XSS allows attackers to inject malicious scripts into web pages viewed by other users, enabling session hijacking, data theft, and defacement.
Cross-Site Scripting (XSS) is a code injection vulnerability that occurs when an application includes untrusted data in a web page without proper validation or escaping. An attacker can inject malicious scripts (typically JavaScript) that execute in the context of a victim's browser session.
There are three main types of XSS: Reflected XSS, where the malicious script comes from the current HTTP request; Stored XSS, where the script is permanently stored on the target server (e.g., in a database or comment field); and DOM-based XSS, where the vulnerability exists entirely in client-side code that processes data from an untrusted source.
XSS is one of the most prevalent web vulnerabilities and can have devastating consequences. An attacker exploiting XSS can steal session cookies and authentication tokens, impersonate users and perform actions on their behalf, redirect users to malicious websites, deface web pages, and install keyloggers to capture credentials. Because XSS executes in the trusted context of the vulnerable website, it can bypass same-origin policies and access any data the user can see. In applications handling sensitive data -- financial records, health information, or personal communications -- XSS can lead to massive data breaches and regulatory violations.
The primary defense against XSS is output encoding: escape all untrusted data before inserting it into HTML, JavaScript, CSS, or URL contexts. Use your framework's built-in auto-escaping (React JSX, Vue templates, Angular interpolation) and avoid bypassing it with dangerous APIs. Implement a strict Content Security Policy (CSP) that prevents inline script execution. Validate and sanitize all user input on the server side using allowlists rather than denylists. For rich text, use a proven sanitization library like DOMPurify. Set the HttpOnly flag on session cookies to prevent JavaScript access. Use the X-XSS-Protection header as an additional layer of defense.
Select your framework for a detailed guide on fixing Cross-Site Scripting (XSS) with framework-specific code examples and security checklists.
SQL Injection
SQL Injection allows attackers to interfere with database queries, potentially reading, modifying, or deleting data, and in some cases executing system commands.
Cross-Site Request Forgery (CSRF)
CSRF forces authenticated users to execute unwanted actions on a web application, exploiting the trust a site has in a user's browser.
Insecure Direct Object References (IDOR)
IDOR occurs when an application exposes internal implementation objects (like database IDs) and fails to verify that users are authorized to access the referenced resource.
Broken Authentication
Broken authentication encompasses weaknesses in session management, credential handling, and identity verification that allow attackers to compromise user accounts.
Security Misconfiguration
Security misconfiguration encompasses insecure default settings, open cloud storage, verbose error messages, unnecessary features, and missing security headers.
Sensitive Data Exposure
Sensitive data exposure occurs when applications fail to adequately protect sensitive information like credentials, tokens, personal data, or financial information.
Missing Rate Limiting
Missing rate limiting allows attackers to perform brute force attacks, credential stuffing, API abuse, and denial of service by making unlimited requests.
JWT Vulnerabilities
JWT vulnerabilities include algorithm confusion, missing validation, token leakage, and improper key management that can lead to authentication bypass.
Path Traversal
Path traversal allows attackers to access files and directories stored outside the intended directory by manipulating file path references.
Command Injection
Command injection allows attackers to execute arbitrary operating system commands on the server through vulnerable application interfaces.
Server-Side Request Forgery (SSRF)
SSRF allows attackers to induce the server to make HTTP requests to arbitrary destinations, potentially accessing internal services, metadata APIs, and private networks.
File Upload Vulnerabilities
Insecure file upload handling can allow attackers to upload malicious files including web shells, malware, or files that exploit server-side processing.
Insecure Deserialization
Insecure deserialization allows attackers to manipulate serialized objects to achieve remote code execution, replay attacks, injection, or privilege escalation.
Row Level Security (RLS) Bypass
RLS bypass vulnerabilities occur when database-level access policies are missing, misconfigured, or can be circumvented, exposing data across tenant boundaries.
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