What Is an XSS Attack

By Jayden Andrews. June 6, 2020

Cross-Site Scripting (also known as XSS) attack is a sort of injection that is a malicious script inserted into an otherwise credible, benign website. Attackers, using the XSS technique utilizes a web program to distribute malicious code, often disguised as a browser side script.

The flaws associated with this form of attack vary and may occur from a web application utilizing input from the end-user within the results generated before encoding or corroborating it. In most cases, actual attacks happen when a user visits a malicious web page or application which has the execution rights for the malicious code. After that, the web page or app becomes a carrier for the delivery of the malicious script to the victim’s browser platform.

The commonly used vulnerable carriers for XSS attacks include chat boards, webpages, forums, and apps.

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How it works

Websites or apps considered to be vulnerable to XSS attack utilize user input for the generated output. The victim’s browser must then infer this user input. The possibility of an XSS attack is mainly witnessed on VBScript, CSS, and ActiveX. Nevertheless, it is also quite familiar in JavaScript due to JavaScript’s fundamental purpose to almost any browser experience.

Attackers can utilize XSS and distribute it to an unsuspecting user. The browser of the end-user can’t tell if the script is apprehensive, hence executing it. Since your browser would have executed the script thinking it’s from a reputable source, malicious code will then gain access to all cookies, tokens, as well as other sensitive data stored by the browser and the site. XSS script has the capability of rewriting HTML page content. Since the attacker is using the XSS vulnerability to execute discretionary JavaScript in a victim’s browser, the webpage’s or application’s security becomes compromised. The idea of XSS injection being a user issue is a misconception.

Just like any other vulnerability related to security, your users get affected as much as you do. Moreover, the XSS attack can also be utilized to obliterate a website’s image rather than targeting the users. Attackers can manipulate web content and even redirect browser traffic to another website (often, the ones with malicious code).

The Nature of an XSS Attack

An XSS attack happens when:

1. Data penetrates a web application via a shady source, commonly a web request.
2. Data is bundled with dynamic content and distributed to web users without corroborating for malicious content.

Commonly, distributed malicious content imitate a JavaScript’s form of a segment, but also not limited to HTML, Flash, or other sorts of code executable by the browser. The variety of XSS-based attacks is unlimited. However, the similarities between most of the XSS attacks is that they often transmit sensitive information, such as cookies, as well as session data, back to the orchestrator. They also tend to redirect the user to websites monitored by attackers, or executing malicious activities on the victim’s machine under the aspect of the vulnerable web page.

XSS Attack Category

Cross-Site Scripting can be classified into two pigeonholes namely stored and reflected. Nevertheless, there is also a third that is less common to the public called DOM-based XSS which will be discussed later on in this article. As for now, let’s start with by defining the two common ones mentioned in the few lines above.

Stored XSS Attacks

This is a form of XSS injection intended to permanently store the script in the victim’s servers like in the chat forum, comment field, or database etc. This type of XSS attack is also known as Type-I or Persistent XSS.

Reflected XSS Attacks

In this form of attack, script injected reflect off the victim’s web server. An example of Reflected XSS attack can be portrayed in a message error, search results, as well as other outputs inclusive of all input, consigned to the victim’s server as a portion of the request. When the user is deceived into opening a malicious link or visiting an untrustworthy site, the injected code is then carried to the vulnerable site before reflecting the whole process back to the victim’s browser. The browser will then execute the code since it will be coming from a “trusted” server. This form of attack is also known as Type-II or Non-Persistent XXS attack.

Other XSS Vulnerability Forms

In 2005, on top of the Stored and Reflected XSS attacks, a third type was discovered by Amit Klein who then named it DOM-Based XSS. The attack carries out a somewhat complex procedure through executing an attack payload due to modifying the DOM ambience in the target’s browser. This leads to the client’s code running strangely. To lighten this further, the page’s HTTP response doesn’t change. However, the side code of the client included in the web page runs differently since there will be some mods inflicted to the DOM environment. This is varied from the two common attacks mentioned above where the attack payload is shown at the response page because of the side flaw in the server.

Consequences Related to XSS Attack

Regardless of the type of XSS attack used, the consequences are the same. The only difference lies in the means used to deliver the payload to the server. Moreover, you mustn’t be fooled into believing the read-only or brochureware website is immune to XSS attacks.

The severity of the inflicted damage to the user by an XSS injection varies from annoyance, disruptive, to account compromisation. However, the most severe attack that a victim may suffer include exposure of the victim’s personal data such as session cookie which creates security loopholes, allowing the orchestrator to take over the victim’s account. Other detrimental XSS attacks may cause exposure of victim’s essential files, Trojan program installation, browser hijacks leading to the user being redirected to malicious sites and altered content presentation.

An XSS vulnerability that allows the attacker to modify press release content may damage a company’s image, making clients lose confidence. Moreover, it can cause a decline in the stock prices of the company. If an XSS attack makes a pharmaceutical website its victim, dosage information can be changed, leading to an overdose.

How Can Detect if You’re Vulnerable

It’s difficult to detect and get rid of XSS flaws from a web page or application. Regardless, the most ideal approach to detect flaws is to execute a code security review, as well as to scan all the places that can allow HTTP input request to find its way to the HTML output.

Various HTML tags can be used as a carrier for malicious JavaScript. Therefore, it’s ideal to opt for tools such as Nessus and Nikto to search for flaws in a website even though they can only scratch the surface; because if a particular part of the site is compromised, the possibility of having other problems is high.

Therefore, we advise running several tests using different trusted software to detect your site’s vulnerability. A tool such as Acunetix vulnerability scanner can be useful, and thanks to its user-friendly interface, it can run an automated website scan quite easily. Moreover, the tool is packaged with a dedicated XSS scanner module.

XSS Attack Example

The occurrence of XSS attacks can be anywhere, however, it’s more popular in forums where users’ comments are not regulated. This allows malicious users to distribute unscrupulous material targeting other valid users. Here are some of the XSS attack example codes:

Example 1

<% String eid = request.getParameter(“eid”); %>

Employee ID: <%= eid %>

In the code shown above, for it to be execute correctly, there must be a standard alphanumeric text in the eid. If meta-characters or source code is included in the value contained by eid, then, the web browser will execute the code during the showcase of an HTTP response. At first, this may appear as a minor vulnerability, especially if you consider chances of someone entering a URL that can execute malicious code in their system. However, attackers also think from a defensive perspective. Therefore, instead of distributing a raw URL that users will think twice before clicking for obvious reasons, attackers utilize emails as well as other social techniques to deceive victims into clicking and visiting the URL. When the targeted users click on the link, they unknowingly reflect the malicious code via vulnerable web applications, reversing it to their systems.

Example 2

<%…
Statement stmt = conn.createStatement();
ResultSet rs = stmt.executeQuery(“select * from emp where id=”+eid);
if (rs != null) {
rs.next();
String name = rs.getString(“name”);
%>

Employee Name: <%= name %>

The code you are seeing above represents a Stored XSS attack form. Just like in example 1, code executions can only work if there are well-behaved name values, without doing anything to avoid exploits if they aren’t. So, this code also may be seemingly less dangerous since the value name can only be read from a database, whose data is monitored by the application. Nevertheless, if the user-supplied data is where the value name originates, then it means the whole database is likely to be used as a pipeline for malicious material.

How to Prevent the Risk of an XSS Attack

One thing for sure, there is no foolproof method of preventing XSS attacks. However, despite not being easy, it’s possible to keep safe from an XSS vulnerability even though it requires strict measures. Moreover, the applied techniques commonly rely on the subtype of XSS attack, context usage of user input, as well as the compute framework. Regardless, there are generic methods to follow to keep your web application secure which include:

Step 1: Enhance Awareness

Educate everyone involved in the management and building of your website or application. Each member should be fully aware of the risks related to XSS vulnerabilities. Offer your developers intensive security training and arm them with educational content such as this article.

Step 2: Take all User Input with a Grain of Salt

Although most of the users are saints and even clueless about XSS attacks, you never know who might be the spoiled bean. Therefore, consider all user input as suspicious and never let your guard down. Keep in mind that any user input that can be used as HTML output is a potential XSS risk.

Step 3: HTML Sanitization

Parse and clean HTML using a reputable library if the user input contains HTML. Consider your development language when selecting the library.

Step 4: Content Security Policy (CSP)

Apply Content Security Policy, a header for HTTP response which allows you to affirm dynamic resources permitted to lade relying on the source of a request.

Step 5: Scan Regularly

Carry out scans regularly since XSS can also be introduced by developers via external modules. Therefore, you must scan your web applications often using a trusted scanner such as Acunetix which can scan every build.

Use a VPN as an Added Layer of Protection

A Virtual Private Network can function as an added layer of protection against XSS attacks. Since XSS attacks are different from web vectors in the sense that they don’t directly aim for the application itself but at the users of the application; a successful attack can lead to devastating results that may tarnish the name of the business. Now, if an attack has been successfully launched, users can be misled into voluntarily submitting private information, with the session cookies revealed. This, in turn, enables the orchestrator to impersonate victims using their sensitive data, leading to damaged career profiles or financial loss.

However, with a VPN, the targeted users can still be exposed, but to a lesser extent since their data, as well as online activities will be masked. This makes it difficult for perpetrators to obtain valuable information. Some services such as NordVPN offer special features like Cybersec that create an extra digital Kevlar to block reflected XSS. Moreover, this type of feature is kept updated with the list of suspicious sites that are likely to trigger an attack. Note that, for a VPN to be more effective, the user must be using the latest browser since newer versions come equipped with security features that assist in detecting, as well as blocking XSS attacks.

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