Insecure file upload handling can allow attackers to upload malicious files including web shells, malware, or files that exploit server-side processing.
File Upload Vulnerabilities occur when an application allows users to upload files without adequate validation of the file's type, content, size, or name. Attackers can exploit these weaknesses to upload malicious files that are then executed by the server or served to other users.
Attack vectors include: uploading web shells (server-side scripts that provide remote access); uploading files with double extensions (e.g., `shell.php.jpg`) that are executed by misconfigured servers; exploiting image processing libraries by uploading crafted images that trigger buffer overflows or command injection (ImageTragick); uploading oversized files for denial of service; uploading HTML/SVG files containing JavaScript for stored XSS; and path traversal in upload filenames to write files to arbitrary locations.
Unrestricted file upload can lead to complete server compromise if an attacker uploads and executes a web shell. Even in modern cloud-hosted environments, malicious uploads can serve malware to users, enabling drive-by attacks. Uploaded SVG or HTML files can execute JavaScript in the application's origin, effectively creating stored XSS. Large file uploads can be used for denial of service or to incur significant storage costs. Files with embedded malicious content (EXIF data, Office macros, PDF JavaScript) can compromise users who download and open them.
Validate file types using both the Content-Type header and actual file content (magic bytes), not just the file extension. Implement allowlists of permitted file types. Set strict file size limits. Generate random filenames on the server (never use user-supplied filenames). Store uploaded files outside the web root or in a separate storage service. Serve uploaded files with Content-Disposition: attachment and appropriate Content-Type headers. Use a CDN or separate domain for serving user content to isolate from the application's origin. Scan uploaded files for malware. Process images through a re-encoding step to strip malicious content. Implement virus scanning for document uploads.
Select your framework for a detailed guide on fixing File Upload Vulnerabilities with framework-specific code examples and security checklists.
Cross-Site Scripting (XSS)
XSS allows attackers to inject malicious scripts into web pages viewed by other users, enabling session hijacking, data theft, and defacement.
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.
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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