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Consider a template fragment of the form <form ...> <input name="query" value="%(query)s"> </form>
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If an attacker is able to cause the variable query to contain, for example cookies"><script>evil-script;</script> then, after substitution, this will result in the HTML <form ...> <input name="query" value="cookies"><script>evil-script;</script>"> </form> That is, the attacker is able to close the quote and insert a script tag that will be executed by the browser.
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Attribute-Injection Attacks
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A variation of the attack in the previous section is possible if the attribute s value is not enclosed in quotes in the template. Consider a template fragment in which the attribute s value is not enclosed in quotes, for example <img src=%(image_url)s> Suppose the attacker is able to cause the variable image_url to contain http://www.examplesite.org/ onerror=evil-script; After substitution, this will result in the HTML fragment <img src=http://www.examplesite.org/ onerror=evil-script;> Browsers are usually lenient in their parsing of HTML attributes, and assume that an attribute whose value is not enclosed in quotes ends at the first whitespace character or the end of the tag. Thus, the preceding HTML will be parsed as an <img> tag with two attributes (i.e., the attacker was able to inject an additional attribute). The ability to inject an arbitrary attribute can often be exploited to execute arbitrary script. In the preceding example, the attacker arranged to inject an onerror attribute, which specifies an error handler in the form of a JavaScript snippet that the browser evaluates if evaluation of the tag resulted in an error condition. In the example, the attacker forces the error condition by supplying a URL that does not resolve into an image document (i.e., the URL can be a valid, resolvable URL that returns an HTML or other non-image document). Besides the onerror handler, other handler attributes, such as onload, or handlers for various DOM events, such as onmouseover, may be usable in an exploit (though the latter usually requires user interaction to be triggered). It should be noted that this attribute-injection attack did not require the injection of any HTML metacharacters (angle brackets or quotes) that would be commonly escaped or filtered. We also note that it is quite possible to craft malicious script payloads without using quote characters (it may be tempting to assume that it is difficult for an attacker to do anything damaging without being able to specify string constants for instance, to refer to their server s URL).
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Any string that is possibly derived from untrusted data and is inserted into the value of an HTML tag s attribute must be HTML-escaped using the HTML escape function introduced in Section 10.5.1. Furthermore, the attribute s value must be enclosed in double quotes.
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The entire attribute value must be enclosed in quotes to prevent attribute-injection attacks. First, it is necessary to escape the quote character that is used to delimit the attribute s value to prevent the closing the quote attack. While the HTML specification allows either double or single quotes to be used to enclose attributes, it is advisable to decide on a convention and use one type of quote throughout the application. It is nevertheless advisable to use an HTML escaping function that escapes both types of quotes, in case of deviation from the convention. Second, it is necessary to escape the ampersand character. Older versions of the Netscape browser support so-called JavaScript entities (see Netscape s JavaScript Guide ). This allows a string of the form &{javascript_expression}; to be used within attributes; the expression is evaluated and the entire entity expression is replaced with the result of this evaluation. An attacker who is able to inject ampersand and curly-brace characters into an attribute could be able to execute malicious script. While non-escaped angle brackets in attribute values do not result in XSS vulnerabilities in popular browsers, it is safest to escape them nevertheless. This also ensures that the resulting HTML is well-formed and allows you to use the same HTML-escaping function as elsewhere.
10.5.4. URL Attributes (href and src)
Attributes such as href and src take URLs as arguments. Depending on the tag they are associated with, the URL may be interpreted, de-referenced, or loaded at the time the browser interprets the tag (e.g., <img src=...> tags), or loaded only when the user performs an action (e.g., <a href=...> tags). If the value of the URL attribute is computed dynamically and may be influenced by a attacker, the attacker can make the URL refer to a resource that we did not intend. This could result in all kinds of problems (e.g., page spoofing), but may in particular result in injection of malicious script.
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