> For the complete documentation index, see [llms.txt](https://playbook.sidthoviti.com/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://playbook.sidthoviti.com/devsecops/secure-coding/code-review-examples/command-injection.md).

# Command Injection

**Example 1: Python**

**Vulnerable Code:**

```python
pythonCopy codeos.system("ls " + request.args.get('dir'))
```

**Reason for vulnerability:** User input is directly used in the command, allowing command injection.

**Fixed Code:**

```python
pythonCopy codedir = request.args.get('dir')
if not is_valid_directory(dir):
    abort(400)
os.system("ls " + dir)
```

**Reason for fix:** Validate and sanitize user input before using it in the command.

**Example 2: Java**

**Vulnerable Code:**

```java
javaCopy codeString dir = request.getParameter("dir");
Runtime.getRuntime().exec("ls " + dir);
```

**Reason for vulnerability:** User input is directly used in the command, allowing command injection.

**Fixed Code:**

```java
javaCopy codeString dir = request.getParameter("dir");
if (!isValidDirectory(dir)) {
    throw new IllegalArgumentException("Invalid directory");
}
Runtime.getRuntime().exec("ls " + dir);
```

**Reason for fix:** Validate and sanitize user input before using it in the command.

***

### Python Example

#### Vulnerable Code:

```python
pythonCopyimport os

def run_command(command):
    os.system(command)

def process_user_input(user_input):
    run_command(f"echo {user_input} >> log.txt")
```

#### Reason for Vulnerability:

This code directly incorporates user input into a shell command, allowing injection of arbitrary commands.

#### Fixed Code:

```python
pythonCopyimport subprocess
import shlex

def run_command(command, args):
    subprocess.run([command] + args, check=True)

def process_user_input(user_input):
    run_command("echo", [user_input, ">>", "log.txt"])
```

#### Reason for Fix:

The fixed code uses `subprocess.run` with a list of arguments, which prevents shell injection. It also uses `shlex.quote` for extra safety when constructing commands.

***

### Java Example

#### Vulnerable Code:

```java
javaCopyimport java.io.IOException;

public class PingUtility {
    public static String ping(String host) throws IOException {
        Process process = Runtime.getRuntime().exec("ping " + host);
        // Read and return process output
    }
}
```

#### Reason for Vulnerability:

This code directly incorporates user input into a shell command, allowing injection of arbitrary commands.

#### Fixed Code:

```java
javaCopyimport java.io.IOException;

public class PingUtility {
    public static String ping(String host) throws IOException {
        if (!isValidHostname(host)) {
            throw new IllegalArgumentException("Invalid hostname");
        }
        ProcessBuilder pb = new ProcessBuilder("ping", host);
        Process process = pb.start();
        // Read and return process output
    }

    private static boolean isValidHostname(String host) {
        return host.matches("^[a-zA-Z0-9.-]+$");
    }
}
```

#### Reason for Fix:

The fixed code validates the hostname and uses ProcessBuilder to safely construct the command.

### Ruby Example

#### Vulnerable Code:

```ruby
rubyCopydef execute_command(command)
  `#{command}`
end

puts execute_command(ARGV[0])
```

#### Reason for Vulnerability:

This code directly executes user-provided input as a shell command, allowing arbitrary command execution.

#### Fixed Code:

```ruby
rubyCopyrequire 'open3'

def execute_command(command, *args)
  raise ArgumentError, "Invalid command" unless ['ls', 'cat', 'echo'].include?(command)
  stdout, stderr, status = Open3.capture3(command, *args)
  raise "Command failed: #{stderr}" unless status.success?
  stdout
end

begin
  puts execute_command(ARGV[0], *ARGV[1..-1])
rescue ArgumentError, RuntimeError => e
  puts "Error: #{e.message}"
end
```

#### Reason for Fix:

The fixed code uses a whitelist of allowed commands and Open3.capture3 for safer command execution.


---

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