Input and Output Validation for the Web in Java

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If the author parameter consists of only standard alphanumeric characters, such as the string Jane Smith, the HTTP response might take the following form:
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HTTP/11 200 OK Set-Cookie: author=Jane Smith
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However, because the value of the cookie contains unvalidated user input, the response will maintain this form only if the value submitted for author does not include any CR and LF characters If an attacker submits a malicious string, such as Wiley Hacker\r\n\r\nHTTP/11 200 OK\r\n, the HTTP response will be split into two responses of the following form:
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HTTP/11 200 OK Set-Cookie: author=Wiley Hacker HTTP/11 200 OK
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Many Web browsers and Web proxies will mishandle a response that looks like this An attacker might be able to use the vulnerability to do the following: Provide malicious content to the victim browser This is similar to a re ected cross-site scripting attack Confuse a Web proxy This can result in the Web proxy sending the second HTTP response to a different user, or in the Web proxy sending a different user s data to the attacker Defending against HTTP response splitting is similar to defending against cross-site scripting Because HTTP response splitting vulnerabilities occur when an application includes unvalidated data in HTTP headers, one logical approach is to validate data immediately before they leave the application However, because Web applications often have complex and intricate code for generating responses dynamically, this method is prone to errors of omission (missing validation) An effective way to mitigate this risk is to also perform input validation for values that will be used as part of HTTP headers As always, a whitelist is preferable to a blacklist HTTP
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headers rarely require a great variety of characters, so a whitelist is almost always feasible If you must blacklist, the CR and LF characters are at the heart of an HTTP response splitting attack, but other characters, such as the colon (:) and equals sign (=), have special meaning in response headers as well Many application servers attempt to limit an application s exposure to HTTP response splitting vulnerabilities by automatically blacklisting CR and LF in methods that control HTTP headers We prefer not to rely on the application server in this respect Open Redirects Phishing involves luring potential victims to an attacker-controlled Web site masquerading as a trustworthy site, such as a bank or e-commerce site, that many users are likely to recognize Attackers typically target victims with authentic-looking e-mail messages that appear to originate from the target organization and inform the recipients that they must visit a link included in the e-mail to perform some action, such as verifying their online banking credentials When victims visit the site, the attacker harvests their credentials, which can then be used to defraud the victims For a more detailed discussion of phishing and ways to prevent it, we refer you to Phishing and Countermeasures [Jakobsson and Myers, 2006] But how does an attacker bait a victim into visiting a fake site One approach is to use a cross-site scripting vulnerability to inject malicious content into a legitimate site, but that requires a carefully crafted attack From a phisher s point of view, an even easier approach is a link that actually takes victims to a legitimate Web site but then immediately forwards them on to another site controlled by the attacker that harvests the sensitive information That s exactly what an open redirect enables Example 97 shows a bit of code that will happily forward a victim s browser on to whatever URL the attacker has provided The developer tried to do the right thing: She created a whitelist that correctly prevents HTTP response splitting attacks, but it still allows any URL through The best way to defend against such attacks is with a level of indirection Use the request parameter only to look up the URL in a table, as shown in Example 98
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