PHP is a powerful and versatile scripting language widely used for web development. However, it also includes several functions that, if misused or left unrestricted, can pose significant security risks. This article provides an in-depth explanation of some potentially dangerous PHP functions, why they are considered risky, and how to handle them securely.

1. exec()

The exec() function executes a shell command and returns the last line of the output.

Example:

$output = exec("ls -l");

Risks:

  • Allows arbitrary command execution, leading to remote code execution (RCE).
  • Attackers can inject malicious commands.

Mitigation:

  • Avoid using exec() unless absolutely necessary.
  • Validate and sanitize user input rigorously.
  • Disable the function in php.ini: 
    disable_functions = exec

2. passthru()

The passthru() function executes a shell command and outputs the raw result.

Example:

passthru("cat /etc/passwd");

Risks:

  • Directly outputs sensitive command results, increasing the risk of information leakage.

Mitigation:

  • Use safer alternatives or disable the function: 
    disable_functions = passthru

3. shell_exec()

The shell_exec() function executes a command and returns the complete output as a string.

Example:

$result = shell_exec("df -h");

Risks:

  • Can lead to command injection if user input is not validated.
  • Exposes server resources to malicious commands.

Mitigation:

  • Avoid use, or sanitize inputs and disable it: 
    disable_functions = shell_exec

4. system()

The system() function executes a command and outputs the result immediately.

Example:

system("whoami");

Risks:

  • Similar to exec() and shell_exec(), it can be exploited for RCE.

Mitigation:

  • Disable the function unless required: 
    disable_functions = system

5. proc_open()

The proc_open() function opens a process resource and provides more control over input/output streams.

Example:

$process = proc_open('ls -la', $descriptorspec, $pipes);

Risks:

  • Provides extensive control over processes, which attackers can exploit for advanced payloads.

Mitigation:

  • Disable it if not needed: 
    disable_functions = proc_open

6. popen()

The popen() function opens a process file pointer, allowing interaction with the process.

Example:

$handle = popen("ls -la", "r");

Risks:

  • Similar to proc_open(), it can be abused for malicious activities.

Mitigation:

  • Disable it unless necessary: 
    disable_functions = popen

7. curl_exec() and curl_multi_exec()

These functions execute cURL sessions, enabling HTTP requests to external servers.

Example:

$ch = curl_init();
curl_setopt($ch, CURLOPT_URL, "http://example.com");
curl_exec($ch);
curl_close($ch);

Risks:

  • Allows interactions with external servers, which can be used to exfiltrate data.

Mitigation:

  • Use only with trusted URLs.
  • Disable these functions if external requests are not required: 
    disable_functions = curl_exec,curl_multi_exec

8. parse_ini_file()

The parse_ini_file() function parses .ini configuration files.

Example:

$config = parse_ini_file("config.ini");

Risks:

  • Can reveal sensitive data if .ini files are not properly protected.

Mitigation:

  • Restrict access to .ini files via web server configurations.
  • Disable the function if unnecessary: 
    disable_functions = parse_ini_file

9. show_source()

The show_source() function outputs the source code of a file.

Example:

show_source("example.php");

Risks:

  • Exposes PHP source code, which can lead to the disclosure of sensitive logic or credentials.

Mitigation:

  • Disable this function in production environments: 
    disable_functions = show_source

Why Are These Functions Dangerous?

These functions allow direct interaction with the operating system, external systems, or the application’s internal files. If improperly secured, they can be exploited for:

  • Command Injection
  • Remote Code Execution
  • Data Exfiltration
  • Information Leakage

General Best Practices

  1. Disable Unnecessary Functions: Use the disable_functions directive in php.ini to turn off dangerous functions: 
    disable_functions = exec,passthru,shell_exec,system,proc_open,popen,curl_exec,curl_multi_exec,parse_ini_file,show_source
  2. Validate User Input: Always sanitize and validate input data to prevent injection attacks.
  3. Use a Web Application Firewall (WAF): Tools like ModSecurity can block malicious requests.
  4. Keep PHP Updated: Regular updates ensure known vulnerabilities are patched.