The software does not neutralize or incorrectly neutralizes user-controlled input for alternate script syntax. 1000 699 Weakness ChildOf 79 888 Category ChildOf 896 Implementation Confidentiality Integrity Availability Read application data Execute unauthorized code or commands Implementation Resolve all input to absolute or canonical representations before processing. Implementation Carefully check each input parameter against a rigorous positive specification (white list) defining the specific characters and format allowed. All input should be neutralized, 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. Implementation Output Encoding Use and specify an output encoding that can be handled by the downstream component that is reading the output. Common encodings include ISO-8859-1, UTF-7, and UTF-8. When an encoding is not specified, a downstream component may choose a different encoding, either by assuming a default encoding or automatically inferring which encoding is being used, which can be erroneous. When the encodings are inconsistent, the downstream component might treat some character or byte sequences as special, even if they are not special in the original encoding. Attackers might then be able to exploit this discrepancy and conduct injection attacks; they even might be able to bypass protection mechanisms that assume the original encoding is also being used by the downstream component. The problem of inconsistent output encodings often arises in web pages. If an encoding is not specified in an HTTP header, web browsers often guess about which encoding is being used. This can open up the browser to subtle XSS attacks. Implementation With Struts, write all data from form beans with the bean's filter attribute set to true. Implementation Identify and Reduce Attack Surface 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 more recent versions of Internet Explorer and Firefox), this attribute can prevent the user's session cookie from being accessible to malicious client-side scripts that use document.cookie. This is not a complete solution, since HttpOnly is not supported by all browsers. More importantly, XMLHTTPRequest and other powerful browser technologies provide read access to HTTP headers, including the Set-Cookie header in which the HttpOnly flag is set. Defense in Depth In the following example, an XSS neutralization routine checks for the lower-case "script" string but does not account for alternate strings ("SCRIPT", for example). Java public String preventXSS(String input, String mask) { return input.replaceAll("script", mask); } CVE-2002-0738 XSS using "&={script}". Alternate XSS syntax 199 PLOVER Sean Eidemiller Cigital 2008-07-01 added/updated demonstrative examples Eric Dalci Cigital 2008-07-01 updated Time_of_Introduction CWE Content Team MITRE 2008-09-08 updated Name, Relationships, Taxonomy_Mappings CWE Content Team MITRE 2009-07-27 updated Related_Attack_Patterns CWE Content Team MITRE 2010-06-21 updated Demonstrative_Examples, Description, Name, Potential_Mitigations CWE Content Team MITRE 2010-12-13 updated Demonstrative_Examples CWE Content Team MITRE 2011-03-29 updated Potential_Mitigations CWE Content Team MITRE 2011-06-01 updated Common_Consequences CWE Content Team MITRE 2012-05-11 updated Relationships CWE Content Team MITRE 2012-10-30 updated Potential_Mitigations Alternate XSS Syntax Failure to Sanitize Alternate XSS Syntax