Format String Injection
ID |
php.format_string_injection |
Severity |
low |
Resource |
Injection |
Language |
Php |
Tags |
CWE:134, NIST.SP.800-53, PCI-DSS:6.5.1 |
Description
Format String Injection occurs when user-controlled input is used directly in formatting functions without proper validation, potentially leading to memory corruption, information leaks, or arbitrary code execution.
Rationale
Format string vulnerabilities in PHP arise when functions such as printf, sprintf, or vfprintf process untrusted input as a format string. Attackers can manipulate format specifiers (%s, %x, %n, etc.) to read or write arbitrary memory locations, leading to severe security risks.
Here’s a simple example illustrating the problem:
<?php
// Vulnerable code
$user_input = $_GET['data'];
$result = sprintf($user_input, "Injected"); // Dangerous: User input controls the format string
echo $result;
?>In this scenario, if the user input includes something like "%x %x %x", the program may try to interpret these as format specifiers, leading to unexpected results, or even a crash depending on the logic afterward. The severity of this vulnerability can depend on the ability of the attacker to manipulate the format string to cause info leaks or stack manipulation under specific circumstances.
Remediation
To mitigate format string injection vulnerabilities, always treat user input as data rather than a format string. This can be achieved by explicitly specifying the format string in functions that support formatting.
Consider a fixed example:
<?php
// Secure implementation
$user_input = $_GET['data'];
$result = sprintf("%s", $user_input); // Safe: Explicit format specifier
echo $result;
?>In this corrected example, user input is appended directly to the string. Though this is a simplistic approach, it avoids potential vulnerabilities related to format string specifiers altogether.