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About CWE ID 78/77
Improper Neutralization of Special Elements used in an OS Command ('OS Command Injection')
Improper Neutralization of Special Elements used in a Command ('Command Injection')
The vulnerability arises when an application fails to properly sanitize or validate user input that is used to construct a system command.
Impact
System compromise
Data loss or damage
Unauthorized data access
Denial of service
Example with Code Explanation
PHP
Let us consider an example case and understand the CWE 78/77 with context of Vulnerable code and Mitigated code.
Vulnerable Code
<?php
$filename = $_POST['filename'];
// Execute the "cat" command on the specified file
$cmd = "cat " . $filename;
$output = shell_exec($cmd);
// Display the file contents to the user
echo "<pre>" . $output . "</pre>";
?>
This code is vulnerable to command injection as user input for filename is concatenated into a string and then passed directly to the shell_exec() function without any input validation or sanitization. This can allow an attacker to inject additional commands into the cmd variable, potentially leading to arbitrary code execution on the underlying system.
Some of the ways the Vulnerable code can be mitigated is:
Whitelist: Use a whitelist approach to restrict the filenames that can be accessed.
Input validation: We need to ensure that the filename parameter only contains valid characters and is in a valid format.
Input sanitization: We need to sanitize the filename parameter to remove any special characters that could be used to inject commands or modify the command. For example, we can use the escapeshellarg() function to escape any special characters in the filename parameter.
Use Safe API: Use a safer function like file_get_contents() to read the contents of the file instead of executing a shell command. This approach removes the need for shell injection altogether and is more secure.
Mitigated Code
<?php
// Check if the request method is POST
if ($_SERVER['REQUEST_METHOD'] !== 'POST') {
http_response_code(405); // Method Not Allowed
exit('Only POST requests are allowed.');
}
// Check if the filename parameter is provided
if (!isset($_POST['filename'])) {
http_response_code(400); // Bad Request
exit('Please provide a filename parameter.');
}
// Get the filename parameter
$filename = $_POST['filename'];
// Define a whitelist of allowed file names
$whitelist = array("file1.txt", "file2.txt", "file3.txt");
// Check if the filename parameter is in the whitelist
if (!in_array($filename, $whitelist)) {
http_response_code(403); // Forbidden
exit('Access denied.');
}
// Construct the full path to the file
$file_path = "/path/to/files/" . $filename;
// Check if the file exists
if (!file_exists($file_path)) {
http_response_code(404); // Not Found
exit('File not found.');
}
// Use file_get_contents to read the contents of the file
$content = file_get_contents($file_path);
// Display the file contents to the user
echo "<pre>" . htmlspecialchars($content, ENT_QUOTES, 'UTF-8') . "</pre>";
?>
The Mitigated code does the following:
The code does not use any potentially vulnerable functions that take user input and execute it as operating system commands.
The code uses a whitelist approach to restrict the filenames that can be accessed.
The code performs basic input validation on the filename parameter to ensure that it is provided and not empty.
Java
Vulnerable Code
import java.io.*;
public class CommandExecutor {
public static void main(String[] args) throws IOException {
String command = args[0];
String line = "";
// Execute the specified command using Runtime.exec()
Process process = Runtime.getRuntime().exec(command);
// Read the output of the command from the input stream
BufferedReader inputReader = new BufferedReader(new InputStreamReader(process.getInputStream()));
while ((line = inputReader.readLine()) != null) {
System.out.println(line);
}
}
}
The Vulnerable code reads a command to execute from the first command-line argument and passes it to the Runtime.exec() method without any input validation or sanitation. An attacker could supply a malicious command as the argument, potentially allowing them to execute arbitrary code on the system.
Some of the ways the Vulnerable code can be mitigated is:
Input validation: Validate and sanitize user input before using it to execute commands. This includes checking the input for any unexpected or invalid characters and only allowing a predefined set of characters.
Least privilege: When executing commands or accessing files, ensure that the process has the minimum necessary permissions. This reduces the risk of attackers gaining access to sensitive information or resources.
Use a whitelist of allowed commands: Instead of validating user input to block certain characters, you can create a whitelist of allowed commands that the application can execute. This way, even if an attacker manages to inject code, it won't execute because it's not in the allowed list of commands.
Mitigated Code
import java.io.*;
import java.util.*;
public class Example {
public static void main(String[] args) {
// Check if the request method is POST
if (!"POST".equals(request.getMethod())) {
response.sendError(HttpServletResponse.SC_METHOD_NOT_ALLOWED, "Only POST requests are allowed.");
return;
}
// Check if the filename parameter is provided
String filename = request.getParameter("filename");
if (filename == null || filename.isEmpty()) {
response.sendError(HttpServletResponse.SC_BAD_REQUEST, "Please provide a filename parameter.");
return;
}
// Validate the filename parameter
if (!isValidFilename(filename)) {
response.sendError(HttpServletResponse.SC_FORBIDDEN, "Access denied.");
return;
}
// Construct the full path to the file
String filePath = "/path/to/files/" + filename;
// Check if the file exists and is a regular file
File file = new File(filePath);
if (!file.exists() || !file.isFile()) {
response.sendError(HttpServletResponse.SC_NOT_FOUND, "File not found.");
return;
}
// Use a buffered reader to read the contents of the file
BufferedReader reader = null;
try {
reader = new BufferedReader(new FileReader(file));
String line = null;
StringBuilder sb = new StringBuilder();
while ((line = reader.readLine()) != null) {
sb.append(line).append("\n");
}
response.getWriter().write(sb.toString());
} catch (IOException e) {
response.sendError(HttpServletResponse.SC_INTERNAL_SERVER_ERROR, "Internal server error.");
} finally {
if (reader != null) {
try {
reader.close();
} catch (IOException e) {
// ignore
}
}
}
}
private static boolean isValidFilename(String filename) {
// Define a whitelist of allowed file names
List<String> whitelist = Arrays.asList("file1.txt", "file2.txt", "file3.txt");
// Check if the filename parameter is in the whitelist
return whitelist.contains(filename);
}
}
The Mitigated code does the following:
The code only allows access to a predefined list of files and doesn't allow access to any other file on the server.
The code ensures that the provided file exists and is a regular file before attempting to read it. It also uses a buffered reader to read the contents of the file, which is a safe way to read files.
It checks if the request method is POST, checks if the filename parameter is provided, and validates the filename parameter.
C
Vulnerable Code
#include <stdio.h>
#include <string.h>
int main(int argc, char **argv) {
char filename[100];
FILE *fp;
if (argc != 2) {
printf("Usage: %s <filename>\n", argv[0]);
return 1;
}
strcpy(filename, argv[1]);
fp = fopen(filename, "r");
if (fp == NULL) {
printf("Unable to open file: %s\n", filename);
return 1;
}
// read and process file contents
fclose(fp);
return 0;
}
The code is vulnerable as the program takes a filename as a command-line argument and attempts to open the file using fopen(). However, there is no validation or sanitization of the filename input, so an attacker could craft a malicious filename that includes special characters or escape sequences to bypass any input filtering and access or modify sensitive files on the system.
Some of the ways the vulnerable code can be mitigated is:
Whitelisting can be used to restrict the input to only accept certain values.
One of the most important mitigations is to validate any input that is being used in a potentially dangerous function such as strcpy() or fopen()
Sanitization involves removing or escaping any potentially dangerous characters or sequences from input before it is used in a function.
Mitigated Code
#include <stdio.h>
#include <string.h>
#include <ctype.h>
int main(int argc, char **argv) {
char filename[100];
FILE *fp;
const char *allowed_filenames[] = { "file1.txt", "file2.txt", "file3.txt", NULL };
int i;
if (argc != 2) {
printf("Usage: %s <filename>\n", argv[0]);
return 1;
}
// validate filename against whitelist
for (i = 0; allowed_filenames[i] != NULL; i++) {
if (strcmp(argv[1], allowed_filenames[i]) == 0) {
break;
}
}
if (allowed_filenames[i] == NULL) {
printf("Invalid filename: %s\n", argv[1]);
return 1;
}
// validate filename against allowed characters
for (i = 0; argv[1][i] != '\0'; i++) {
if (!isalnum(argv[1][i]) && argv[1][i] != '.' && argv[1][i] != '_') {
printf("Invalid character in filename: %c\n", argv[1][i]);
return 1;
}
}
fp = fopen(argv[1], "r");
if (fp == NULL) {
printf("Unable to open file: %s\n", argv[1]);
return 1;
}
// read and process file contents
fclose(fp);
return 0;
}
The Mitigated code does the following:
The user-controlled input is the filename passed as a command-line argument. However, this input is validated against a whitelist of allowed filenames and checked to ensure that it contains only alphanumeric characters, periods, and underscores.
The code does not use any shell metacharacters that could be used to execute arbitrary commands. For example, the code does not use system() or any other function that could be used to execute a shell command.
The code does not use system-level commands that could be vulnerable to command injection. The only system-level function called is fopen(), which is a library function that opens a file for reading.