Category Archives: PHP

PHP

Exception Handling

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Exception handling is used to change the normal flow of the code execution if a specified error (exceptional) condition occurs. This condition is called an exception.

Exceptions or errors (more on that see below) end code execution, unless they are handled with try-catch.

try {
// code that throws an Exception
} catch (Exception $ex) {
// Exception is handled, code
continues
}

Several catch statements may be used to differentiate between several kinds of exceptions and errors.

try {
// code that throws an Exception
} catch (CustomExceptionClass $ex) {
// custom Exception is handled
} catch (Exception $ex) {
// Exception is handled
} catch (Error $err) {
// Error is handled
}

Several errors may be handled with the same code.

try {
// code that throws an Exception
} catch (Exception | CustomException $ex)
{
// custom Exception and Exception
}

Optional finally block contains code that runs after the trycatch block, no matter whether an error was caught or not

This is what normally happens when an exception is triggered:

  • The current code state is saved
  • The code execution will switch to a predefined (custom) exception handler function
  • Depending on the situation, the handler may then resume the execution from the saved code state, terminate the script execution or continue the script from a different location in the code
PHP

Error Class

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• Since php 7, many errors php reports now throw an error exception, not a fatal error as before.

• Subclasses of error exist for the specific type of error, such as parseerror or typeerror.

• To facilitate backwards compatibility, error is not derived from exception.

• Both error and exception implement the throwable interface

PHP

Escape Output

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• One of two fundamental security rules: (1) filter and validate all input; (2) escape output

• Always escape outside data unless previously filtered

• Typical output formats that require escaping when containing user data: html, json, sql

•  Never rely on client side (javascript) filtering

• Functions used to escape data before outputting within html: htmlspecialchars() htmlentities() strip_tags()

PHP

Input Filtering

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• Input is everything that comes as part of the http request

• Some data does not seem to be input, but may contain data originating from the user, thus must be considered as input (ex: session data that was originally supplied by the user)

Character Set

• Risk

 attack vectors may employ a non-standard char set (ex: utf-8 encoded) that may be missed by filtering, but executed by the browser

• counter

 Use the same char set for filtering as the target procedure

 convert charsets prior to filtering content-type: text/html; charset=”utf-8″

 use php’s filter extension

 use filters native to the database (ex: db quoting functions)

PHP

Email Injection

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Email injection is a security vulnerability that can occur in Internet applications that are used to send email messages. It is the email equivalent of HTTP Header Injection. Like SQL injection attacks, this vulnerability is one of a general class of vulnerabilities that occur when one programming language is embedded within another.

Counter Measures

• Do not provide open relays

• Open the smtp port only if essential

• Use a ‘tarpits’ technique to slow requests as a means of dissuading attacks

PHP

escapeshellcmd

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escapeshellcmd() escapes any characters in a string that might be used to trick a shell command into executing arbitrary commands. This function should be used to make sure that any data coming from user input is escaped before this data is passed to the exec() or system() functions, or to the backtick operator.

Following characters are preceded by a backslash: &#;`|*?~<>^()[]{}$\, \x0A and \xFF. ‘ and ” are escaped only if they are not paired. In Windows, all these characters plus % and ! are replaced by a space instead.

<?php
 $command = './configure '.$_POST['configure_options'];
 $escaped_command = escapeshellcmd($command);
 system($escaped_command);
 ?>
PHP

allow_url_include

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The PHP option allow_url_include normally allows a programmer to include() a remote file (as PHP code) using a URL rather than a local file path. For security reasons, this feature should be disabled. If a script claims to require this feature, you should look into alternative software, as the use of this feature indicates serious design flaws.

There are a number of reasons why URL includes should always be avoided:

It’s insecure – if your application can be tricked into including content from a URL outside itself (and there are a number of common ways this can happen), an attacker can force your application to start running code from their own web site.

It’s inefficient – if your PHP script includes content from a URL, then the web server must make HTTP requests to generate the page. This makes your page load much slower than necessary, especially if the site you’re loading content from is responding slowly.

It’s unreliable, for the same reasons – if the web server you are loading content from occasionally fails to respond, your web site also sometimes fails to load properly.

It’s usually unnecessary – in most cases, allow_url_include can be avoided either by including the content directly (if it is being loaded from a domain you host) or by loading and printing the content without evaluating it as PHP.

PHP

Remote Code Injection

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Remote code injections attempt to run the attacker’s code on a server, often by exploiting the functionality of the include or require functions.

The eval(), exec(), system(), and shell_exec() functions are vulnerable to remote code injections.

Include / Require attacks occur when including and executing files (possible from remote servers and includes remote code execution)

Counter Measures

• Check data against a whitelist

• Remove paths using basename()

• Set allow_url_include = off in php.ini that helps somewhat but not sufficient, as some attack vectors remain open