div); d.appendChild(document.createTextNode(str)); return d.innerHTML;}Request forgeryAlso known as Session riding is related to session hijacking attacks, in which an attacker captures a users session token and thereby uses that application as that user.Request Forgery vulnerabilities comes in two flavors:On-site Request Forgery (OSRF)Cross-Site Request Forgery (CSRF)OSRFAttack for exploiting stored XSS vulnerabilities.OSRF vulnerabilities can exist even in situations where XSS is not possible.Consider a message board application that lets users submit items that are viewed by other users. Messages are submitted using a request like the following:POST /submit.phpHost: wahh-app.comContent-Length: 34type=question&name=daf&message=foo

ContinuedThis request results in the following being added to the messages page: daf fooCSRFAttacker creates the innocuous-looking website causes the users browser to submit a request directly to the vulnerable application to perform some unintended action that is beneficial to the attacker.CSRF attacks are one-way only.Consider an application in which administrators can create new user accounts using requests like the following:POST /auth/390/NewUserStep2.ashx HTTP/1.1Host: mdsec.netCookie: SessionId=8299BE6B260193DA076383A2385B07B9Content-Type: application/x-www-form-urlencodedContent-Length: 83realname=daf&username=daf&userrole=admin&password=letmein1&confirmpassword=letmein1ContinuedThis request has three key features that make it vulnerable to CSRF attacks:The request performs a privileged action. In the example shown, the request creates a new user with administrative privileges.The application relies solely on HTTP cookies for tracking sessions. No session-related tokens are transmitted elsewhere within the request.The attacker can determine all the parameters required to perform the action. Aside from the session token in the cookie, no unpredictable values need to be included in the requestContinuedAttacker can construct a web page that makes a cross-domain request to the vulnerable application containing