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Failure to Sanitize Script-Related HTML Tags in a Web Page (Basic XSS) Status: Incomplete Weakness ID: 80 (Weakness Variant)Description Summary The web application fails to adequately filter user-controlled input and sanitize its own output for any special characters, such as "<", ">", and "&". Extended Description This may allow such characters to be treated as control characters, which are executed client-side in the context of the user's session. Although this can be classified as an injection problem, the more pertinent issue is the failure to convert such special characters to respective context-appropriate entities before displaying them to the user. Likelihood of Exploit High to Very High Weakness Ordinalities Primary (where the weakness exists independent of other weaknesses) Causal Nature Explicit (an explicit
weakness resulting from behavior of the developer) Potential Mitigations 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 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, a guestbook comment isn't properly sanitized for script-related tags before being displayed in a client browser. Java Example: <% for (Iterator i = guestbook.iterator(); i.hasNext(); ) { Entry e = (Entry) i.next(); %> <p>Entry #<%= e.getId()
%></p> <p><%= e.getText()
%></p> <% } %> Observed Examples
Relationships
Taxonomy Mappings
Applicable Platforms Languages All Time of Introduction  ImplementationRelated Attack Patterns
White Box Definitions A weakness where the code path has: 1. start statement that accepts input from HTML page 2. end statement that publishes a data item to HTML where a. the input is part of the data item and b. the input is undesirable Where "input is undesirable" is defined through the following scenarios: 1. input not validated 2. input incorrectly validated Content 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 KDM Analytics. 2008-08-01. (External) added/updated white box definitions CWE Content Team. MITRE. 2008-09-08. (Internal) updated Relationships, Taxonomy_Mappings, Weakness_Ordinalities CWE Content Team. MITRE. 2008-10-14. (Internal) updated Description Previous Entry Names Basic XSS (changed 2008-04-11) |
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