Alexis Kestler
Dear reader,
Cross-site scripting (XSS) attacks remain one of the most dangerous threats facing web applications today. As per the latest statistics, XSS was the most reported web app vulnerability in 2021 accounting for over 30% of submissions.
| Vulnerability Type | 2021 submissions |
|---|---|
| Cross-Site Scripting | 30% |
| Injection | 13% |
| Broken Authentication | 12% |
| Sensitive Data Exposure | 12% |
This shows the prevalence of XSS flaws, which allow attackers to execute malicious scripts and code in a victim‘s browser. The consequences of such attacks can be devastating, ranging from data theft to account takeovers.
As web developers, we have a responsibility to secure our applications against XSS and other critical risks. This is where automated vulnerability scanners come in.
In this comprehensive guide, I‘ll provide my insights on XSS scanning tools – both free and paid – that I have used extensively for spotting XSS flaws early.
I‘ll share details on how these scanners work, key features to look for, and best practices for integrating scanning into your processes. My goal is to help you make the most informed decision for choosing an XSS scanner tailored to your application security needs.
So let‘s get started!
What Exactly is XSS and Why Should You Be Concerned?
Before we explore the tools, it‘s important to understand what XSS is, how it works, and its implications.
XSS or cross-site scripting refers to security flaws that allow attackers to inject malicious client-side scripts (usually JavaScript) into web pages. The injected scripts can then access cookies, session tokens, or other sensitive information retained by the victim‘s browser and send them to the attacker.
Magecart attackers heavily utilize XSS to steal payment card data entered into compromised online stores. Attackers have also used XSS to bypass CSRF protection, steal login credentials, and takeover accounts.
There are three main types of XSS attacks:
1. Stored XSS
This is the most dangerous type of XSS. Here the malicious script comes from the website‘s database and gets executed whenever the compromised page is loaded.
For example, an attacker can post malicious JavaScript in website comments. When users load the page, the script gets executed in their browsers enabling the attack.
Stored XSS can be used for persistent attacks until the vulnerability is fixed.
2. Reflected XSS
In this type, the malicious script is part of the victims request to the web application. The input gets reflected and executed in the response page returned to the victim.
For example, an attacker can craft a malicious link with embedded JavaScript and trick users into clicking it. The users become victims when the JavaScript executes on the response page.
Reflected XSS enables one-time attacks per malicious link click.
3. DOM-Based XSS
Here the vulnerability exists in client-side JavaScript code rather than server-side code. The attack payload gets executed as part of the web page‘s DOM (Document Object Model).
For example, directly writing user input to the DOM via innerHTML or using unvalidated URL parameters to modify the DOM enables this attack.
Dom-based XSS can be challenging to detect but scanners are getting better at discovering such flaws.
The bottom line is XSS can seriously impact your business – from data theft to compliance violations and loss of customer trust. Let‘s now dive into how we can uncover these flaws.
Techniques for Detecting XSS Vulnerabilities Early
The first step towards securing your apps is identifying XSS vulnerabilities proactively.
Here are some of the main techniques that can be used:
Manual Testing
This involves manually testing inputs by injecting exploit payloads like snippets of JavaScript code, special characters etc. and analyzing the application‘s response.
For example, entering the payload <script>alert(‘XSS Vulnerability‘)</script> into search forms or other inputs to check if it gets executed.
Manual testing requires expertise and is time-consuming. But in the hands of an experienced tester, it can uncover flaws that automated scanners sometimes miss.
Automated Scanners
Automated vulnerability scanners crawl web applications to identify pages and inputs vulnerable to XSS and other weaknesses.
They work by automatically injecting hundreds of malicious payloads and analyzing the response to detect flaws. This scales testing and reduces manual effort significantly.
However, scanners may miss flaws that require completing complex workflows or multi-step processes to exploit.
Web Application Firewalls
Web application firewalls (WAFs) provide runtime protection against XSS and other attacks.
They analyze web traffic for suspicious patterns and block requests containing malicious payloads. WAFs prevent many XSS attacks but do not eliminate the need to fix underlying flaws.
Code Review
Reviewing source code for improper encoding, lack of input validation, unsafe DOM manipulation etc. can reveal XSS vulnerabilities that evade scanners.
But manual code review requires access to source code and is unscalable for large complex applications.
By combining multiple such techniques, organizations can thoroughly uncover XSS issues before they are exploited by attackers.
Next, let‘s explore some of the top open source and commercial XSS scanning tools available.
Top XSS Vulnerability Scanners
Over the years, I have extensively used various free and commercial scanners for detecting XSS flaws in complex web applications.
Here are some of my top recommendations:
1. Burp Suite
Burp Suite is arguably the most popular web security testing tool used by professionals.
Burp Scanner serves as the engine that performs automated vulnerability detection. It can identify many types of weaknesses from XSS to SQLi with a high degree of accuracy.
For detecting XSS, Burp inserts payloads that can trigger JavaScript execution in responses and analyzes for evidence of vulnerability. Its smart payload selection parses responses to identify insertion points and attack surface.
I find Burp particularly effective in thoroughly testing complex endpoints such as APIs that receive JSON input. Fine-tuning scan settings and payload lists allows detecting obscure XSS issues.
Useful features of Burp for XSS include:
- Powerful payload encoder to bypass filters via encoding and obfuscation
- Handy attack payloads for exploiting XSS quickly
- Ability to reissue requests manually via repeater
- Extensibility via Python plugins
Both the free and paid professional versions include comprehensive XSS scanning capabilities making Burp Suite an essential tool for web security testing.
2. OWASP ZAP
The free and open source OWASP ZAP is a fantastic option loved by many security testers and developers.
It offers several useful features for finding XSS vulnerabilities:
- Passive and active scanning modes
- Custom payload generators
- Context-aware scanning for minimized false positives
- Handy attack vectors for DOM-based XSS testing
- Automated form detection
- Fuzzing capabilities for enhanced test coverage
ZAP also makes it easy to test HEADLESS services like REST APIs thanks to its OpenAPI integration. Useful automation and reporting capabilities provide added value.
For most small applications, ZAP itself is sufficient for comprehensive XSS testing. It can be further enhanced via community-contributed add-ons.
3. Acunetix
Acunetix by Invicti is an advanced automated web scanner loved by security professionals.
It leverages sophisticated fuzzing and vulnerability detection algorithms to identify thousands of vulnerability types with a high degree of accuracy.
For detecting XSS, Acunetix intelligently inserts specially crafted attack vectors into parameters and analyzes the server‘s response. This enables it to find all types of XSS flaws including the tricky DOM-based variety.
Useful features include:
- Automated crawling to discover new pages, forms and inputs
- Smart payload insertion based on advanced context analysis
- Custom payload lists and vulnerability triggers
- Detailed vulnerability reports with PoC exploits
Acunetix also offers WAF integration, CI/CD scanning, and useful automation capabilities. It‘s one of the most powerful commercial scanners available.
4. Rapid7 InsightAppSec
Rapid7 InsightAppSec provides dynamic scanning capabilities as part of the Insight Platform suite.
It leverages the powerful ScanEngine used by popular tools like Nexpose to find XSS and over 30 other vulnerability types.
InsightAppSec generates smart input values tailored to application specifics to maximize detection accuracy. Useful automation features like continuous scanning simplify maintaining web security over time.
It also integrates seamlessly with bug trackers and WAFs to streamline remediation. The scanner is available as SaaS and on-prem deployment.
5. IBM AppScan
IBM AppScan is an enterprise-grade dynamic scanner designed for large complex applications.
It combines static, dynamic, interactive, and configuration analysis techniques to provide comprehensive testing. Intelligent algorithms precisely detect vulnerabilities while minimizing false positives.
For XSS, AppScan inserts malicious values into parameters and analyzes response patterns to identify potential injection flaws. Mutation fuzzing and attack replication further enhance coverage.
Useful features include prioritized and actionable findings along with guidance for fixing flaws. Developer-friendly design simplifies web security testing.
6. sqlmap
sqlmap is a popular open source injection scanner specialized for SQLi testing.
In addition to SQLi, sqlmap can also detect some XSS and command injection flaws by analyzing responses to specially crafted payloads.
It utilizes techniques like heuristics analysis, inference, statistical analysis etc. to achieve high speed and accuracy in detecting flaws.
While sqlmap focuses mainly on SQLi, its XSS capabilities provide added value during web security testing. It runs on Linux, MacOS and Windows.
7. Xenotix
The Xenotix XSS Exploit Framework is an open source scanner designed specifically for XSS vulnerabilities.
It performs comprehensive fuzzing and vulnerability analysis of web applications to uncover even obscure XSS flaws.
Smart payload generation, context-aware scanning, and response analysis help Xenotix achieve excellent detection rates. It also includes an exploit framework for PoC testing of found issues.
Xenotix simplifies validating results from other scanners and successfully exploiting the detected XSS vulnerabilities.
8. Detectify
Detectify offers a highly user-friendly web vulnerability scanning service accessible directly from your browser.
It combines automated scanning with manual verification by security researchers to minimize false positives. Smart algorithms precisely identify vulnerabilities including XSS.
Useful features include risk-based prioritization of findings, seamless integration with bug trackers, and the ability to schedule periodic rescans.
Pricing starts at $29/month for weekly scans of a single site making Detectify very affordable for SMBs and individuals.
9. Intruder
Intruder is another cloud-based vulnerability scanner providing easy and accurate testing.
It leverages Breach And Attack Simulation (BAS) technology to identify vulnerabilities with a low false-positive rate.
Intruder automatically crawls web apps to detect pages and input vectors. These are then subjected to thousands of simulated attacks to uncover vulnerabilities like XSS.
Useful automation features, slick UI, and integrations with tools like JIRA simplify web security testing.
10. ImmuniWeb
ImmuniWeb offers AI-powered web security testing available as SaaS or an on-prem product.
It combines automated and manual testing to provide accurate and trusted results. The AI engine mimics human reasoning and logic to minimize false findings.
ImmuniWeb detects 4000+ web vulnerabilities including all forms of XSS. It also provides useful virtual patching and WAF rule generation capabilities.
The scanner is used by 500+ global organizations and available under flexible pricing plans.
Key Factors for Choosing an XSS Scanner
When evaluating XSS scanning tools, keep an eye out for the following essential capabilities:
Comprehensive Vulnerability Coverage
The scanner must detect all types of XSS flaws – stored, reflected, and DOM-based – in diverse contexts from HTML to JavaScript, APIs, headers etc.
Both simple and complex multi-step XSS vulnerabilities should be detectable.
Sophisticated Scanning Logic
Intelligent payload generation, advanced response analysis, contextual fuzzing and other smart techniques help maximize detection rates and accuracy.
Look for context-aware scanning tailored to your tech stack – say Node.js, React, Angular etc.
Low False Positive Rate
Minimal false alarms save time wasted chasing and confirming non-existent flaws.
Prioritization and risk analysis also helps focus on findings that are highly likely to be exploitable.
Easy Authentication Handling
The scanner should support logging into web apps via form submissions, HTTP authentication etc. This provides access to restricted pages and features.
Using a proxy like Burp simplifies managing scanner authentication.
Developer-Friendly Reporting
Reports should provide technical details and PoC examples to aid developers in reproducing and fixing vulnerabilities.
Integration with defect trackers streamlines remediation tracking.
Customizable Scans
Flexibility in fine-tuning aspects like scan scope, vulnerability coverage, throughput etc. is necessary for optimizing scans.
Following Best Practices for Maximum Effectiveness
Here are some tips and best practices I follow when leveraging XSS scanners for securing web applications:
Crawl and Map Application Scope
Spider or crawl your web app to identify all pages, forms, inputs etc. This helps configure the scanner to test the right attack surface comprehensively.
Use tools like OWASP ZAP to generate an application site map for importing into scanners.
Configure Authentication
Ensure the scanner can correctly handle login forms, HTTP authentication etc. to access restricted pages and features.
Using a proxy is helpful in this regard. Recording login sequences simplifies configuring scanner auth.
Carefully Select Policies
Tune scan policies, payload lists etc. according to application specifics to maximize detection chances while minimizing disruptive false positives.
Leverage custom payload lists where possible.
Monitor Scanning Traffic
Keep an eye on requests and responses made during scanning to gain visibility into the tool‘s logic and improve future configurations.
Validate Key Findings
Exploit some reported issues manually to confirm they are truly valid XSS flaws and not false alarms.
Review your defensive mechanisms – can the payload actually be delivered to users?
Supplement With Manual Testing
While automated scanners do the heavy lifting, also manually test complex pages and attack scenarios like DOM XSS.
Leverage tools like Burp Suite to complement scanner capabilities.
Integrate Scanning into SDLC
Schedule periodic scans, ideally every sprint. Scan early in QA rather than just before production deployment.
Configure CI/CD integration to bake security into the development pipeline.
Implementing Robust Defenses Against XSS
While scanners help uncover flaws, you need robust defensive measures to truly minimize XSS risk:
Validate and Sanitize User Input
Decoding, validating and sanitizing all input on the server-side is crucial before outputting to prevent XSS attacks.
Use allowlists over denylists where possible. Avoid blacklist-based filters prone to bypasses.
Encode Dangerous Output
Encode special chars like < > & in your web app‘s output using libraries like OWASP ESAPI Encoder.
Enable HTTP-Only Cookies
This prevents client-side access to cookies via JavaScript and mitigates cookie theft.
Adopt Content Security Policy
CSP restricts sources of scripts and code execution thereby limiting XSS risk. It‘s supported by all modern browsers.
Limit DOM Manipulation
Avoid directly inserting user input into the DOM. Use textContent instead of innerHTML where possible.
Use Security Headers
Headers like X-XSS-Protection enable browser anti-XSS defenses.
Virtual Patch with WAF
Detecting and blocking XSS payloads at runtime via a WAF provides temporary protection until code fixes can be implemented.
Final Thoughts
I hope this detailed guide gives you clarity in choosing an XSS scanner that meets your application security needs without breaking the bank.
The key takeaways are:
-
XSS attacks remain highly prevalent and dangerous necessitating robust testing.
-
Combining automated scanners with other techniques provides comprehensive vulnerability detection.
-
Numerous free and commercial scanners are available – open source ZAP and Burp Suite offer excellent capability.
-
Fine-tuning scanners and following best practices is key to maximizing detection accuracy.
-
Scanning must be supplemented with secure coding practices, validation, sanitization and other defenses.
Please feel free to reach out if you have any other questions as you embark on securing your web apps against the XSS threat. I‘ll be happy to offer any guidance based on my own experience.
Stay safe out there!
