Related
How do I prevent XSS (cross-site scripting) using just HTML and PHP?
I've seen numerous other posts on this topic but I have not found an article that clear and concisely states how to actually prevent XSS.
Basically you need to use the function htmlspecialchars() whenever you want to output something to the browser that came from the user input.
The correct way to use this function is something like this:
echo htmlspecialchars($string, ENT_QUOTES, 'UTF-8');
Google Code University also has these very educational videos on Web Security:
How To Break Web Software - A look at security vulnerabilities in
web software
What Every Engineer Needs to Know About Security
and Where to Learn It
One of the most important steps is to sanitize any user input before it is processed and/or rendered back to the browser. PHP has some "filter" functions that can be used.
The form that XSS attacks usually have is to insert a link to some off-site javascript that contains malicious intent for the user. Read more about it here.
You'll also want to test your site - I can recommend the Firefox add-on [XSS Me]. Looks like Easy XSS is now the way to go.
Cross-posting this as a consolidated reference from the SO Documentation beta which is going offline.
Problem
Cross-site scripting is the unintended execution of remote code by a web client. Any web application might expose itself to XSS if it takes input from a user and outputs it directly on a web page. If input includes HTML or JavaScript, remote code can be executed when this content is rendered by the web client.
For example, if a 3rd party side contains a JavaScript file:
// http://example.com/runme.js
document.write("I'm running");
And a PHP application directly outputs a string passed into it:
<?php
echo '<div>' . $_GET['input'] . '</div>';
If an unchecked GET parameter contains <script src="http://example.com/runme.js"></script> then the output of the PHP script will be:
<div><script src="http://example.com/runme.js"></script></div>
The 3rd party JavaScript will run and the user will see "I'm running" on the web page.
Solution
As a general rule, never trust input coming from a client. Every GET parameter, POST or PUT content, and cookie value could be anything at all, and should therefore be validated. When outputting any of these values, escape them so they will not be evaluated in an unexpected way.
Keep in mind that even in the simplest applications data can be moved around and it will be hard to keep track of all sources. Therefore it is a best practice to always escape output.
PHP provides a few ways to escape output depending on the context.
Filter Functions
PHPs Filter Functions allow the input data to the php script to be sanitized or validated in many ways. They are useful when saving or outputting client input.
HTML Encoding
htmlspecialchars will convert any "HTML special characters" into their HTML encodings, meaning they will then not be processed as standard HTML. To fix our previous example using this method:
<?php
echo '<div>' . htmlspecialchars($_GET['input']) . '</div>';
// or
echo '<div>' . filter_input(INPUT_GET, 'input', FILTER_SANITIZE_SPECIAL_CHARS) . '</div>';
Would output:
<div><script src="http://example.com/runme.js"></script></div>
Everything inside the <div> tag will not be interpreted as a JavaScript tag by the browser, but instead as a simple text node. The user will safely see:
<script src="http://example.com/runme.js"></script>
URL Encoding
When outputting a dynamically generated URL, PHP provides the urlencode function to safely output valid URLs. So, for example, if a user is able to input data that becomes part of another GET parameter:
<?php
$input = urlencode($_GET['input']);
// or
$input = filter_input(INPUT_GET, 'input', FILTER_SANITIZE_URL);
echo 'Link';
Any malicious input will be converted to an encoded URL parameter.
Using specialised external libraries or OWASP AntiSamy lists
Sometimes you will want to send HTML or other kind of code inputs. You will need to maintain a list of authorised words (white list) and un-authorized (blacklist).
You can download standard lists available at the OWASP AntiSamy website. Each list is fit for a specific kind of interaction (ebay api, tinyMCE, etc...). And it is open source.
There are libraries existing to filter HTML and prevent XSS attacks for the general case and performing at least as well as AntiSamy lists with very easy use.
For example you have HTML Purifier
In order of preference:
If you are using a templating engine (e.g. Twig, Smarty, Blade), check that it offers context-sensitive escaping. I know from experience that Twig does. {{ var|e('html_attr') }}
If you want to allow HTML, use HTML Purifier. Even if you think you only accept Markdown or ReStructuredText, you still want to purify the HTML these markup languages output.
Otherwise, use htmlentities($var, ENT_QUOTES | ENT_HTML5, $charset) and make sure the rest of your document uses the same character set as $charset. In most cases, 'UTF-8' is the desired character set.
Also, make sure you escape on output, not on input.
Many frameworks help handle XSS in various ways. When rolling your own or if there's some XSS concern, we can leverage filter_input_array (available in PHP 5 >= 5.2.0, PHP 7.)
I typically will add this snippet to my SessionController, because all calls go through there before any other controller interacts with the data. In this manner, all user input gets sanitized in 1 central location. If this is done at the beginning of a project or before your database is poisoned, you shouldn't have any issues at time of output...stops garbage in, garbage out.
/* Prevent XSS input */
$_GET = filter_input_array(INPUT_GET, FILTER_SANITIZE_STRING);
$_POST = filter_input_array(INPUT_POST, FILTER_SANITIZE_STRING);
/* I prefer not to use $_REQUEST...but for those who do: */
$_REQUEST = (array)$_POST + (array)$_GET + (array)$_REQUEST;
The above will remove ALL HTML & script tags. If you need a solution that allows safe tags, based on a whitelist, check out HTML Purifier.
If your database is already poisoned or you want to deal with XSS at time of output, OWASP recommends creating a custom wrapper function for echo, and using it EVERYWHERE you output user-supplied values:
//xss mitigation functions
function xssafe($data,$encoding='UTF-8')
{
return htmlspecialchars($data,ENT_QUOTES | ENT_HTML401,$encoding);
}
function xecho($data)
{
echo xssafe($data);
}
<?php
function xss_clean($data)
{
// Fix &entity\n;
$data = str_replace(array('&','<','>'), array('&','<','>'), $data);
$data = preg_replace('/(&#*\w+)[\x00-\x20]+;/u', '$1;', $data);
$data = preg_replace('/(&#x*[0-9A-F]+);*/iu', '$1;', $data);
$data = html_entity_decode($data, ENT_COMPAT, 'UTF-8');
// Remove any attribute starting with "on" or xmlns
$data = preg_replace('#(<[^>]+?[\x00-\x20"\'])(?:on|xmlns)[^>]*+>#iu', '$1>', $data);
// Remove javascript: and vbscript: protocols
$data = preg_replace('#([a-z]*)[\x00-\x20]*=[\x00-\x20]*([`\'"]*)[\x00-\x20]*j[\x00-\x20]*a[\x00-\x20]*v[\x00-\x20]*a[\x00-\x20]*s[\x00-\x20]*c[\x00-\x20]*r[\x00-\x20]*i[\x00-\x20]*p[\x00-\x20]*t[\x00-\x20]*:#iu', '$1=$2nojavascript...', $data);
$data = preg_replace('#([a-z]*)[\x00-\x20]*=([\'"]*)[\x00-\x20]*v[\x00-\x20]*b[\x00-\x20]*s[\x00-\x20]*c[\x00-\x20]*r[\x00-\x20]*i[\x00-\x20]*p[\x00-\x20]*t[\x00-\x20]*:#iu', '$1=$2novbscript...', $data);
$data = preg_replace('#([a-z]*)[\x00-\x20]*=([\'"]*)[\x00-\x20]*-moz-binding[\x00-\x20]*:#u', '$1=$2nomozbinding...', $data);
// Only works in IE: <span style="width: expression(alert('Ping!'));"></span>
$data = preg_replace('#(<[^>]+?)style[\x00-\x20]*=[\x00-\x20]*[`\'"]*.*?expression[\x00-\x20]*\([^>]*+>#i', '$1>', $data);
$data = preg_replace('#(<[^>]+?)style[\x00-\x20]*=[\x00-\x20]*[`\'"]*.*?behaviour[\x00-\x20]*\([^>]*+>#i', '$1>', $data);
$data = preg_replace('#(<[^>]+?)style[\x00-\x20]*=[\x00-\x20]*[`\'"]*.*?s[\x00-\x20]*c[\x00-\x20]*r[\x00-\x20]*i[\x00-\x20]*p[\x00-\x20]*t[\x00-\x20]*:*[^>]*+>#iu', '$1>', $data);
// Remove namespaced elements (we do not need them)
$data = preg_replace('#</*\w+:\w[^>]*+>#i', '', $data);
do
{
// Remove really unwanted tags
$old_data = $data;
$data = preg_replace('#</*(?:applet|b(?:ase|gsound|link)|embed|frame(?:set)?|i(?:frame|layer)|l(?:ayer|ink)|meta|object|s(?:cript|tyle)|title|xml)[^>]*+>#i', '', $data);
}
while ($old_data !== $data);
// we are done...
return $data;
}
You are also able to set some XSS related HTTP response headers via header(...)
X-XSS-Protection "1; mode=block"
to be sure, the browser XSS protection mode is enabled.
Content-Security-Policy "default-src 'self'; ..."
to enable browser-side content security. See this one for Content Security Policy (CSP) details: http://content-security-policy.com/
Especially setting up CSP to block inline-scripts and external script sources is helpful against XSS.
for a general bunch of useful HTTP response headers concerning the security of you webapp, look at OWASP: https://www.owasp.org/index.php/List_of_useful_HTTP_headers
Use htmlspecialchars on PHP. On HTML try to avoid using:
element.innerHTML = “…”;
element.outerHTML = “…”;
document.write(…);
document.writeln(…);
where var is controlled by the user.
Also obviously try avoiding eval(var),
if you have to use any of them then try JS escaping them, HTML escape them and you might have to do some more but for the basics this should be enough.
The best way to protect your input it's use htmlentities function.
Example:
htmlentities($target, ENT_QUOTES, 'UTF-8');
You can get more information here.
How do I prevent XSS (cross-site scripting) using just HTML and PHP?
I've seen numerous other posts on this topic but I have not found an article that clear and concisely states how to actually prevent XSS.
Basically you need to use the function htmlspecialchars() whenever you want to output something to the browser that came from the user input.
The correct way to use this function is something like this:
echo htmlspecialchars($string, ENT_QUOTES, 'UTF-8');
Google Code University also has these very educational videos on Web Security:
How To Break Web Software - A look at security vulnerabilities in
web software
What Every Engineer Needs to Know About Security
and Where to Learn It
One of the most important steps is to sanitize any user input before it is processed and/or rendered back to the browser. PHP has some "filter" functions that can be used.
The form that XSS attacks usually have is to insert a link to some off-site javascript that contains malicious intent for the user. Read more about it here.
You'll also want to test your site - I can recommend the Firefox add-on [XSS Me]. Looks like Easy XSS is now the way to go.
Cross-posting this as a consolidated reference from the SO Documentation beta which is going offline.
Problem
Cross-site scripting is the unintended execution of remote code by a web client. Any web application might expose itself to XSS if it takes input from a user and outputs it directly on a web page. If input includes HTML or JavaScript, remote code can be executed when this content is rendered by the web client.
For example, if a 3rd party side contains a JavaScript file:
// http://example.com/runme.js
document.write("I'm running");
And a PHP application directly outputs a string passed into it:
<?php
echo '<div>' . $_GET['input'] . '</div>';
If an unchecked GET parameter contains <script src="http://example.com/runme.js"></script> then the output of the PHP script will be:
<div><script src="http://example.com/runme.js"></script></div>
The 3rd party JavaScript will run and the user will see "I'm running" on the web page.
Solution
As a general rule, never trust input coming from a client. Every GET parameter, POST or PUT content, and cookie value could be anything at all, and should therefore be validated. When outputting any of these values, escape them so they will not be evaluated in an unexpected way.
Keep in mind that even in the simplest applications data can be moved around and it will be hard to keep track of all sources. Therefore it is a best practice to always escape output.
PHP provides a few ways to escape output depending on the context.
Filter Functions
PHPs Filter Functions allow the input data to the php script to be sanitized or validated in many ways. They are useful when saving or outputting client input.
HTML Encoding
htmlspecialchars will convert any "HTML special characters" into their HTML encodings, meaning they will then not be processed as standard HTML. To fix our previous example using this method:
<?php
echo '<div>' . htmlspecialchars($_GET['input']) . '</div>';
// or
echo '<div>' . filter_input(INPUT_GET, 'input', FILTER_SANITIZE_SPECIAL_CHARS) . '</div>';
Would output:
<div><script src="http://example.com/runme.js"></script></div>
Everything inside the <div> tag will not be interpreted as a JavaScript tag by the browser, but instead as a simple text node. The user will safely see:
<script src="http://example.com/runme.js"></script>
URL Encoding
When outputting a dynamically generated URL, PHP provides the urlencode function to safely output valid URLs. So, for example, if a user is able to input data that becomes part of another GET parameter:
<?php
$input = urlencode($_GET['input']);
// or
$input = filter_input(INPUT_GET, 'input', FILTER_SANITIZE_URL);
echo 'Link';
Any malicious input will be converted to an encoded URL parameter.
Using specialised external libraries or OWASP AntiSamy lists
Sometimes you will want to send HTML or other kind of code inputs. You will need to maintain a list of authorised words (white list) and un-authorized (blacklist).
You can download standard lists available at the OWASP AntiSamy website. Each list is fit for a specific kind of interaction (ebay api, tinyMCE, etc...). And it is open source.
There are libraries existing to filter HTML and prevent XSS attacks for the general case and performing at least as well as AntiSamy lists with very easy use.
For example you have HTML Purifier
In order of preference:
If you are using a templating engine (e.g. Twig, Smarty, Blade), check that it offers context-sensitive escaping. I know from experience that Twig does. {{ var|e('html_attr') }}
If you want to allow HTML, use HTML Purifier. Even if you think you only accept Markdown or ReStructuredText, you still want to purify the HTML these markup languages output.
Otherwise, use htmlentities($var, ENT_QUOTES | ENT_HTML5, $charset) and make sure the rest of your document uses the same character set as $charset. In most cases, 'UTF-8' is the desired character set.
Also, make sure you escape on output, not on input.
Many frameworks help handle XSS in various ways. When rolling your own or if there's some XSS concern, we can leverage filter_input_array (available in PHP 5 >= 5.2.0, PHP 7.)
I typically will add this snippet to my SessionController, because all calls go through there before any other controller interacts with the data. In this manner, all user input gets sanitized in 1 central location. If this is done at the beginning of a project or before your database is poisoned, you shouldn't have any issues at time of output...stops garbage in, garbage out.
/* Prevent XSS input */
$_GET = filter_input_array(INPUT_GET, FILTER_SANITIZE_STRING);
$_POST = filter_input_array(INPUT_POST, FILTER_SANITIZE_STRING);
/* I prefer not to use $_REQUEST...but for those who do: */
$_REQUEST = (array)$_POST + (array)$_GET + (array)$_REQUEST;
The above will remove ALL HTML & script tags. If you need a solution that allows safe tags, based on a whitelist, check out HTML Purifier.
If your database is already poisoned or you want to deal with XSS at time of output, OWASP recommends creating a custom wrapper function for echo, and using it EVERYWHERE you output user-supplied values:
//xss mitigation functions
function xssafe($data,$encoding='UTF-8')
{
return htmlspecialchars($data,ENT_QUOTES | ENT_HTML401,$encoding);
}
function xecho($data)
{
echo xssafe($data);
}
<?php
function xss_clean($data)
{
// Fix &entity\n;
$data = str_replace(array('&','<','>'), array('&','<','>'), $data);
$data = preg_replace('/(&#*\w+)[\x00-\x20]+;/u', '$1;', $data);
$data = preg_replace('/(&#x*[0-9A-F]+);*/iu', '$1;', $data);
$data = html_entity_decode($data, ENT_COMPAT, 'UTF-8');
// Remove any attribute starting with "on" or xmlns
$data = preg_replace('#(<[^>]+?[\x00-\x20"\'])(?:on|xmlns)[^>]*+>#iu', '$1>', $data);
// Remove javascript: and vbscript: protocols
$data = preg_replace('#([a-z]*)[\x00-\x20]*=[\x00-\x20]*([`\'"]*)[\x00-\x20]*j[\x00-\x20]*a[\x00-\x20]*v[\x00-\x20]*a[\x00-\x20]*s[\x00-\x20]*c[\x00-\x20]*r[\x00-\x20]*i[\x00-\x20]*p[\x00-\x20]*t[\x00-\x20]*:#iu', '$1=$2nojavascript...', $data);
$data = preg_replace('#([a-z]*)[\x00-\x20]*=([\'"]*)[\x00-\x20]*v[\x00-\x20]*b[\x00-\x20]*s[\x00-\x20]*c[\x00-\x20]*r[\x00-\x20]*i[\x00-\x20]*p[\x00-\x20]*t[\x00-\x20]*:#iu', '$1=$2novbscript...', $data);
$data = preg_replace('#([a-z]*)[\x00-\x20]*=([\'"]*)[\x00-\x20]*-moz-binding[\x00-\x20]*:#u', '$1=$2nomozbinding...', $data);
// Only works in IE: <span style="width: expression(alert('Ping!'));"></span>
$data = preg_replace('#(<[^>]+?)style[\x00-\x20]*=[\x00-\x20]*[`\'"]*.*?expression[\x00-\x20]*\([^>]*+>#i', '$1>', $data);
$data = preg_replace('#(<[^>]+?)style[\x00-\x20]*=[\x00-\x20]*[`\'"]*.*?behaviour[\x00-\x20]*\([^>]*+>#i', '$1>', $data);
$data = preg_replace('#(<[^>]+?)style[\x00-\x20]*=[\x00-\x20]*[`\'"]*.*?s[\x00-\x20]*c[\x00-\x20]*r[\x00-\x20]*i[\x00-\x20]*p[\x00-\x20]*t[\x00-\x20]*:*[^>]*+>#iu', '$1>', $data);
// Remove namespaced elements (we do not need them)
$data = preg_replace('#</*\w+:\w[^>]*+>#i', '', $data);
do
{
// Remove really unwanted tags
$old_data = $data;
$data = preg_replace('#</*(?:applet|b(?:ase|gsound|link)|embed|frame(?:set)?|i(?:frame|layer)|l(?:ayer|ink)|meta|object|s(?:cript|tyle)|title|xml)[^>]*+>#i', '', $data);
}
while ($old_data !== $data);
// we are done...
return $data;
}
You are also able to set some XSS related HTTP response headers via header(...)
X-XSS-Protection "1; mode=block"
to be sure, the browser XSS protection mode is enabled.
Content-Security-Policy "default-src 'self'; ..."
to enable browser-side content security. See this one for Content Security Policy (CSP) details: http://content-security-policy.com/
Especially setting up CSP to block inline-scripts and external script sources is helpful against XSS.
for a general bunch of useful HTTP response headers concerning the security of you webapp, look at OWASP: https://www.owasp.org/index.php/List_of_useful_HTTP_headers
Use htmlspecialchars on PHP. On HTML try to avoid using:
element.innerHTML = “…”;
element.outerHTML = “…”;
document.write(…);
document.writeln(…);
where var is controlled by the user.
Also obviously try avoiding eval(var),
if you have to use any of them then try JS escaping them, HTML escape them and you might have to do some more but for the basics this should be enough.
The best way to protect your input it's use htmlentities function.
Example:
htmlentities($target, ENT_QUOTES, 'UTF-8');
You can get more information here.
I have a data javascript file, which is being dynamically added to website via some custom code.
This file comes from a third party vendor, who could potentially add malicious code in the file
Before this file is added to the website, I would like to parse through it, and look for malicious code, such as redirects or alerts, that inherently get executed upon a files inclusion in the project/website.
For example, my js file could look like this :
alert ('i am malicious');
var IAmGoodData =
[
{ Name :'test', Type:'Test2 },
{ Name :'test1', Type:'Test21' },
{ Name :'test2', Type:'Test22' }
]
I load this file into a object via a XMLHttpRequest call, and when this call returns, I can use the variable (which is my file text) and search it for words:
var client = new XMLHttpRequest();
client.open('GET', 'folder/fileName.js');
client.onreadystatechange = function()
{
ScanText(client.responseText);
}
client.send();
function ScanText(text)
{
alert(text);
var index = text.search('alert'); //Here i can search for keywords
}
The last line would return index of 0, as the word alert is found at index 0 in the file.
Questions:
Is there a more efficient way to search for keywords in the file?
What specific keywords should i be searching for to prevent malicious code being run? ie redirects, popups, sounds etc.....
Instead of having them include var IAmGoodData =, make them simply provide JSON (which is basically what the rest of the file is, or seems to be). Then you parse it as JSON, using JSON.parse(). If it fails, they either didn't follow the JSON format well, or have external code, and in either case you would ignore the response.
For example, you'd expect data from the external file like:
[
{ Name :'test', Type:'Test2' },
{ Name :'test1', Type:'Test21' },
{ Name :'test2', Type:'Test22' }
]
which needs to be properly serialized as JSON (double quotes instead of single quotes, and double quotes around the keys). In your code, you'd use:
var json;
try {
json = JSON.parse(client.responseText);
catch (ex) {
// Invalid JSON
}
if (json) {
// Do something with the response
}
Then you could loop over json and access the Name and Type properties of each.
Random Note:
In your client.onreadystatechange callback, make sure you check client.readyState === 4 && client.status === 200, to know that the request was successful and is done.
This is extremely difficult to do. There are no intrinsically malicious keywords or functions in JavaScript, there are malicious applications. You could be getting false positives for "malicious" activity and prevent a legitimate code with a real purpose from being executed. And at the same time, anyone with a little bit of imagination could bypass any "preventive" method you may implement.
I'd suggest you look for a different approach. This is one of those problems (like CAPTCHA) in which it's trivial for a human to solve while for a machine is practically impossible to do so. You could try having a moderator or some human evaluator to interpret the code and accept it.
You should have them provide valid JSON rather than arbitrary Javascript.
You can then call JSON.parse() to read their data without any risk of code execution.
In short, data is not code, and should not be able to contain code.
You shouldn't. The user should be allowed to type whatever they want, and it's your job to display it.
It all depends on where it is being put, of course:
Database: mysql_real_escape_string or equivalent for whatever engine you're using.
HTML: htmlspecialchars in PHP, createTextNode or .replace(/</g,"<") in JavaScript
JavaScript: json_encode in PHP, JSON.stringify in JavaScript.
At the end of the day, just don't be Yahoo
I have an application where users have somewhat of a settings page, and some of those settings allow URLs to be entered into them. I want to be able to run a check using javascript to make sure the URL they entered is valid and a real URL.
When they click submit, it will either do what it's supposed to do, or if there's an error in the URL, pop up with an alert and say "Invalid URL entered!".
Is there a way to do this with JS? I also want this script to work with http:// and https:// as well as www. in the URL, and every domain extension (.com, .tv, etc.). This also needs to be done with JavaScript, not jQuery.
Can anyone show me how to do this? Thanks.
It seems you're wanting to do two things:
1) Determine that the URL is valid (i.e. correctly formatted)
For this a regular expression will work well, this approach will also allow you to retrieve various parts of the URL if that's something you'd like to do.
this has been discussed here: What is the best regular expression to check if a string is a valid URL?.
2) Determine that the URL is real (i.e. if someone where to follow it they'd find something)
This is more tricky, but you could attempt an AJAX request to the URL and if it fails or times out assume it's down. There may be some limitations to this approach due to XSS security features on sites.
If that's a problem you could create a service of your own design that runs on a server that your JavaScript makes a request to, passing it the URL, and it responds with a failure or success.
Here's an example:
verify.js
function verifyURL (url) {
// with jQuery
$.getJSON('check-url.cgi', { url : url }, function (res) {
console.log(res); // display server response
if ( res.status == 'success' ) {
// URL is real
} else {
// URL is not real
}
});
}
check-url.cgi
#!/usr/bin/env perl
use v5.10;
use strict;
use warnings;
use CGI qw(:standard);
use JSON::XS;
use LWP::UserAgent;
my $ua = LWP::UserAgent->new;
$ua->agent("URL Checker/0.1");
my $url = param('url');
my $req = HTTP::Request->new(GET => $url);
my $res = $ua->request($req);
my $status = $res->is_success ? 'success' : 'failure';
print header('applicaton/json'), encode_json { status => $status };
boolean canModify = UTIL.hasSecurity("PFTMODFY") && something;
function name() {
I need to pass a value false to something when the page loads.
}
window.onLoad = name
How can i pass a value from JavaScript to JSP on the page load
I think you mean it the other way around, have server-side code output a value that JavaScript can see at page-load time.
Sending from your server code (JSP) to your client code (JavaScript)
Just output it like you would anything else, e.g.:
<%
boolean canModify = UTIL.hasSecurity("PFTMODFY") && something;
%>
var canModify = <%=canModify%>;
// ^ ^
// | +-- server-side variable
// +-- client-side variable
When the JSP actually runs, the script code returned to the client will simply be
var canModify = false;
or
var canModify = true;
That's a boolean; if you need to output a string, you need to put the quotes around it and be careful that you escape anything within it that should be escaped inside a JavaScript literal string (like a backslash, for instance, and whatever quote character you're using).
Sending from your client code (JavaScript) to your server code (JSP)
But if you really mean you want to send a value back to the server on page load (which seems odd, but hey), you'd have to use Ajax for that. If you're going to be doing Ajax stuff, I'd recommend using a library like jQuery, Closure, Prototype, YUI, or any of several others as they can dramatically simplify the process for you. For instance, using jQuery, this client-side code sends a value back to the server:
jQuery.get("/your/page/url", {"name": "value"});
(or jQuery.post for things that make changes). Using Prototype, it'd be:
new Ajax.Request("/your/page/url", {
method: "GET", // or, of couse, "POST"
parameters: {"name": "value"}
});
All of the libs I mentioned above have similarly easy-to-use mechanisms for sending data to the server. Note that this mechanism is bound by the Same Origin Policy.
You don't need a library for this, of course — anything a library can do, you can do yourself. The features I've listed above are all wrappers for the XMLHttpRequest object (Wikipedia, MSDN, MDC). See the linked resources for examples.
Non-Ajax hack
I shouldn't have said you need to use Ajax for this, more like you want to. :-) If, for some reason, you really didn't want to use Ajax you could do it by adding something to the page with JavaScript that triggers a retrieval from the server, and then include your value as a flag in that request. For instance:
var link = document.createElement('link');
link.setAttribute('rel', 'stylesheet');
link.src = "/path/to/jsp?name=" + encodeURIComponent(value);
document.getElementsByTagName('head')[0].appendChild(link);
Your JSP would do whatever it needs to do with the query string, and then return anything that's valid in a stylesheet (/* */, for instance). Or if you don't want to use a style sheet link, use an img tag and return a transparent pixel; etc.
But I wouldn't do that. I'd use Ajax.