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Status: Draft Weakness ID: 87 (Weakness Variant)Description Summary The software fails to adequately filter user-controlled input for alternate script syntax. Potential Mitigations Resolve all filtered input to absolute or canonical representations before processing. Carefully check each input parameter against a rigorous positive specification (white list) defining the specific characters and format allowed. All input should be sanitized, not just parameters that the user is supposed to specify, but all data in the request, including tag attributes, hidden fields, cookies, headers, the URL itself, and so forth. A common mistake that leads to continuing XSS vulnerabilities is to validate only fields that are expected to be redisplayed by the site. We often encounter data from the request that is reflected by the application server or the application that the development team did not anticipate. Also, a field that is not currently reflected may be used by a future developer. Therefore, validating ALL parts of the HTTP request is recommended. This involves "HTML Entity Encoding" all non-alphanumeric characters from data that was received from the user and is now being written to the request. With Struts, you should write all data from form beans with the bean's filter attribute set to true. Additionally, to help mitigate XSS attacks against the user's session cookie, set the session cookie to be HttpOnly. In browsers that support the HttpOnly feature (such as Internet Explorer), this attribute prevents the user's session cookie from being accessed by client-side scripts, including scripts inserted due to a XSS attack. Demonstrative Examples In the following example, an XSS sanitization routine checks for the lower-case "script" string but fails to account for alternate strings ("SCRIPT", for example). Java Example: public String sanitize(String input, String mask) { return input.replaceAll("script", mask); }
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Applicable Platforms Languages All Time of Introduction ImplementationContent History Submissions PLOVER. (Externally Mined) Modifications Sean Eidemiller. Cigital. 2008-07-01. (External) added/updated demonstrative examples Eric Dalci. Cigital. 2008-07-01. (External) updated Time_of_Introduction CWE Content Team. MITRE. 2008-09-08. (Internal) updated Name, Relationships, Taxonomy_Mappings Previous Entry Names Alternate XSS Syntax (changed 2008-09-09) |
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