How Can You Modify Request Parameters with Servlet Filters Without Altering Source Code?

Modifying Request Parameters with Servlet Filters

Despite issues with an existing web application, the source code cannot be altered. To address an XSS vulnerability, a servlet filter is considered to sanitize parameters. However, the lack of a setParameter method in ServletRequest poses a challenge.

Solution 1: HttpServletRequestWrapper

One approach is to utilize the HttpServletRequestWrapper class. By subclassing this class and overriding the getParameter method, you can intercept and return the sanitized value. This wrapped request can then be passed to chain.doFilter instead of the original.

Solution 2: Request Attributes

A cleaner solution involves modifying the original servlet/JSP to accept a request attribute instead of a parameter. The filter would examine the parameter, sanitize it, and set the attribute using request.setAttribute. This method avoids the need for subclassing or spoofing, but requires modifications to other application components.

Considerations

Using HttpServletRequestWrapper complies with the servlet API. However, some servlet containers may raise objections if you attempt to pass an altered request to doFilter.

The request attribute approach offers a more elegant solution but requires additional code changes. Ultimately, the choice depends on the specific application and the level of access to the application code.

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