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A cross-site scripting attack occurs when a web application executes a script that the attacker supplied to end users. This flaw can be found anywhere in an application where user input has been taken but not properly encoded.
If the input is not properly encoded and sanitized, this injected malicious script will be sent to users. And a browser has no way to know that it should not trust a script. When the browser executes the script, a malicious action is performed on the client side. Most of the times, XSS is used to steal cookies and steal session tokens of a valid user to perform session hijacking.
You see a search box on almost all websites. With this search box, you can search to find anything available on the website. This search form looks something like this. On the search. Whatever a person searches for, it will be displayed on the web page along with search results. Now think what happens if an attacker tries to inject malicious script from this side.
If web application has nothing implemented to encode input and filter malicious scripts, it will take input as it is and then print on webpage where it will be called.
So, at the keyword place, it will look like this:. Suppose there is a website with a messaging feature. In this website, users can send messages to their contacts. A basic form will look something like this:. When this form is submitted, the message will be stored in the database. Another person will see the message when he opens the message from the inbox. Suppose an attacker has sent some cookie-stealing script in the message. This script will be stored on the website as a message.
Non-persistent XSS is also known as reflected cross-site vulnerability. It is the most common type of XSS. In this, data injected by attacker is reflected in the response. If you take a look at the examples we have shown above, the first XSS example was a non-persistent attack.
Persistent cross-site scripting is also known as stored cross-site scripting. It occurs when XSS vectors are stored in the website database and executed when a page is opened by the user. Every time the user opens the browser, the script executes. In the above examples, the second example of messaging a website was a persistent XSS attack. Persistent XSS is more harmful that non-persistent XSS, because the script will automatically execute whenever the user opens the page to see the content.
On the client side, the HTTP response does not change but the script executes in malicious manner. Attacks using malicious CSS. These typically define their own escaping system. For example, in Twig, you can use the e filter, with an argument defining the context:. Some other template engines, such as Jinja and React, escape dynamic content by default which effectively prevents most occurrences of XSS. We recommend reviewing escaping features closely when you evaluate whether to use a given template engine or framework.
If you directly concatenate user input into template strings, you will be vulnerable to server-side template injection which is often more serious than XSS. In PHP there is a built-in function to encode entities called htmlentities. You should call this function to escape your input when inside an HTML context.
The function should be called with three arguments:. When in a JavaScript string context, you need to Unicode-escape input as already described. Here is some code to do that in PHP:. If your input is inside a JavaScript string, you need an encoder that performs Unicode escaping.
Here is a sample Unicode-encoder:. Web developers would often use location. If an event handler is properly quoted, breaking out requires the corresponding quote.
However, we have intentionally made this rule quite broad because event handler attributes are often left unquoted. Please note this is an aggressive encoding policy that over-encodes. If there is a guarantee that proper quoting is accomplished then a much smaller character set is needed.
In a Web 2. One strategy is to make an AJAX call to get the values, but this isn't always performant. Often, an initial block of JSON is loaded into the page to act as a single place to store multiple values. This shall instruct the browser not misunderstand the context and execute injected script. Consider the Yahoo JavaScript Serializer for this task. This technique has the advantage that HTML entity encoding is widely supported and helps separate data from server side code without crossing any context boundaries.
The JavaScript that reads the span can live in an external file, thus making the implementation of CSP enforcement easier. Rule 4 is for when you want to put untrusted data into a style sheet or a style tag. CSS is surprisingly powerful, and can be used for numerous attacks. Therefore, it's important that you only use untrusted data in a property value and not into other places in style data.
You should stay away from putting untrusted data into complex properties like url , behavior , and custom -moz-binding. You should also not put untrusted data into IE's expression property value which allows JavaScript. You will have to ensure that URLs only start with http not javascript and that properties never start with "expression".
If attribute is quoted, breaking out requires the corresponding quote. All attributes should be quoted but your encoding should be strong enough to prevent XSS when untrusted data is placed in unquoted contexts. Please note that we recommend aggressive CSS encoding and validation to prevent XSS attacks for both quoted and unquoted attributes. All attributes should be quoted. Note that entity encoding is useless in this context. If untrusted input is meant to be placed into href , src or other URL-based attributes, it should be validated to make sure it does not point to an unexpected protocol, especially javascript links.
URLs should then be encoded based on the context of display like any other piece of data. If your application handles markup -- untrusted input that is supposed to contain HTML -- it can be very difficult to validate.
Encoding is also difficult, since it would break all the tags that are supposed to be in the input. Therefore, you need a library that can parse and clean HTML formatted text.
Customization of the safe list only affects encoders sourced via DI. If you directly access an encoder via System. Default then the default, Basic Latin only safelist will be used. The general accepted practice is that encoding takes place at the point of output and encoded values should never be stored in a database. Encoding at the point of output allows you to change the use of data, for example, from HTML to a query string value.
It also enables you to easily search your data without having to encode values before searching and allows you to take advantage of any changes or bug fixes made to encoders. Validation can be a useful tool in limiting XSS attacks. For example, a numeric string containing only the characters won't trigger an XSS attack. Validation becomes more complicated when accepting HTML in user input. Parsing HTML input is difficult, if not impossible.
Markdown, coupled with a parser that strips embedded HTML, is a safer option for accepting rich input. Never rely on validation alone. Always encode untrusted input before output, no matter what validation or sanitization has been performed. Feedback will be sent to Microsoft: By pressing the submit button, your feedback will be used to improve Microsoft products and services.
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