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Last updated
Local File Inclusion (LFI) attacks are not only about exploiting vulnerabilities but also about crafting sophisticated payloads to bypass security measures such as input filters, especially those implemented using regular expressions (regex). Let's delve into the intricacies of LFI attacks with advanced payloads:
1. Directory Traversal: Traditional LFI attacks often leverage directory traversal sequences to navigate the server's file system. For example, "../../../../../etc/passwd" aims to traverse multiple directory levels and include the "/etc/passwd" file, revealing sensitive user account information.
2. Null Byte Injection: Exploiting certain languages or platforms that interpret null bytes as string terminators, attackers can append a null byte ("%00") to the end of a file path to bypass input filters and include arbitrary files. For instance, "../../../../../etc/passwd%00" terminates the string before it reaches the ".php" extension, evading regex-based filters.
3. Character Encoding: Attackers can obfuscate payloads using character encoding techniques to evade detection. For example, encoding the directory traversal sequence "../../../../../" as its URL-encoded equivalent "%2e%2e%2f" makes it less recognizable to regex-based filters, allowing attackers to bypass input validation checks.
4. Double URL Encoding: By applying double URL encoding, attackers can further obfuscate payloads to circumvent detection mechanisms. For instance, double encoding the null byte ("%00") as "%2500" makes it even more challenging for regex-based filters to detect and block malicious payloads.
5. Regular Expression Bypass: Crafty attackers may exploit weaknesses in regex-based input filters by using alternative syntax or character classes. For example, instead of using the dot (".") metacharacter to represent any character, attackers may use character classes like "[^/]" to match any character except a forward slash ("/"), effectively bypassing regex-based filters.
In a practical scenario, let's consider a web application vulnerable to LFI attacks with regex-based input filters. The application restricts file inclusion to files with the ".php" extension using the regex pattern "/.php$/".
To bypass the regex-based filter, an attacker employs a null byte injection payload:
By appending "%00" to the file path, the attacker terminates the string before it reaches the ".php" extension, evading the input filter and successfully including the "/etc/passwd" file.
In this example, the null byte injection payload serves as a potent technique for bypassing regex-based input filters and executing successful LFI attacks. By mastering advanced payloads and understanding the intricacies of input filtering mechanisms, attackers can exploit LFI vulnerabilities with precision and effectiveness.