One of the most frequent targets are websites that allow users to share content, including blogs, social networks, video sharing platforms and message boards. Register Now. The embedded tags become a permanent feature of the page, causing the browser to parse them with the rest of the source code every time the page is opened. The attacker adds the following comment: Great price for a great item!
Meanwhile, the visitor, who may never have even scrolled down to the comments section, is not aware that the attack took place. Unlike a reflected attack, where the script is activated after a link is clicked, a stored attack only requires that the victim visit the compromised web page. This increases the reach of the attack, endangering all visitors no matter their level of vigilance.
A web application firewall WAF is the most commonly used solution for protection from XSS and web application attacks. WAFs employ different methods to counter attack vectors. In the case of XSS, most will rely on signature based filtering to identify and block malicious requests.
Imperva cloud WAF is offered as a managed service, regularly maintained by a team of security experts who are constantly updating the security rule set with signatures of newly discovered attack vectors. Imperva crowdsourcing technology automatically collects and aggregates attack data from across its network, for the benefit of all customers. XSS flaws can be difficult to identify and remove from a web application.
The best way to find flaws is to perform a security review of the code and search for all places where input from an HTTP request could possibly make its way into the HTML output. Nessus, Nikto, and some other available tools can help scan a website for these flaws, but can only scratch the surface.
If one part of a website is vulnerable, there is a high likelihood that there are other problems as well. An attacker can steal cookie data via Javascript even when document. If we need to hide against web application filters we may try to encode string characters, e. We may encode our script in base64 and place it in META tag.
This way we get rid of alert totally. More information about this method can be found in RFC Cross-site scripting attacks may occur anywhere that possibly malicious users are allowed to post unregulated material to a trusted website for the consumption of other valid users.
The most common example can be found in bulletin-board websites which provide web based mailing list-style functionality. The code in this example operates correctly if eid contains only standard alphanumeric text. You can start by referring them to this page. In such cases, use a trusted and verified library to parse and clean HTML.
Choose the library depending on your development language, for example, HtmlSanitizer for. If you do, such cookies will not be accessible via client-side JavaScript. CSP is an HTTP response header that lets you declare the dynamic resources that are allowed to load depending on the request source. You should regularly scan your web applications using a web vulnerability scanner such as Acunetix. If you use Jenkins, you should install the Acunetix plugin to automatically scan every build.
Frequently asked questions How does Cross-site Scripting work? Note that about one in three websites is vulnerable to Cross-site scripting. Learn more about the current state of web security. For example, an attacker may use it to steal user credentials and log in to your website as that user. If that user is an administrator, the attacker gains control over your website. See an example of a dangerous XSS attack from the past.
To discover Cross-site Scripting, you may either perform manual penetration testing or first use a vulnerability scanner. If you use a vulnerability scanner, it will save you a lot of time and money because your penetration testers can then focus on more challenging vulnerabilities. To protect against Cross-site Scripting, you must scan your website or web application regularly or at least after every chance in the code.
Then, your developers must correct the code to eliminate the vulnerability. Opportunities for exploiting XSS vulnerabilities are not limited to executable scripts. If an attacker has an Internet server, malicious scripts can be loaded directly from it. An attacker could deploy the following script to capture keystrokes:.
The script here implements a keystroke interceptor that saves the corresponding character and timestamp to the internal buffer. It also implements a function that sends data stored in the buffer twice per second to the evil. In order to embed this keylogger script on a target web page, actors can use the following attack vector:. After the exploit is triggered, the user's keystrokes on the web page will be redirected to the attacker server:.
The screenshot shows entries from the event log on the attacker server. In this example, the user has typed "James" on the keyboard. These records show keystrokes presented in JSON format: the field "k" contains a character and the field "t" contains the corresponding timestamp. From these examples and attack vectors, it is clear that a successful XSS attack on a vulnerable web application gives attackers a very powerful tool.
With XSS, attackers have the capability to:. Cross-site scripting attacks can also be leveraged for financial benefit in more indirect ways.
Impact typically depends on the type of XSS vulnerability for example, stored or reflected , difficulty of implementation, and whether it requires authentication perhaps not everybody has access to the page in question. Other factors include what, if any, additional actions are required from the user; whether the attack is triggered reliably; and what exactly could a potential attacker gain. If the site does not contain private information because of there being no authentication or distinction between users , then the impact is minimal.
A website might have stored XSS, resulting in High impact. However, if you need a certain level of access to visit that site, then the impact is reduced to Medium. It's also important to mention that in any case, impact depends on the author's assessment of criticality—researchers have their own viewpoints. XSS vulnerabilities can be of high severity, but typically they receive scores below those given to other types of attacks.
Severity levels are those valid as of the vulnerability publication date. A vulnerability in Wonderware Information Server allows attackers to inject arbitrary code into a web page viewed by other users or to bypass client-side security in web browsers. The attack can be initiated remotely and no authentication is required for successful exploitation. The best way to test your own application, or one for which you have source code, is by combining manual and automated techniques.
Static code analysis should be able to detect a number of XSS vulnerabilities. How well detection works depends heavily on the scanner.
Different scanners vary in vectors and techniques, so some will be more reliable than others, and none of them will be perfect. For example, there is a chance that a manual tester will be able to find issues that a black-box scanner missed. Another hazard to bear in mind is the possibility of false positives. Combining techniques and tools will improve the outcome, but certain issues will still take manual work to identify.
Here is a video from our colleague with an example. This particular case involves vulnerabilities in the Acorn JavaScript parser. Many other parsers have this vulnerability as well. If the parser can't recognize the JavaScript code in any part of the page, then this code will not be correctly passed to the analyzer. This means that by tricking the parser, it is possible to make a successful XSS attack that bypasses the scanner entirely.
The specific code that is not recognized by the Acorn parser as of the time of the video's release is presented below. In both cases, the program did not find potential XSS vulnerabilities in the code. We can conclude that manual testing is likely the most effective method—as long you know what you're doing.
Trial and error are unavoidable. But if you inject code, check the resulting HTML page, and see what happens after you change the vector, you are sure to find things.
BurpSuite or Acunetix can automate this process. After automatic verification, make sure to manually check filtering on any form of text input. The next step is analyzing the JavaScript code of the project. BlueClosure can automatically test the entire frontend. After eliminating vulnerabilities that can be found automatically, pay special attention to where the application displays user input and where it's passed to the server and subsequently being saved to the database.
Then consider not only the JavaScript code, but all parts of the system as a whole. For example, some elements involve turning user input data into links or other hypertext elements. Embedding a link like "javascript: alert 1 " in a website field in a user profile is a very frequent vector in successful attacks. Any parser that converts text to HTML potentially opens the door to malicious code. To get a better understanding of XSS vulnerabilities, let's analyze each of the major threat vectors.
This one is a relatively simple XSS script. It can be placed as an external script reference external payload or embedded within the script tag itself. These are embedded in many different tags. This list is not comprehensive. Standalone scanners are mostly self-written by pentesters, who adjust them as needed.
Most XSS testing tools are a part of larger, more comprehensive vulnerability scanners. Research shows that most vulnerabilities are caused by errors in source code. Therefore, it is important that your cybersecurity arsenal include a comprehensive source code analysis solution, such as PT Application Inspector.
This is an enterprise solution that combines static, dynamic, and interactive approaches to make testing robust and thorough. For a more detailed look at how PT AI performs in practice, we've prepared a quick rundown of its capabilities at the end of this article. From a technical point of view, XSS is an injection-class vulnerability, in which the attacker manipulates the logic of the web application in a browser.
So to prevent such vulnerabilities, one needs to thoroughly check any data that enters the application from the outside. To do so, the application must implement a number of approaches, which we describe here. User input is initially presented in string form. This data should be transformed into objects of a specified type.
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