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CHAPTER 7 s CLIENT-STATE MANIPULATION
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7.4. JavaScript
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JavaScript is a scripting language that can be used to write scripts that interact with web pages. JavaScript is a language that is separate and distinct from Java, but derives its name from its Java-like syntax. JavaScript code can be included within HTML web pages, and the code is executed by a JavaScript interpreter once downloaded to the web browser. We cover JavaScript in this chapter for two reasons: (1) sometimes programmers rely on JavaScript for tasks that they should not, and (2) sometimes attackers can use JavaScript to help construct attacks. We illustrate how using JavaScript carelessly can give rise to a security vulnerability in this section, and provide a description of how attackers can use JavaScript to construct more attacks in 10. In the following example, we show some JavaScript that can be used to help compute the price of an order: <HTML> <HEAD> <TITLE>Order Pizza</TITLE> </HEAD> <BODY> <FORM ACTION="submit_order" METHOD="GET" NAME="f"> How many pizzas would you like to order <INPUT TYPE="text" NAME="qty" VALUE="1" onKeyUp="computePrice();"> <INPUT TYPE="hidden" NAME="price" VALUE="5.50"><BR> <INPUT TYPE="submit" NAME ="Order" VALUE="Pay"> <INPUT TYPE="submit" NAME ="Cancel" VALUE="Cancel"> <SCRIPT> function computePrice() { f.price.value = 5.50 * f.qty.value; f.Order.value = "Pay $" + f.price.value } </SCRIPT> </BODY> </HTML> The preceding pizza order form looks similar to ones used earlier in the chapter, with just a few differences that help the browser compute the price of the order. First, the form has been given a name attribute that specifies that its name is f. The form is given a name so that JavaScript code elsewhere in the HTML can refer to the components of the form, such as the text field qty, which contains the user-specified number of pizzas to order; and the submit button named Order, which the user can click to execute the order. Second, an onKeyUp handler has been added to the qty text field. The onKeyUp handler tells the browser to call the computePrice() JavaScript function whenever the user has made a change to the qty text field. Third, the definition of the computePrice() JavaScript function has been included in the HTML using the <SCRIPT> tag. The computePrice() function first updates the value of the hidden price field based on the quantity the user has selected, and then updates the order submit button to read Pay $X, where X is the computed price. The page rendered by the browser for the preceding HTML and JavaScript code is shown in Figure 7-3.
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CHAPTER 7 s CLIENT-STATE MANIPULATION
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Figure 7-3. JavaScript HTML order page In the preceding example, the client browser computes the price to be paid based on the number of pizzas the user would like to order. However, as you learned before, you cannot trust the client! A malicious user could simply save the HTML page to disk (as we illustrated earlier in this chapter), delete the JavaScript from the HTML page, substitute 10000 for the quantity and 0 for the price, and submit the form. Alternatively, a malicious user could also just submit an HTTP request such as GET /submit_order qty=1000&price=0&Order=Pay and completely bypass the price computation done by the JavaScript! The solution to eliminating the problem of not being able to trust the client, in this case, is to do the price computation on the server, and charge the user the price that is computed by the server. While JavaScript can be used to make the web page more interactive for the client, any data validation or computations done by the JavaScript cannot be trusted by the server. The computations must be redone on the server to ensure security.
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CHAPTER
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SQL Injection
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n this chapter, you will see that exploiting buffer overflow vulnerabilities in C programs is not the only way for an attacker to take control of a running system. Rather, an attacker might exploit a different class of vulnerabilities that can arise when untrusted data is evaluated in the context of a command or query language. Here, you ll study SQL injection vulnerabilities as an example of this class of security issues. SQL injection vulnerabilities can affect applications that use untrusted input in an SQL query made to a database back end without taking precautions to sanitize the data. SQL injection is a type of a more general class of vulnerabilities, referred to as command injection vulnerabilities. In general, command injection vulnerabilities can arise when untrusted (e.g., end-user supplied) data is inserted into a query or command, and specially crafted malicious input can cause the command interpreter or query processor to misinterpret part of the supplied data as a command, or otherwise alter the intended semantics of the command or query. In addition to SQL queries, this issue can occur if an application executes shell commands, makes queries to an LDAP server, uses XPath expressions to extract data from an XML document, interprets untrusted data as part of an XSLT style sheet, and so forth.
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