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.
But even this model provides a way to inject malicious JavaScript. This is often done via hash links used for navigation across the SPA. For a detailed explanation of this type of cross-site scripting complete with code examples, see our article about DOM-based XSS. As with all injection attacks, the root cause of cross-site scripting vulnerabilities is insufficient validation and sanitization of user inputs. To prevent XSS security vulnerabilities, you need to apply context-dependent output encoding.
In some cases, it may be enough to encode HTML special characters such as opening and closing tags , but in general, correctly applied URL encoding will be more secure. Until fairly recently, most browsers had built-in XSS filters to catch at least some reflected XSS attempts, but these were limited in scope and effectiveness and, in any case, could be evaded by more advanced attackers. Browser-side XSS filters if any should therefore be considered a second line of defense at best to minimize the impact of existing vulnerabilities.
Tempting though it may seem, developers should not use input blacklisting in effect, manual filtering , as there are also many ways to bypass it. Another practice to avoid is manually stripping potentially dangerous function names and characters from input strings, as this only makes it harder for XSS filters to recognize dangerous payloads.
Again, the only recommended way to prevent cross-site scripting vulnerabilities is to consistently apply context-dependent encoding. Cross-site scripting vulnerabilities make up the majority of all web application security issues that Netsparker identifies every day in customer websites and web applications. Because the Netsparker scanner incorporates a full modern web browser engine, it can use its proprietary Proof-Based Scanning technology to simulate vulnerability exploitation and automatically confirm most XSS vulnerabilities with no need for manual verification — and no risk of false positives.
Netsparker was built with accurate automation in mind and provides out-of-the-box integration with popular issue trackers, so you can if necessary automatically create developer tickets for security issues with no manual work for the security team. This is why Netsparker vulnerability reports include all the remediation guidance that developers need to understand the issue and correctly fix it by addressing the root cause.
This helps to avoid partial fixes that lead to cross-site scripting vulnerabilities resurfacing in the future and also fosters more secure coding practices to improve your web application security in the long run. Technical Content Writer at Invicti. The criminals attach their malicious code on top of the legitimate website, essentially deceiving browsers into executing their malware whenever the site is loaded.
Depending on how the code is injected, the malicious content may not even be on the actual web page itself but rather as a transient element that only appears to be part of the website at the time of exploitation.
This can create the illusion that the actual website is compromised when it isn't. There are different ways to trigger an XSS attack. For example, the execution could be triggered automatically when the page loads or when a user hovers over specific page elements, such as hyperlinks.
In some cases, XSS is performed more directly, such as in an email message. Some XSS attacks do not have a specific target; the attacker simply exploits a vulnerability in the application or site, taking advantage of anyone unfortunate enough to fall victim. Depending on the scale of the attack, user accounts may be compromised, Trojan horse programs activated, and page content modified, misleading users into divulging their private data.
Session cookies could be revealed, enabling a perpetrator to impersonate valid users and abuse their private accounts. A successful cross-site scripting attack can have devastating consequences for an online business's reputation and its relationship with its customers. Unfortunately, the flaws which allow XSS attacks to succeed are quite widespread. XSS primarily targets JavaScript because of the language's tight integration with most browsers.
This ability to exploit commonly used platforms makes XSS attacks both dangerous and widespread. By exploiting XSS vulnerabilities, an attacker can perform malicious actions, such as:. Attackers can trick users into entering credentials on a fake form, which provides all the information to the attacker. Once they obtain user credentials, attacks could use them to commit identity theft or financial fraud. Stored cross-site scripting Persistent XSS. Stored XSS occurs when user-supplied input is stored and then rendered within a web page.
Typical entry points for stored XSS include message forums, blog comments, user profiles, and username fields. After successful authentication, the server generates a string that uniquely identifies the current user session. This string is passed in a response header, in the form of cookie data. On subsequent visits to the server, the cookie data will be automatically included in the request.
This data will be used by the server to determine whether the request comes from a legitimate user. Naturally, the security of session cookies then becomes critical. Any interception of this information would enable impersonating a legitimate user. One of the classic ways to transfer session cookie data to an attacker is to send an HTTP request from the user's web browser to an attacker-controlled server.
In this case, the request is generated by JavaScript that is embedded on a vulnerable web page. The cookie data is then transmitted in the parameters of this request. One example of an attack vector could be the following:. In this example, the user's web browser creates an image object in the DOM model. After that, it tries to load the image from the address specified in the src tag. The browser then sends the cookie data to the attacker's site with the corresponding HTTP request handler:.
In this case, an attacker only needs to listen to incoming connections, or else configure event logs and obtain cookie data from the log files this is described later in more detail. JavaScript is a very capable programming language. An attacker can use these abilities, combined with XSS vulnerabilities, simultaneously as part of an attack vector. So instead of XSS being a way just to obtain critical user data, it can also be a way to conduct an attack directly from the user's browser.
These requests can be used to send comments or to conduct financial transactions:. By exploiting an XSS vulnerability with this attack vector, malicious actors can transfer any specified amount of money to their accounts. This allows an attacker to change how the website appears to the user, such as by creating fake input forms. If a vulnerable web application permits modifications to the DOM model, an attacker could inject a fake authentication form into the web page by using the following attack vector:.
Any credentials that a user enters in this form will be sent as a POST request to the evil. 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.
0コメント