Web Applications in Java

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Continued 11 Other limits, such as a maximum length, imposed other problems and required tight code, no spaces, obfuscated names, reusable functions, etc There were a few other complications and things to get around This was not by any means to be a straightforward process, and none of this was meant to cause any damage or piss anyone off This was in the interest of interest It was interesting and fun!
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Preventing Cross-Site Scripting Preventing cross-site scripting means limiting the ways in which users can affect an application s output In essence, this means the application must perform output validation or output encoding This is conceptually similar to egress ltering on a rewall, which allows only approved types of outbound traf c to reach the Internet [Brenton, 2006] As an extra precaution, we recommend doing input validation for cross-site scripting, too Good output validation is just like good input validation: Whitelisting is the way to go Blacklisting to prevent XSS is highly error prone because different Web browsers, different versions of the same browser, and even the same browser con gured to use different character encodings can respond differently to the huge number of corner cases that occur in HTML Here are just a few ways to represent < in HTML (See the OWASP Guide to Building Secure Web Applications [OWASP, 2005] for 60 more)
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It s a lost cause to try to guess at what a Web browser will interpret as a tag For example, many versions of Internet Explorer will interpret the following two lines of code as a single <script> tag:
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To make matters worse, imagine trying to identify all the different places that a browser might allow JavaScript to appear without requiring a <script> tag (See the sidebar The First XSS Worm for several examples) If output that contains special characters needs to be rendered in the browser, you must encode the special characters to remove their signi cance In JSPs, you can start by using the JSTL <c:out> tag, which, by default, will escape >, <, &, ', and " In a Servlet, you can use a javanetURLEncoder object to transform any characters outside of the whitelist a z, A Z, 0 9, -, *, , and _ into their hexadecimal form If the application output must include user-speci ed tags, tailor your whitelist to allow only exactly the tags that you are willing to accept no attributes, no extra whitespace, just the tag in its most vanilla form Examples 95 and 96 repair the cross-site scripting errors illustrated in previous examples We have added both input validation and output encoding In Example 95, we make two changes to prevent cross-site scripting First, we implement input validation using a whitelist: The request parameter must match a regular expression built for validating identifiers Second, we use the JSTL <c:out> tag to perform output encoding Even if there is a way to sneak around the input validation , the name parameter won t cause any grief when it is displayed in the browser because metacharacters will be HTML encoded Example 96 also implements both input validation and output encoding We check the name returned from the database against a regular expression (input validation) and URL-encode it (output encoding)
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Example 95 The code from Example 92 revised to include input validation and output encoding to prevent XSS
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<c:if test="${paramsayHello}"> <!-- Let's welcome the user ${paramname} --> <% String name = requestgetParameter("name"); if (!ID_REGEXmatcher(name)matches()) { throw new ValidationException("invalid name"); } %> Hello <c:out value="${paramname}"/>! </c:if>
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Example 96 The code from Example 94 revised to include input validation and output encoding to prevent XSS
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String query = "select * from emp where id= "; PreparedStatement stmt = connprepareStatement(query); stmtsetString(1, eid); ResultSet rs = stmtexecuteQuery(); if (rs != null) { rsnext(); String name = rsgetString("name"); if (!NAME_REGEXmatcher(name)matches()) { throw new ValidationException("invalid emp name"); }
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outprintln("Employee Name: "+URLEncoderencode(name, "UTF8")); }
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HTTP Response Splitting HTTP response splitting is similar to cross-site scripting, but an HTTP response splitting vulnerability allows an attacker to write data into an HTTP header This can give the attacker the ability to control the remaining headers and the body of the current response or even craft an entirely new HTTP response These capabilities lead to a number of potential attacks with a variety of deleterious effects, including cross-user defacement, Web and browser cache poisoning, cross-site scripting, and page hijacking [Klein, 2004] First, we take a look at an HTTP response splitting vulnerability and then further examine the rami cations To mount a successful exploit, the application must allow input that contains CR (carriage return, also given by %0d or \r) and LF (line feed, given by %0a or \n) characters into the header These characters not only give attackers control of the remaining headers and body of the HTTP response, but they also allow for the creation of a second, entirely new, HTTP response The following code segment reads a parameter named author from an HTTP request and sets it in a cookie as part of the HTTP response (Cookies are transmitted in a header eld in an HTTP response)
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String author = requestgetParameter("author"); Cookie cookie = new Cookie("author", author); cookiesetMaxAge(cookieExpiration); responseaddCookie(cookie);
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