XSS Attacks (Cross-site scripting): Know the Risk and Prevention
Cross-site scripting (XSS) is a security vulnerability that allows an attacker to inject malicious code into a website. This code is then executed in the browser of users visiting the site, allowing the attacker to steal sensitive information like session cookies, log keystrokes, or perform other malicious actions.
XSS attacks are one of the most common web application security risks today. They can impact any website or web application that displays user-supplied data without properly sanitizing it first. Fortunately, XSS vulnerabilities are also among the easiest to remediate once properly understood.
Key Takeaways:
- XSS allows attackers to inject malicious scripts into a vulnerable website to be executed on visiting users’ browsers.
- The injected code can be used to steal sensitive data like session cookies or perform other malicious actions.
- XSS attacks occur because web apps fail to properly sanitize user-supplied data before displaying it.
- Proper output encoding and validation of inputs can prevent XSS vulnerabilities.
- XSS attacks come in three main forms: reflected, stored, and DOM-based.
- Tools like browser extensions and web vulnerability scanners can help detect XSS flaws.
- OWASP provides a simple XSS prevention cheat sheet to help developers stop XSS.
The Basics of Cross-Site Scripting (XSS) Attacks
Cross-site scripting has been a well-known threat to web applications for over two decades. The first public references to XSS vulnerabilities emerged around the year 2000. Since then, XSS has become one of the most frequently reported and exploited web vulnerabilities.
XSS attacks enable threat actors to inject client-side scripts into web applications viewed by other users. The term “cross-site scripting” comes from the fact that the attacking code often originates from a domain different from the vulnerable web app that executes it.
For example, a malicious script injected into Website A could be used to steal sensitive data from users of that site and transmit it back to Website B, which the attacker controls. The script executes inside a victim’s browser after they visit the compromised site, making XSS a “client-side” attack.
The most dangerous aspect of XSS is that it often allows attackers to bypass security controls like the same-origin policy. Websites normally restrict how scripts loaded from one origin can interact with resources from another. However, XSS payloads originate directly in the victim’s browser, bypassing those constraints and granting the injected code full access.
Attackers often exploit XSS to steal session cookies containing authentication or authorization tokens. By hijacking a victim’s session, the attacker can gain unauthorized access to private account information. XSS vulnerabilities can affect any website that displays unfiltered user input.
XSS Attack Payloads
The most basic XSS attack payloads involve injecting a short script that alerts the value of the cookie associated with the vulnerable site:
<script>alert(document.cookie);</script>
This proof-of-concept payload demonstrates the vulnerability by popping up an alert box displaying the victim’s cookie. More advanced real-world XSS attacks would substitute something more malicious than a simple alert box.
Some common payloads include:
- Sending the cookie to an external domain controlled by the attacker
- Overwriting the cookie to hijack a victim’s session
- Logging keystrokes and mouse movements to capture sensitive data
- Performing web scraping or content injection to deface the website
- Redirecting the victim’s browser to a phishing site or malware download
The most devastating XSS attacks happen when the vulnerable website has elevated privileges in the browser. For example, XSS on a banking domain could enable credentials theft or fraudulent money transfers. Similarly, XSS in a web-based email client could allow compromising private communications.
XSS Attack Methods
Attackers need to inject their malicious scripts into a website for them to trigger when users visit. There are three primary methods of conducting XSS attacks:
Reflected XSS
Reflected XSS involves injecting code into vulnerable website inputs that get reflected in the site’s responses. For example, an attacker may submit JavaScript in a search box or product ID parameter. If the site displays search terms or product details without encoding special characters, it will render the script instead of showing it harmlessly.
Stored XSS
Stored XSS is more dangerous than reflected XSS. It involves submitting malicious code that gets permanently stored on the vulnerable website’s databases and servers. For example, an attacker may post a comment containing malicious JavaScript. Later, visitors to that page will have the script executed in their browsers.
DOM-Based XSS
DOM-based XSS exploits client-side vulnerabilities in how the website’s JavaScript handles data. Instead of relying on server-side weaknesses, the attack payload modifies the DOM environment in the victim’s browser to trigger malicious code execution. For example, JavaScript can be used to write malicious code into an element’s inner HTML.
Real-World XSS Attack Scenarios
Some real-world examples of XSS attacks include:
- Social media sites—Attackers have leveraged stored XSS on social sites like Twitter and Facebook to craft tweets or posts with embedded malicious scripts that compromise viewers.
- Online retailers – Stored XSS vulnerabilities in product names/descriptions have allowed attackers to craft listings that hijack victims’ sessions when viewed.
- News sites – Attackers have exploited reflected XSS in the comment sections of news sites by tricking users into clicking links carrying a malicious payload.
- Forums/bulletin boards – XSS in forums is common since users can often submit unfiltered HTML input that gets reflected to other users.
- Browser extensions – Chrome, Firefox, and other browser extensions that take URL inputs without sanitization are vulnerable to reflected XSS attacks if tricked into visiting a malicious link.
- Webmail clients—Due to the sensitivity of the data involved, Web-based email services are prime targets for XSS. XSS could let an attacker read messages, forge emails, or spy on communications.
- Intranet sites – Internal corporate apps that fail to sanitize inputs can be vulnerable to XSS, enabling attacks on the organization’s private network and resources.
Anatomy of an XSS Attack
At a high level, here are the steps involved in carrying out a cross-site scripting attack that steals a victim’s session cookie:
- Discovery – The attacker probes the target website to detect inputs that are vulnerable to XSS. This may be done manually or via automated web vulnerability scanners.
- Injection—The attacker crafts a malicious script and injects it into a vulnerable website parameter, such as the search box, a comment field, the site URL, etc.
- Execution – A victim browses the compromised site, which displays the attacker’s payload. The malicious script executes in the victim’s browser due to the site’s failure to sanitize inputs.
- Transmission – The injected script sends the victim’s cookie or other sensitive data back to the attacker’s server for harvesting.
- Escalation – Having obtained a valid session cookie, the attacker can now access the victim’s account on the vulnerable site to steal private data, deface content, etc.
- Replication – The attacker repeats the process to compromise as many victims as possible. Additional payloads may be injected to maximize impact.
Vulnerable Website Elements
Nearly any element of a web page that includes unfiltered user input can be vulnerable to XSS. Some common sources include:
- Search input boxes
- Login/registration forms
- Comment submission forms
- User profile input fields
- URL parameters
- Shopping carts/product listings
- Webmail clients
- Feed reader apps
- Site/article sharing features
- Browser extension URL parameters
- API endpoints
- File uploads processing
- Dynamic database queries
- Document parsers like CSV or XML handlers
- Markdown processors
- Template engines
- Any other location where untrusted data gets embedded into HTML
Again, the key factor is whether user-controllable input is properly sanitized before outputting it into the HTML document. If not, there is potential for XSS abuse.
XSS Attack Impact
Depending on the nature of the vulnerable site and the attacker’s goals, real-world XSS attacks can enable:
- Session hijacking – Stealing session cookies grants the attacker unauthorized access to the victim’s account.
- Account takeover – XSS payloads may reset passwords or register new accounts with the victim’s credentials.
- UI redressing – The site’s user interface can be modified or hidden via injected CSS/JavaScript.
- Keylogging – Logging keystrokes can reveal sensitive data entered by the victim after being compromised.
- UI spoofing – Fake UI dialogs can trick users into entering confidential data like passwords.
- Browser exploitation – Malicious scripts may exploit browser vulnerabilities to install malware.
- Denial-of-service – Resource exhaustion attacks can take down vulnerable sites.
- Dominating users – Long-running scripts can lock up the browser, preventing normal use.
- Scraping sensitive data – Attackers can extract private user info displayed in the DOM through XSS payloads.
- Spreading worm payloads – Self-propagating scripts can infect across large user bases.
- Defacing/data destruction – Injected code can modify site content, like deleting databases.
Who is at Risk?
Any organization or individual operating a website is potentially at risk of XSS attacks. Major web properties tend to devote significant resources to security and suffer fewer incidents as a result. Smaller sites, custom web apps, intranet sites, and personal blogs have proven more susceptible historically.
Here are some categories of websites commonly impacted by XSS:
- Personal blogs and small business sites – Those are often built on top of vulnerable web software like outdated CMS platforms.
- Retailer websites – Product listings, search boxes, and shopping carts provide dangerous XSS vectors. Attackers can hijack user accounts for fraud.
- Social networks – Social sites contain many vectors, like embedded posts, messages, and comments that are rendered without context.
- News and media sites – User-submitted comments are prime XSS attack vectors if not filtered. They can be used to spread fake news or deface content.
- Forums and bulletin boards – contain XSS risks similar to comment sections but often have elevated browser permissions, increasing potential impact.
- Banking and financial sites – The high value of financial account takeovers makes banks prime targets if XSS flaws exist.
- Government agencies – Public-sector sites contain sensitive citizen data, making them high-value targets. Legacy apps heighten XSS risk.
Detecting XSS Vulnerabilities
Failure to proactively detect and remediate XSS flaws early can lead to devastating data breaches down the road. Some ways to identify vulnerable XSS entry points include:
Manual Testing
The simplest technique is to manually test inputs like search boxes to verify they are properly sanitizing special characters like < and > before output. Attempt injecting a basic script like <script>alert(‘XSS’)</script> to validate inputs reflect it vs. encoding the tags.
Automated Scanners
Automated web vulnerability scanners from vendors like Acunetix, Netsparker, and Burpsuite can crawl sites for reflected and stored XSS flaws. Scanners submit payloads designed to trigger popups if scripts are not properly sanitized.
Code Review
Reviewing codebases to verify proper output encoding and validation routines are implemented provides assurance XSS was addressed during development. This requires access to the source code.
Browser Extensions
Browser add-ons like XSS Hunter automatically injects payloads into inputs while browsing sites. They alert if the payload successfully executes, indicating a vulnerability.
Bug Bounties
Bug bounty programs incentivize external security researchers to find and report flaws like XSS. They often discover creative attack vectors that elude automated tools.
Preventing XSS Attacks
Comprehensive XSS mitigations require a combination of secure coding practices and risk-based input validation.
Escaping Untrusted Data
Escape special characters like <, >, (, ), #, & when outputting untrusted data into HTML. This prevents payload code snippets from executing. The specific escaping technique depends on the output context:
- HTML body: Encode for HTML entities, e.g., < becomes <
- HTML attribute values: Encode for HTML attributes, e.g., < becomes <
- JavaScript data values: Encode for JavaScript literals, e.g., < becomes <
- CSS property values: Encode for CSS, e.g., < becomes \3c
- URLs: Encode for URL formatting, e.g., < becomes %3c
Context-aware escaping libraries like OWASP Java Encoder provide robust implementations for common platforms and template engines.
Validating Inputs
Validation complements encoding by only allowing known-good input formats. Approaches include:
- Setting strong type constraints, e.g., numeric fields reject non-digit entries.
- Allow-listing valid input sets, e.g., dropdown menus vs. free-text fields.
- Limiting string lengths and character sets to the required minimum.
- Rejecting unsanitized HTML or JavaScript input in contexts where it is unnecessary.
- Applying centralized input validation routines vs. relying on individual developers.
Content Security Policy
CSPs provide a browser-enforced whitelist of allowed scripts. This can mitigate the impact of any DOM-based XSS by preventing the injection of unauthorized scripts.
HTTP-Only Cookies
Enable the HTTP-only flag on cookies to prevent access from client-side JavaScript. This reduces the impact of stolen cookies via XSS.
Virtual Patches
Apply temporary filters targeting detected XSS vectors until root causes can be fixed. For example, sanitize search inputs known to be vulnerable.
Limit Browser Permissions
Reduce the browser capabilities available to third-party JavaScript to limit what injected scripts can exploit.
Security Testing
Conduct vulnerability scans and penetration testing focused on confirming XSS vulnerabilities are identified and resolved before production release.
OWASP XSS Prevention Cheat Sheet
The OWASP XSS Prevention Cheat Sheet provides web developers with a summary guide for preventing XSS vulnerabilities:
- Rule #1: Always escape untrusted data based on output context using a standard library like OWASP Java Encoder. Do this at the last moment before rendering output.
- Rule #2: Use appropriate XSS protections in frameworks like CSP, CSRF tokens, SecurityHeaders, Content-Type headers, etc. Let frameworks handle common mitigations.
- Rule #3: Positive input validation is useful for detecting malicious/abnormal payloads but is not sufficient for XSS prevention. Encoding is still required.
- Rule #4: Presume all HTML, JavaScript, and URL input contains malicious code that requires encoding. Always sanitize based on context before rendering.
- Rule #5: Validate for data types, length limits, and allow-list where possible as an additional defense beyond encoding.
- Rule #6: Do not rely exclusively on blacklisting. Attempting to block all XSS vectors is unrealistic and prone to gaps. Whitelisting and encoding are preferred.
- Rule #7: URL escape all URL arguments and encode URL parameters placed into HTML to prevent DOM XSS.
- Rule #8: JavaScript source files should be served with Content-Security-Policy: script-src ‘self’ to avoid DOM XSS vulnerabilities.
- Rule #9: Escape dynamic JavaScript variables, objects, and functions contained in HTML. Misuse can lead to DOM XSS.
- Rule #10: Use a standard DOM-based XSS filter provided by your framework as an added defense for legacy browsers.
The Future of XSS Attacks
XSS has been a known and preventable threat for over 20 years, yet attacks continue to occur at high frequencies. This is often due to legacy web applications built before awareness of XSS risks. Modern web development frameworks and automated detection tools provide simpler ways to avoid XSS today.
However, DOM-based XSS and mashup architectures with dynamic JavaScript still introduce ways for XSS to slip in. As web apps grow in complexity, client-side XSS seems likely to remain an evolving exposure requiring ongoing vigilance to address emerging attack vectors.
Providing web developers with robust XSS prevention education is critical so these coding mistakes cease to occur in new applications. For legacy apps, organizations should act quickly to identify and eliminate dangerous XSS flaws through scanning, patching, and refactoring vulnerable components. By understanding root causes and applying core defenses like context-aware output encoding, XSS can be relegated to the history books.
Frequently Asked Questions About XSS Attacks
What is the main cause of XSS vulnerabilities in web applications?
The primary cause of XSS is failing to properly sanitize and escape untrusted data before including it in HTML output. This allows injected scripts to execute in victim browsers instead of displaying harmlessly.
What are the three main types of XSS attacks?
The main types are reflected XSS, stored XSS, and DOM-based XSS:
- Reflected XSS – injected directly from the victim’s request into immediate site response.
- Stored XSS is injected into site databases to be executed when persisted data is later viewed.
- DOM-Based XSS – payloads modify client-side JavaScript environments like document DOM to trigger vulnerabilities.
What are the common locations where XSS vulnerabilities appear?
Vulnerable XSS entry points include search forms, login forms, comments, URL parameters, API endpoints, document parsers, dynamic database queries, and anywhere else untrusted inputs get rendered without escaping.
What are examples of real-world XSS attacks?
Prime targets include social sites, retailers, news sites, webmail clients, forums, browser extensions, banking sites, and intranet apps. Attack goals range from session hijacking to keylogging to UI spoofing.
How can XSS vulnerabilities be detected?
Detection methods include manual injection testing, automated vulnerability scanning, code review, browser extensions, and bug bounty programs.
How can developers prevent XSS vulnerabilities in web applications?
Key prevention measures include:
- Context-aware output encoding.
- Positive input validation.
- Httponly cookies.
- Content Security Policy.
- Virtual patching.
- Limiting browser permissions.
- Security testing.
Why do XSS attacks remain so common despite being well-known for decades?
Legacy web apps still contain vulnerable code. XSS continues to be problematic when developers are not properly trained on secure coding practices or utilize vulnerable frameworks and components.
What is the future outlook for XSS attacks?
Increased use of modern frameworks and automated detectors should reduce XSS flaws in new web apps. However, complex client-side JavaScript will continue to provide openings for DOM-based XSS, requiring ongoing vigilance.
Jinu Arjun
