Resolve all input to absolute or canonical representations before
Carefully check each input parameter against a rigorous positive
specification (whitelist) 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
Strategy: 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
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.
With Struts, write all data from form beans with the bean's filter
attribute set to true.
Strategy: 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.