Cross Site Scripting Attacks Xss Exploits And Defense Pdf Writer

cross site scripting attacks xss exploits and defense pdf writer

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Stay tuned! When you imagine a cyberattack, what do you think of, exactly?

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This article provides a simple positive model for preventing XSS using output encoding properly. While there are a huge number of XSS attack vectors, following a few simple rules can completely defend against this serious attack. This article does not explore the technical or business impact of XSS. Suffice it to say that it can lead to an attacker gaining the ability to do anything a victim can do through their browser. Both reflected and stored XSS can be addressed by performing the appropriate validation and encoding on the server-side.

Watch What You Write : Preventing Cross-Site Scripting by Observing Program Output

Cross-Site Scripting XSS is probably the most common singular security vulnerability existing in web applications at large. XSS occurs when an attacker is capable of injecting a script, often Javascript, into the output of a web application in such a way that it is executed in the client browser. HTML has no shortage of locations where executable Javascript can be injected and browsers have even managed to add more. The injection is sent to the web application via any means of input such as HTTP parameters.

This is particularly true of PHP where poor information has overshadowed all other attempts to educate programmers. In addition, because XSS examples in the wild are of the simple variety programmers are not beyond justifying a lack of defenses when it suits them. This gives them complete control of the user experience. Back in my Introduction, I noted that trusting any data not created explicitly by PHP in the current request should be considered untrusted.

This sentiment extends to the browser which sits separately from your web application. The fact that the browser trusts everything it receives from the server is itself one of the root problems in Cross-Site Scripting.

We can extend this even further to the Javascript environment a web application introduces within the browser. Client side Javascript can range from the very simple to the extremely complex, often becoming client side applications in their own right.

These client side applications must be secured like any application, distrusting data received from remote sources including the server-hosted web application itself , applying input validation, and ensuring output to the DOM is correctly escaped or sanitised. While a distinct attack in its own right, UI Redress is tightly linked with Cross-Site Scripting since both leverage similar sets of vectors.

Sometimes it can be very hard to differentiate the two because each can assist in being successful with the other. When such attacks are intended to trick a user into clicking on an injected button or link it is usually referred to as Clickjacking. Investigating this further, the attacker sets up an account, spams all topics in reach, and uses the following markup in their signature which is attached to all of their posts:.

By some miracle, the forum software includes this signature as-is in all those spammed topics for all the forum users to load into their browsers.

The results should be obvious from the Javascript code. The attacker is injecting an iframe into the page which will appear as a teeny tiny dot zero sized at the very bottom of the page attracting no notice from anyone. This is a simple example but feel free to extend it. Perhaps the attacker would like to know the username associated with this cookie?

Perhaps they also need information about your browser to handle a Fingerprint defense of the session too? This simple attack has a lot of repercussions including potentially gaining control over the forum as an administrator. All cookies containing sensitive data should be tagged with the HttpOnly flag which prevents Javascript from accessing the cookie data. The principle you should remember, however, is that if the attacker can inject Javascript, they can probably inject all conceivable Javascript.

We could encapsulate this in a check to only run for a moderator, i. They block the logging of cookies by an attacker but do not actually prevent their use during an XSS attack. Furthermore, an attacker would prefer not to leave bread crumbs in the visible markup to arouse suspicion unless they actually want to be detected. Next time you see an example using the Javascript alert function, substitute it with a XMLHttpRequest object to avoid being underwhelmed.

XSS attacks can be categorised in two ways. The first lies in how malicious input navigates the web application. Input to an application can be included in the output of the current request, stored for inclusion in the output of a later request, or passed to a Javascript based DOM operation. This gives rise to the following categories:. Reflection can occur with error messages, search engine submissions, comment previews, etc. Getting a user to click untrusted links may require a bit of persuasion and involve emailing the target, mounting a UI Redress attack, or using a URL Shortener service to disguise the URL.

Social services are particularly vulnerable to shortened URLs since they are commonplace in that setting. Be careful of what you click! A Stored XSS attack is when the payload for the attack is stored somewhere and retrieved as users view the targeted data. While a database is to be expected, other persistent storage mechanisms can include caches and logs which also store information for long periods of time.

DOM-based XSS can be either reflected or stored and the differentiation lies in how the attack is targeted. Most attacks will strike at the immediate markup of a HTML document. There may also be security vulnerabilities in Javascript libraries or their usage which can also be targeted. An XSS attack is successful when it can inject Context. The goal of an attacker is to take data destined for one of these Contexts and make browser interpret it as another Context.

For example, consider the following:. It may seem unimportant to get so hooked up on Context but consider this:. Now, I was very careless with the above example because I know some readers will be desperate to get to the point of using escaping. This is the importance of understanding Context correctly.

Each Context requires a different method of escaping because each Context has different special characters and different escaping needs. You cannot just throw htmlspecialchars and htmlentities at everything and pray that your web application is safe. What went wrong in the above is that the browser will always unesape HTML Attributes before interpreting the context.

We ignored the fact there were TWO Contexts to escape for. By not recognising that our attribute encompassed two Contexts, we escaped it as if it was only one: a HTML Attribute.

A common mistake to make. The lesson here is that Context matters. In an XSS attack, the attacker will always try to jump out of the current Context into another one where Javascript can be executed. Late implementation of defenses can be a costly affair.

That said, Input Validation is limited to knowing what the immediate usage of an untrusted input is and cannot predict where that input will finally be used when included in output. Practically all free text falls into this category since we always need to allow for valid uses of quotes, angular brackets and other characters.

Therefore, validation works best by preventing XSS attacks on data which has inherent value limits. An integer, for example, should never contain HTML special characters. An option, such as a country name, should match a list of allowed countries which likewise will prevent XSS payloads from being injected. Input Validation can also check data with clear syntax constraints. In fact, all URLs derived from untrusted input must be validated for this very reason. We cover Input Validation in greater detail in Chapter 2.

Escaping data on output is a method of ensuring that the data cannot be misinterpreted by the currently running parser or interpreter. The obvious examples are the less-than and greater-than sign that denote element tags in HTML.

If we allowed these to be inserted by untrusted input as-is, it would allow an attacker to introduce new tags that the browser would render. As the replacement of such special characters suggests, the intent is to preserve the meaning of the data being escaped. Escaping simply replaces characters with special meaning to the interpreter with an alternative which is usually based on a hexadecimal representation of the character or a more readable representation, such as HTML named entities, where it is safe to do so.

As my earlier diversion into explaining Context mentioned, the method of escaping varies depending on which Content data is being injected into.

Applying the wrong escaping strategy to a Context can result in an escaping failure, opening a hole in a web applications defenses which an attacker may be able to take advantage of. PHP does not supply all the necessary escaping functionality out of the box and some of what it does offer is not as safe as popularly believed.

You can find an Escaper class which I designed for the Zend Framework, which offers a more approachable solution, here. This rule refers to injecting data in sensitive areas of HTML which offer an attacker the opportunity to influence markup parsing and which do not ordinarily require escaping when used by a programmer.

Consider the following examples where [ Each of the above locations are dangerous. Allowing data within script tags, outside of literal strings and numbers, would let an attack inject Javascript code. Data injected into HTML comments might be used to trigger Internet Explorer conditionals and other unanticipated results. Data injected into this content must be HTML escaped.

For all other attributes, however, you have the following two choices:. The second option also applies where attribute quoting style may be in doubt.

For example, it is perfectly valid in HTML5 to use unquoted attribute values and examples in the wild do exist. Ere on the side of caution where there is any doubt. Javascript data values are basically strings. The root element in all our discussions about Cross-Site Scripting has been that the browser unquestionably executes all the Javascript it receives from the server whether it be inline or externally sourced.

On receipt of a HTML document, the browser has no means of knowing which of the resources it contains are innocent and which are malicious. What if we could change that? Consider the following:. The browser will now grab scripts from this source but completely ignore all others. It also means that all inline scripts, i. You can add other resource directives, e. The format of the header value is very simple. The URL value is matched based on the information given.

The same thinking applies to paths, ports, URL scheme, etc. If you create a whitelist of a particular type of resource, anything not on that whitelist is ignored. For example:.

The above will limit the source for all resources to the current domain but add an exception for script-src to allow the jQuery CDN.

Cross Site Scripting Attacks: Xss Exploits and Defense

Web applications that allow users to store data are potentially exposed to this type of attack. This chapter illustrates examples of stored cross site scripting injection and related exploitation scenarios. Stored XSS occurs when a web application gathers input from a user which might be malicious, and then stores that input in a data store for later use. The input that is stored is not correctly filtered. Since this vulnerability typically involves at least two requests to the application, this may also called second-order XSS.

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The XSS is manipulated input parameters of an application with the aim of obtaining an output determined than usual to the operation of the system. Despite being a security issue in somewhat old, yet still appear new attack vectors and techniques that make is in constant evolution. The Cross-site scripting attack XSS non persistent; is a type of code injection in which it does not run with the web application, but arises when the victim load a particular URL in the context of the browser. If is "logged" on the application, we could hijack the session that keeps active and go through it. If injecting the sample code you see the session cookie in your browser, the parameter is vulnerable. The code with evasives will execute remotely the script in javascript from attacker's website.

XSS for fun and profit SCG09 (english) pdf

Cross-Site Scripting XSS is probably the most common singular security vulnerability existing in web applications at large. XSS occurs when an attacker is capable of injecting a script, often Javascript, into the output of a web application in such a way that it is executed in the client browser. HTML has no shortage of locations where executable Javascript can be injected and browsers have even managed to add more. The injection is sent to the web application via any means of input such as HTTP parameters. This is particularly true of PHP where poor information has overshadowed all other attempts to educate programmers.

A cross site scripting attack is a very specific type of attack on a web application. It is used by hackers to mimic real sites and fool people into providing personal data. First it discusses the concepts, methodology, and technology that makes XSS a valid concern. It then moves into the various types of XSS attacks, how they are implemented, used, and abused. After XSS is thoroughly explored, the next part provides examples of XSS malware and demonstrates real cases where XSS is a dangerous risk that exposes internet users to remote access, sensitive data theft, and monetary losses.

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XSS Attacks: Cross Site Scripting Exploits and Defense

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Topics include creating effective documents; using themes, templates, and other formatting tools; building

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Web applications that allow users to store data are potentially exposed to this type of attack.

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Anton Rager. Seth Fogie Technical Editor and Co-Author. XSS. Attacks. CROSS SITE SCRIPTING. EXPLOITS AND DEFENSE.