How to Prevent Cross Site Scripting Attacks. Cross site scripting XSS is one of the most dangerous and most often found vulnerabilities related to web applications. Security researchers have found this vulnerability in most of the popular websites, including Google, Facebook, Amazon, Pay. Pal, and many others. If you look at the bug bounty program closely, most of the reported issues belong to XSS. To prevent cross site scripting, browsers also have their own filters, but security researchers always find ways to bypass those filters. This vulnerability is generally used to perform cookie stealing, malware spreading, session hijacking, and malicious redirection. In this attack, the attacker injects malicious Java. Script code into the website so that the browser executes the script and performs action as commanded by the attacker in the script. The vulnerability is easy to find but hard to patch. This is why it can be found in any website if you try. In this post, we will see what a cross site scripting attack is and how to create a filter to prevent it. We will also see few open source libraries that will help you in patching Cross site Script vulnerability in your web application. By Robert Auger v1. 62 Last Modified 42810 About What is Cross Site Request Forgery Who discovered CSRF What can be done with CSRF Is CSRF and Crosssite. Learn about the best database tools for Microsoft SQL Server. Find details and download info on the hottest MS SQL Server freeware, monitoring tools and more. Here is a compiled list of CrossSite Scripting XSS payloads, 298 in total, from various sites. These payloads are great for fuzzing for both reflective and. What is Cross Site Scripting Cross Site Scripting or XSS is one of the most common applicationlayer web attacks. XSS commonly targets scripts embedded in a page. What Is Cross site Scripting A cross site scripting attack is a kind of attack on web applications in which attackers try to inject malicious scripts to perform malicious actions on trusted websites. In cross site scripting, malicious code executes on the browser side and affects users. Cross site scripting is also known as an XSS attack. The first question that comes in mind is why we call it XSS instead of CSS. The answer is simple and known to all who work in web development. In web design, we have cascading style sheet s CSS. So cross site scripting is called XSS so it does not get confused with CSS. Now, back to XSS. A cross site scripting attack occurs when a web application executes a script that the attacker supplied to end users. This flaw can be found anywhere in an application where user input has been taken but not properly encoded. If the input is not properly encoded and sanitized, this injected malicious script will be sent to users. And a browser has no way to know that it should not trust a script. When the browser executes the script, a malicious action is performed on the client side. Most of the times, XSS is used to steal cookies and steal session tokens of a valid user to perform session hijacking. A few XSS examples Example 1. You see a search box on almost all websites. With this search box, you can search to find anything available on the website. This search form looks something like this. On the search. php page where it shows search results, it also lists the search keyword in the form of Search results for Keyword or You Searched for Keyword. On web pages, it is generally coded like this. You Searched for lt GETq. Whatever a person searches for, it will be displayed on the web page along with search results. Now think what happens if an attacker tries to inject malicious script from this side. Search for. lt img srcdata imagegif base. R0l. GODlh. AQABAIAAAAAAAPy. H5. BAEAAAAALAAAAAABAAEAAAIBRAA7 data wp preserve3. Cscript3. EalertE28. XSS2. 0injectionE28. C2. Fscript3. E data mce resizefalse data mce placeholder1 classmce object width2. If web application has nothing implemented to encode input and filter malicious scripts, it will take input as it is and then print on webpage where it will be called. So, at the keyword place, it will look like this. You Searched for lt img srcdata imagegif base. R0l. GODlh. AQABAIAAAAAAAPy. H5. BAEAAAAALAAAAAABAAEAAAIBRAA7 data wp preserve3. Cscript3. EalertE28. XSS2. 0injectionE28. C2. Fscript3. E data mce resizefalse data mce placeholder1 classmce object width2. It will be executed by the browser and it will display an alert box saying XSS injection. Example 2. Suppose there is a website with a messaging feature. In this website, users can send messages to their contacts. A basic form will look something like this. When this form is submitted, the message will be stored in the database. Another person will see the message when he opens the message from the inbox. Suppose an attacker has sent some cookie stealing script in the message. This script will be stored on the website as a message. When the other person tries to read the message, the cookie stealing script will be executed and his session id is now on the attackers side. With a valid session id, the attacker can hijack the other persons account. Ethical Hacking Training Resources. Types of Cross site Scripting Attack. There is no standard classification, but most of the experts classify XSS in these three flavors non persistent XSS, persistent XSS, and DOM based XSS. Non Persistent Cross site scripting attack. Non persistent XSS is also known as reflected cross site vulnerability. It is the most common type of XSS. In this, data injected by attacker is reflected in the response. If you take a look at the examples we have shown above, the first XSS example was a non persistent attack. A typical non persistent XSS contains a link with XSS vector. Persistent cross site scripting attack. Persistent cross site scripting is also known as stored cross site scripting. It occurs when XSS vectors are stored in the website database and executed when a page is opened by the user. Every time the user opens the browser, the script executes. In the above examples, the second example of messaging a website was a persistent XSS attack. Persistent XSS is more harmful that non persistent XSS, because the script will automatically execute whenever the user opens the page to see the content. Googles orkut was vulnerable to persistent XSS that ruined the reputation of the website. 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 users browser. On the client side, the HTTP response does not change but the script executes in malicious manner. This is the most advanced and least known type of XSS. Most of the time, this vulnerability exists because developers do not understand how it works. Reasons Why Cross site Scripting Occurs. The primary reason for cross site script attacks is the trust of developers for users. Developers easily think that users will never try to perform anything wrong, so they create applications without using any extra efforts to filter user input in order to block any malicious activity. Another reason is that this attack has so many variants. Sometimes, an application that properly tries to filter any malicious scripts gets confused and allows a script. In the past few months, we have seen many different kind of XSS vectors that can bypass most of the available XSS filters. So we can never say that a website is fully protected. But we can do our best to filter most of the things, because extraordinary vectors mostly come from responsible security researchers and they will also help you in patching and making your filter smarter. How to Create a good XSS Filter to Block Most XSS Vectors. Before we start creating a XSS filter, I want to say one important thing We can never claim to have a perfect XSS filter. Researchers always find weird ways to bypass filters. But we can try to make a filter that can filter easy and well known XSS vectors. At least you will be safe from script kiddies. If you do not have an understanding of XSS, you cannot patch XSS. You should have an idea how attackers inject scripts. You should have knowledge of XSS vectors. Let us start with basic filters There is a simple rule that you need to follow everywhere Encode every datum that is given by a user. If data is not given by a user but supplied via the GET parameter, encode these data too. Even a POST form can contain XSS vectors. So, every time you are going to use a variable value on the website, try cleaning for XSS. These are the main data that must be properly sanitized before being used on your website. The URLHTTP referrer objects. GET parameters from a form. POST parameters from a form. Window. location. Document. referrerdocument. URLdocument. URLUnencodedcookie dataheaders datadatabase data, if not properly validated on user input. First of all, encode all lt, and. This should be the first step of your XSS filter. Cross Site Tracing OWASPThis is an Attack. To view all attacks, please see the Attack Category page. Last revision mmddyy 1. Description. A Cross Site Tracing XST attack involves the use of Cross site Scripting XSS and the TRACE or TRACK HTTP methods. According to RFC 2. TRACE allows the client to see what is being received at the other end of the request chain and use that data for testing or diagnostic information., the TRACK method works in the same way but is specific to Microsofts IIS web server. XST could be used as a method to steal users cookies via Cross site Scripting XSS even if the cookie has the Http. Only flag set andor exposes the users Authorization header. The TRACE method, while apparently harmless, can be successfully leveraged in some scenarios to steal legitimate users credentials. This attack technique was discovered by Jeremiah Grossman in 2. Http. Only tag that Microsoft introduced in Internet Explorer 6 sp. Java. Script. As a matter of fact, one of the most recurring attack patterns in Cross Site Scripting is to access the document. Tagging a cookie as Http. Only forbids Java. Script to access it, protecting it from being sent to a third party. However, the TRACE method can be used to bypass this protection and access the cookie even in this scenario. Modern browsers now prevent TRACE requests being made via Java. Script, however, other ways of sending TRACE requests with browsers have been discovered, such as using Java. Risk Factors. TBD. Examples. An example using c. URL from the command line to send a TRACE request to a web server on the localhost with TRACE enabled. Notice how the web server responds with the request that was sent to it. X TRACE 1. 27. 0. TRACE HTTP1. 1. User Agent curl7. Open. SSL0. 9. 8r zlib1. Host 1. 27. 0. 0. In this example notice how we send a Cookie header with the request and it is also in the web servers response. X TRACE H Cookie namevalue 1. TRACE HTTP1. 1. User Agent curl7. Open. SSL0. 9. 8r zlib1. Host 1. 27. 0. 0. Cookie namevalue. In this example the TRACE method is disabled, notice how we get an error instead of the request we sent. X TRACE 1. 27. 0. DOCTYPE HTML PUBLIC IETFDTD HTML 2. EN. lt html lt head. Method Not Allowedlt title. Method Not Allowedlt h. The requested method TRACE is not allowed for the URL. lt p. Example Java. Script XMLHttp. Request TRACE request. In Firefox 1. 9. 0. Illegal Value error. In Google Chrome 2. Uncaught Error Security. Error DOM Exception 1. This is because modern browsers now block the TRACE method in XMLHttp. Request to help mitigate XST. XMLHttp. Request. Credentials true send cookie header. TRACE, url, false. Remediation. Apache. In Apache versions 1. Trace. Enable directive to off in the main configuration file and then restart Apache. See Trace. Enable for further information. Trace. Enable off.
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