We live in the era of rapid digital transformation with innovative solutions allowing us to perform a wide range of things faster and easier. As technologies continue advancing, hackers and threat actors create sophisticated counter-approaches by exploiting these futuristic technologies.
Although information security professionals are constantly working to develop practical cybersecurity tools, website users are exposed to new vulnerabilities — with cross-site scripting (XSS) being one of the most common cyber threats.
What is Cross-Site Scripting (XSS)?
Cross-site scripting (XSS) is a code injection security attack that allows a threat actor to insert malicious scripts into web browsers belonging to trusted web applications or websites. Unlike SQL injection and other attack vectors that access the database directly, XSS focuses on the end-user to gain complete control over the operations.
By impersonating users, the bad actor can steal the user’s active session cookie and get access to their confidential data. The victim’s browser cannot distinguish legitimate markup from malicious markup and executes it.
XSS ranks eighth in the list of OWASP Top 10 most critical web application vulnerabilities. Open Web Application Security Project rankings are provided by security experts from all over the world to help developers safeguard any weaknesses in the system and reduce the presence of known risks.
What are types of XSS attacks?
Based on where a hacker feeds malicious input, XSS can be divided into three main categories:
Stored Cross-Site Scripting
Stored XSS or Persistent XSS is the most dangerous type of XSS attack, which can compromise many visitors and lead to devastating consequences. Stored XSS arises when the user’s input is not validated before storing content and rendering it on the web page.
Social networks, forums, blogging websites, user profiles, message boards, and comment sections are common areas where XSS can be stored. Threat actors leverage this vulnerability by injecting XSS payloads on the most visited website pages. Upon clicking on the crafted link and opening the viral page, the victim’s web browser executes the payload client-side.
Below is a clear illustration of a stored XSS vulnerability. A user can leave messages to a message board application, which other users can view:
<p>Hello, this is my message!</p>
The application doesn’t process the data further, making it simple for a fraudster to send a message that targets other users:
<p><script>/* Bad stuff here... */</script></p>
Reflected Cross-Site Scripting
Reflected XSS is the most commonly employed cross-site scripting attack (also known as non-persistent XSS). While performing this attack, a hacker adds malicious code to the end of the URL. Once the user clicks this link in their web browser, the browser executes the reflected XSS payload.
The threat actor usually uses malicious URLs, phishing emails, and other digital social engineering assaults to trick the victim into clicking on the link. As a rule, the following XSS attacks are made using social networks.
To illustrate reflected XSS vulnerability, take a look at the example below:
https://insecure-website.com/status?message=All+is+well. <p>Status: All is well.</p>
The web application doesn’t carry out any additional data processing, enabling a hacker to execute the following attack:
https://insecure-website.com/status?message= <p>Status: <script>/* Bad stuff here... */</script></p>
DOM-Based Cross-Site Scripting
Perpetrators can insert malicious code into a page due to modifying the DOM environment (Document Object Model) when it doesn’t properly filter user input. The attack’s HTML source code and response are identical in DOM-based XSS. The page itself remains the same, but due to the malicious changes made to the DOM environment, the client-side code on the page runs unexpectedly.
DOM-based XSS is entirely a client-side injection vulnerability. Therefore, the parasitic payload never reaches the server. Due to the unique nature of DOM-based XSS, it is difficult for Web Application Firewalls (WAFs) and other web application security tools to detect the attacks.
var search = document.getElementById('search').value; var results = document.getElementById('results'); results.innerHTML = 'You searched for: ' + search;
If the threat actor has control over the input field’s value, they can easily create a malicious value that causes their script to act in an unsafe manner:
You searched for: <img src=1 onerror='/* Bad stuff here... */'>
How can threat actor exploit XSS?
By leveraging XSS vulnerabilities, a threat actor can carry out malicious activities, such as:
- Hijack the user’s session. Session hijacking is a method of taking over a user’s authenticated session. Session cookies contain critical data that may allow a threat actor to masquerade as a legitimate user and obtain the session ID if compromised.
- Perform phishing attacks. Phishing is a common type of cyber attack where the perpetrator injects a form into the vulnerable page and uses that altered form to steal the user’s credential information, typically in the form of passwords, bank account information, and credit card numbers.
- Steal sensitive information. By performing an XSS attack, a perpetrator collects data from the user’s current session. For instance, cross-site scripting attacks on online banking apps can allow attackers to view the current balance, transactional details, personal information, etc.
XSS vulnerabilities foster the escalation of attacks to a more serious level and, if abused by fraudsters, can cause severe damage. XSS attacks can lead to sensitive data exposure, compromise of the victim’s account, and worse.
By creating a fake form, attackers lure web users into giving their login details, which allows them to receive all information about the user. Once the hackers’ cunning intention to deceive their victims is fulfilled, the obtained data can be used for identity theft or financial fraud.
Real-Life XSS Examples
The cross-site scripting attacks have been able to wreak havoc on well-known companies. The following are the most famous examples:
Fortnite is a gaming phenomenon – one of the most popular online battle games with more than 200 million players worldwide. Due to its huge popularity, the game platform became the main target for malicious actors and could face an XSS attack leading to data leakage in early 2019.
Security researchers from Check Point Software Technologies have discovered the cross-site scripting vulnerability in the game that could have allowed hackers to take over gamers’ accounts, access their sensitive information, steal in-game virtual currency, and eavesdrop on their conversations. The experts have privately reported about the identified XSS vulnerabilities to the game developer Epic Games, and the company has fixed the issues.
Having changed the script, the perpetrators sent customer data to their designated server with a domain name similar to British Airways and succeeded in performing credit card skimming. The malicious server had an SSL certificate, so customers fully trusted the server they purchased from.
eBay is one of the earliest and most popular e-commerce platforms. However, it was compromised through a cross-site scripting vulnerability in late 2014 and early 2015. The website used a URL parameter that redirected eBay members to different pages, but the parameter’s value was not checked before including it in the page. Due to this flaw, the attackers could craft a URL containing a parasitic iFrame and inject it into the legitimate eBay website.
Thus, fraudsters could access eBay seller accounts, steal payment details, sell products at a discount, and manipulate eBay listings of valuable products such as vehicles. Although eBay subsequently fixed the issue, recurrent attacks continued until 2017.
In 2011, three separate XSS vulnerabilities were detected on Facebook sites in 10 days. At least two were used to spread viral links and attacks on Facebook users to access or change information on their accounts.
The last XSS issue that came to light was with a Facebook “channel” page for session management, where a code update mistakenly modified the page’s behavior. However, once security experts noticed it, Facebook immediately fixed the flaw.
Detecting Cross-Site Scripting vulnerabilities
The reasons for the occurrence of XSS vulnerabilities are:
- Input validation is not performed.
- Output to the browser is not converted into HTML character entities.
Generally, web scanning tools are used to look for security vulnerabilities in web applications. These automated tools inject malicious scripts into the web application, such as GET or POST variables, URLs, cookies, and other code that could carry out XSS attacks. If the tool succeeds in injecting the script into the webpage, then the site is exposed to XSS. After discovering malware attack vectors, the tool notifies the user of the detected vulnerability.
XSS Prevention & Mitigation
Cross-site Scripting (XSS) can seriously threaten individual users and companies whose websites may be infected. In some cases, it’s challenging to deter the attack. For this reason, you must thoroughly sanitize all untrusted data to keep yourself safe from XSS. To reduce risk to your web application, never allow your browser to process data received as input without validating it.
Specific prevention methods vary depending on the subtype of XSS vulnerability, and user input usage context. However, certain general measures should be taken to safeguard your users against XSS attacks and keep your web application safe:
This is where you ensure the inputs are safe for processing within the code and malformed data does not damage a website, database, and users. By rejecting requests from external parties or sources, input validation prevents users from entering special characters into data entry fields on the websites. Input validation deters injection attacks, data leaks, and compromised systems.
Setting HttpOnly flags
Content Security Policy
Content Security Policy (CSP) is an additional layer of security designed to reduce the severity of any XSS vulnerabilities that may occur. This browser security tool helps repel and mitigate XSS and other client-side attacks.
You should run a robust web vulnerability scanner on your web servers to prevent malicious content from being executed by unaware users or visitors. Pentesters and security experts regularly use XSS scanning tools for web application analysis.
Penetration testing is a cybersecurity technique that uses the hacker perspective to find weaknesses in systems and mitigate cybersecurity risks, enabling organizations to close the identified loopholes before malicious hackers get to them.
Vulnerability scanning is not a substitute for penetration testing. Pentesters imitate the approaches of real-world threat actors, concentrating on security vulnerabilities that cannot be identified automatically.
The QAwerk team consists of professional pentesters who have a good command of advanced tools to conduct thorough security audits and vulnerability assessment.
How to effectively fix XSS errors?
You can take several measures to fix your website if cross-site scripting occurs:
- Find vulnerable code. In the initial phase of cross-site scripting recovery, identify the location of the vulnerability.
- Delete malicious content and backdoors. Once you know where the malware is located, you can remove any malicious content or incorrect data from your database and restore it to a clean state with a robust website malware removal tool. Additionally, you must scan the rest of your website and file systems for backdoors to correct existing errors and maintain your website’s reputation.
- Patch the vulnerability. Perpetrators frequently take advantage of weaknesses in databases, apps, and third-party components. If they pose a real risk, they need to be patched. Once you have identified the vulnerable software, distribute and apply updates to the vulnerable code to remove vulnerabilities that have been flagged.
- Change your credentials. In the event of XSS, it is crucial to immediately reset your passwords and application secrets with a strong password policy once the vulnerability is patched. Make sure there are no backdoors or rogue admin users in the database by clearing up your data to prevent reinfection.
- Set up a WAF. A key component of your security strategy is setting up a powerful web application firewall (WAF) to specifically address XSS by blocking malicious server requests to your website that aim to exfiltrate sensitive data. With a suitable WAF, you can protect against potential threats before patches are made available.
Therefore, it’s crucial to understand where your system has flaws and take preventive measures before an XSS attack seriously harms the website and its visitors. Unless the variables in a web application go through a validation process, they are a potential vulnerability. The most effective protection approaches to deter XSS attacks are: data sanitization, cookie protection, removing HTML tags from strings, vulnerability scanning, and regular penetration testing by professionals.