File Inclusion - Definition & Overview

What is file inclusion?

File inclusion is a web application vulnerability that occurs when an application dynamically loads files based on user-supplied input without adequate validation. When file paths or file names are improperly handled, attackers may manipulate these inputs to access unauthorized files or trigger the execution of unintended content. This can result in the exposure of sensitive server data and, in some cases, enable further compromise of the application or underlying system.

Types of File Inclusion Vulnerabilities

File inclusion vulnerabilities are primarily classified into two types:

Local File Inclusion (LFI)

Local File Inclusion occurs when a web application includes files from the local server based on user input that is not properly validated. Attackers can exploit this weakness to access sensitive system files, configuration data, or application source code. In certain scenarios, LFI may also be leveraged to execute unintended code when combined with other vulnerabilities.

Remote File Inclusion (RFI)

Remote File Inclusion occurs when a web application allows files hosted on external servers to be included through user-controlled input. Attackers exploit this behavior to load and execute malicious scripts from remote sources, such as backdoor shells or malware. Unlike LFI, which targets files on the local server, RFI involves content retrieved from external locations.

How to Detect LFI and RFI Vulnerabilities

File inclusion vulnerabilities can be identified through a combination of code analysis, input evaluation, and monitoring practices:

• Code Review: Examine application logic where user input influences file inclusion or file path construction. Pay close attention to file-handling functions and dynamic include mechanisms.
• Input Evaluation: Assess parameters, URLs, and request fields that accept file names or paths to ensure they do not permit directory traversal patterns or unauthorized references.
• Boundary and Validation Testing: Evaluate how the application handles unexpected input values, special characters, and edge cases to determine whether validation controls are effective.
• File Access Controls: Verify that file inclusion mechanisms restrict access to approved files only, preferably through allowlist-based validation.
• Automated Security Scanning: Use vulnerability scanning tools to identify potential file inclusion weaknesses by analyzing application behavior and responses.
• Log and Error Analysis: Review server logs and application error messages for unusual file access attempts or path-related disclosures that may indicate LFI or RFI exposure.

How to Prevent File Inclusion Vulnerabilities?

Preventing file inclusion vulnerabilities requires enforcing strict controls over how files are referenced and loaded within an application:

• Validate and Control User Input:  Avoid using raw user input directly in file inclusion logic. Where dynamic inclusion is required, map inputs to predefined file references rather than constructing file paths dynamically.
• Restrict File Inclusion Scope: Limit file inclusion to a designated directory and block path traversal techniques that attempt to access files outside approved locations.
• Use Allowlist-Based File Selection: Define an explicit list of permitted files or sources that the application is allowed to include, rather than relying on user-supplied paths.
• Disable Unnecessary Remote Inclusion: Where possible, disable remote file inclusion features at the server or application level to reduce exposure to externally hosted malicious content.