Creating "Secure" PHP Applications, Part 1, Explicit Code & QA

So, who are you, anyway?

Bryan C. Geraghty
Security Consultant at Security PS
@archwisp

I’m a Sr. PHP developer with a systems and security
engineering background - turned application security
consultant

Remember, layers
Simpler is easier to test
Don’t make assumptions
Compromised browser = game over

These are not specifically security activities but they produce more
stable code

Don’t rely on the server’s configuration
Your bootstrap should Absolute Minimum
set the defaults for any  Include paths
PHP core functionality  Error reporting
that your application is
 Time zones
using.
These must be
customizable for every
environment
e.g. (dev, staging, live)

Example Bootstrap
<?php

// All environment configuration for this application is done from this file

ini_set('display_errors', 0);
ini_set('error_reporting', -1);

date_default_timezone_set('UTC');

$rootPath = dirname(__FILE__);

set_include_path(sprintf(
'%s%s%s%s',
$rootPath, PATH_SEPARATOR, $rootPath, '/third-party'
));

require_once('MindFrame2/AutoLoad.php');
MindFrame2_AutoLoad::install();

Don’t use global constants, variables, or registries

 You can’t be certain that nobody else will ever use the
same global constant name
 Tracking down where a value is assigned to a global
variable is usually very difficult
 Global variables remove control of that variable from the
current scope
 Global registries are really just fancy global variables and
the same problems apply

Value Reference Rules
 Values should only be referenced in one of the following
ways:
 A direct reference in the current scope:
$limit = $this->_objectPool->getConfig()-
>getDefaultLimit();
 A parameter:
public function getUsers($limit) {
return $this->_objectPool-
>getMapper(‘User’)->fetchAll($limit);
}

Don’t use function parameter defaults
"There should be one-- and preferably only one --
obvious way to do it." - Tim Peters

 Reduces understanding from the caller perspective
 Often add unnecessary complexity
 Represents the most common use-case
 Other use-cases are often ignored entirely
 Changing the default breaks all implementations
 Adding parameters breaks default implementations

Default parameter example
class User
{
public function __construct($name, $email, $status = self::STATUS_ACTIVE) {
$this->_name = $name;
$this->_email = $email;
$this->_status = $status;
}
}

function createUser($name, $email, $status) {
$user = new User($name, email);
return $this->_objectPoool->getMapper(‘User’)->create($user);
}

Use explicit operators whenever possible
This was actual production code.
if ($sincedate && (int)$sincedate) {
$param["UpdatedSinceDate"] =
date("Y-m-d H:i:s", strtotime(time() - ((int)$sincedate)*60*60);
} elseif ($sincedate && strtotime($sincedate)) {
$param["UpdatedSinceDate"] =
date("Y-m-d H:i:s", strtotime($sincedate));
} elseif(!isset($param["UpdatedSinceDate"])) {
exit ("Invalid param UpdatedSinceDate");
}

Do you think this code worked as the author intended when $sincedate was set to
'2011-05-31'?

Use constants for possible values
Constants cannot be re-defined, so using a constant for configuration limits your code
to only ever be able to run in one configuration in a given stack.

// Bad: This code cannot execute differently in the same stack
define(‘PDO_FETCH_MODE’, 2);
$dbi->fetchAll(PDO_FETCH_MODE);

// Better: The value can change but this literal value means nothing without
// referencing the documentation
$dbi->fetchAll(2);

// Best: Implemented using a meaningful constant which represents a *possible* value
$dbi->fetchAll(PDO::FETCH_ASSOC);

Use wrappers for super-globals
 $_REQUEST, $_GET, $_SESSION, and $_FILES are the most
commonly used PHP super-globals but there are others
 It’s easy to create some serious security problems
 It’s best to use a wrapper for interacting with these
variables

Simulate strong/static types
PHP is dynamically typed, so we cannot do true strong or static typing but we can
put controls in place which provide the same protection.

private function _buildSelectDatabaseTableSql($database_name, $table_name,
array $select_data, array $order_by_columns, $offset, $limit)
{
MindFrame2_Core::assertArgumentIsNotBlank($database_name, 1, 'database_name');
MindFrame2_Core::assertArgumentIsNotBlank($table_name, 2, 'table_name');
MindFrame2_Core::assertArgumentIsInt($offset, 5, 'offset');
MindFrame2_Core::assertArgumentIsInt($limit, 6, 'limit');

$skel = "SELECTn %snFROMn %s.%sn%s%s%s%s;";

$sql = sprintf($skel,
$this->_buildSelectTableSqlFieldClause($table_name),
$this->getSharedModule()->escapeDbElementName($database_name),
$this->getSharedModule()->escapeDbElementName($table_name),
$this->_buildSelectTableSqlJoinClause(
$this->getSharedModule()->getDatabase()->getName(), $table_name),
$this->_buildSelectTableSqlWhereClause($table_name, $select_data),
$this->_buildSelectTableSqlOrderByClause(
$table_name, $order_by_columns),
$this->_buildSelectTableSqlLimitClause($offset, $limit));

return $sql;
}

Example Typing Assertions
public static function assertArgumentIsInt($value, $position, $name)
{
if (!is_int($value)) {
$skel = 'Expected integer value for argument #%d (%s), %s given';
$message = sprintf($skel, $position, $name, gettype($value));
throw new InvalidArgumentException($message);
}
}

public static function assertArgumentIsNotBlank($value, $position, $name)
{
if (trim($value) === '') {
$skel = 'Argument #%d (%s) cannot be blank';
$message = sprintf($skel, $position, $name);

throw new InvalidArgumentException($message);
}
}

Use PHPMD & CodeSniffer
This is the ./bin/codecheck script in MindFrame2
#!/bin/bash

WD="`/usr/bin/dirname $0`/../";
echo Changing to working directory: $WD;
cd $WD;

if [ -z $1 ]; then
DIR='.';
else
DIR=$1;
fi

phpmd --exclude coverage $DIR text codesize,unusedcode,naming,design
phpcs --ignore=coverage --standard=`pwd`/Standards/MindFrame2 $DIR
phpcs --ignore=coverage --report=source --standard=`pwd`/Standards/MindFrame2 $DIR

Example PHPMD Output
bryan@ubuntu-virtual ~/Code/MindFrame2 [130] $ ./bin/codecheck Dbms/Dbi
Changing to working directory: ./bin/../

/home/bryan/Code/MindFrame2/Dbms/Dbi/Distributed.php:128 Avoid unused local
variables such as '$partner'.
variables such as '$data'.
variables such as '$index'.
/home/bryan/Code/MindFrame2/Dbms/Dbi/Distributed.php:240 Avoid unused private
methods such as '_buildReplicaDatabaseSuffix'.
/home/bryan/Code/MindFrame2/Dbms/Dbi/Single.php:25 This class has too many
methods, consider refactoring it.
/home/bryan/Code/MindFrame2/Dbms/Dbi/Single.php:150 Avoid unused parameters such as
'$options'.

Example PHPCS Output
FILE: /home/bryan/Code/MindFrame2/Dbms/Dbi/Single.php
--------------------------------------------------------------------------------
FOUND 6 ERROR(S) AFFECTING 6 LINE(S)
--------------------------------------------------------------------------------
39 | ERROR | Public member variables ("_connection") are forbidden
62 | ERROR | Whitespace found at end of line
160 | ERROR | Line exceeds maximum limit of 80 characters; contains 97
| | characters
298 | ERROR | Line exceeds maximum limit of 80 characters; contains 81
| | characters
--------------------------------------------------------------------------------

Unit testing / TDD
 Test critical functionality at a minimum
 Add edge cases to regression tests as they are discovered
 Test-driven development only works if the entire team is
dedicated to it
 I personally don’t trust code that isn’t regression-tested;
even if it was written by me

Continuous Integration
 Automated system for code analysis and regression
testing
 There are a lot of continuous integration systems for PHP
 If your tests aren’t automated, very few people will know
or care when they break

Deployment
 Manually copying files is riddled with problems
 Tagging releases and re-deploying is a complicated and
time-consuming process
 Deployment systems automate the build and deployment
process

ACLs
MAC
Thread limits
Unwanted services

If you’re interested in an application security career, come talk with
me.

Creating "Secure" PHP Applications, Part 1, Explicit Code & QA

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