SecureReality has put out a very interesting paper titled ``A Study In Scarlet - Exploiting Common Vulnerabilities in PHP'' [Clowes 2001], which discusses some of the problems in writing secure programs in PHP, particularly in versions before PHP 4.1.0. Clowes concludes that ``it is very hard to write a secure PHP application (in the default configuration of PHP), even if you try''.
Granted, there are security issues in any language, but one particular issue stands out in older versions of PHP that arguably makes older PHP versions less secure than most languages: the way it loads data into its namespace. By default, in PHP (versions 4.1.0 and lower) all environment variables and values sent to PHP over the web are automatically loaded into the same namespace (global variables) that normal variables are loaded into - so attackers can set arbitrary variables to arbitrary values, which keep their values unless explicitly reset by a PHP program. In addition, PHP automatically creates variables with a default value when they're first requested, so it's common for PHP programs to not initialize variables. If you forget to set a variable, PHP can report it, but by default PHP won't - and note that this simply an error report, it won't stop an attacker who finds an unusual way to cause it. Thus, by default PHP allows an attacker to completely control the values of all variables in a program unless the program takes special care to override the attacker. Once the program takes over, it can reset these variables, but failing to reset any variable (even one not obvious) might open a vulnerability in the PHP program.
For example, the following PHP program (an example from Clowes) intends to only let those who know the password to get some important information, but an attacker can set ``auth'' in their web browser and subvert the authorization check:
<?php if ($pass == "hello") $auth = 1; ... if ($auth == 1) echo "some important information"; ?>
I and many others have complained about this particularly dangerous problem; it's particularly a problem because PHP is widely used. A language that's supposed to be easy to use better make it easy to write secure programs in, after all. It's possible to disable this misfeature in PHP by turning the setting ``register_globals'' to ``off'', but by default PHP versions up through 4.1.0 default set this to ``on'' and PHP before 4.1.0 is harder to use with register_globals off. The PHP developers warned in their PHP 4.1.0 announcenment that ``as of the next semi-major version of PHP, new installations of PHP will default to having register_globals set to off.'' This has now happened; as of PHP version 4.2.0, External variables (from the environment, the HTTP request, cookies or the web server) are no longer registered in the global scope by default. The preferred method of accessing these external variables is by using the new Superglobal arrays, introduced in PHP 4.1.0.
PHP with ``register_globals'' set to ``on'' is a dangerous choice for nontrivial programs - it's just too easy to write insecure programs. However, once ``register_globals'' is set to ``off'', PHP is quite a reasonable language for development.
The secure default should include setting ``register_globals'' to ``off'', and also including several functions to make it much easier for users to specify and limit the input they'll accept from external sources. Then web servers (such as Apache) could separately configure this secure PHP installation. Routines could be placed in the PHP library to make it easy for users to list the input variables they want to accept; some functions could check the patterns these variables must have and/or the type that the variable must be coerced to. In my opinion, PHP is a bad choice for secure web development if you set register_globals on.
As I suggested in earlier versions of this book, PHP has been trivially modified to become a reasonable choice for secure web development. However, note that PHP doesn't have a particularly good security vulnerability track record (e.g., register_globals, a file upload problem, and a format string problem in the error reporting library); I believe that security issues were not considered sufficiently in early editions of PHP; I also think that the PHP developers are now emphasizing security and that these security issues are finally getting worked out. One evidence is the major change that the PHP developers have made to get turn off register_globals; this had a significant impact on PHP users, and their willingness to make this change is a good sign. Unfortunately, it's not yet clear how secure PHP really is; PHP just hasn't had much of a track record now that the developers of PHP are examining it seriously for security issues. Hopefully this will become clear quickly.
If you've decided to use PHP, here are some of my recommendations (many of these recommendations are based on ways to counter the issues that Clowes raises):
Set the PHP configuration option ``register_globals'' off, and use PHP 4.2.0 or greater. PHP 4.1.0 adds several special arrays, particularly $_REQUEST, which makes it far simpler to develop software in PHP when ``register_globals'' is off. Setting register_globals off, which is the default in PHP 4.2.0, completely eliminates the most common PHP attacks. If you're assuming that register_globals is off, you should check for this first (and halt if it's not true) - that way, people who install your program will quickly know there's a problem. Note that many third-party PHP applications cannot work with this setting, so it can be difficult to keep it off for an entire website. It's possible to set register_globals off for only some programs. For example, for Apache, you could insert these lines into the file .htaccess in the PHP directory (or use Directory directives to control it further):
php_flag register_globals Off php_flag track_vars On
If you must develop software where register_globals might be on while running (e.g., a widely-deployed PHP application), always set values not provided by the user. Don't depend on PHP default values, and don't trust any variable you haven't explicitly set. Note that you have to do this for every entry point (e.g., every PHP program or HTML file using PHP). The best approach is to begin each PHP program by setting all variables you'll be using, even if you're simply resetting them to the usual default values (like "" or 0). This includes global variables referenced in included files, even all libraries, transitively. Unfortunately, this makes this recommendation hard to do, because few developers truly know and understand all global variables that may be used by all functions they call. One lesser alternative is to search through HTTP_GET_VARS, HTTP_POST_VARS, HTTP_COOKIE_VARS, and HTTP_POST_FILES to see if the user provided the data - but programmers often forget to check all sources, and what happens if PHP adds a new data source (e.g., HTTP_POST_FILES wasn't in old versions of PHP). Of course, this simply tells you how to make the best of a bad situation; in case you haven't noticed yet, turn off register_globals!
Set the error reporting level to E_ALL, and resolve all errors reported by it during testing. Among other things, this will complain about un-initialized variables, which are a key issues in PHP. This is a good idea anyway whenever you start using PHP, because this helps debug programs, too. There are many ways to set the error reporting level, including in the ``php.ini'' file (global), the ``.htttpd.conf'' file (single-host), the ``.htaccess'' file (multi-host), or at the top of the script through the error_reporting function. I recommend setting the error reporting level in both the php.ini file and also at the top of the script; that way, you're protected if (1) you forget to insert the command at the top of the script, or (2) move the program to another machine and forget to change the php.ini file. Thus, every PHP program should begin like this:
Filter any user information used to create filenames carefully, in particular to prevent remote file access. PHP by default comes with ``remote files'' functionality -- that means that file-opening commands like fopen(), that in other languages can only open local files, can actually be used to invoke web or ftp requests from another site.
Do not use old-style PHP file uploads; use the HTTP_POST_FILES array and related functions. PHP supports file uploads by uploading the file to some temporary directory with a special filename. PHP originally set a collection of variables to indicate where that filename was, but since an attacker can control variable names and their values, attackers could use that ability to cause great mischief. Instead, always use HTTP_POST_FILES and related functions to access uploaded files. Note that even in this case, PHP's approach permits attackers to temporarily upload files to you with arbitrary content, which is risky by itself.
Only place protected entry points in the document tree; place all other code (which should be most of it) outside the document tree. PHP has a history of unfortunate advice on this topic. Originally, PHP users were supposed to use the ``.inc'' (include) extension for ``included'' files, but these included files often had passwords and other information, and Apache would just give requesters the contents of the ``.inc'' files when asked to do so when they were in the document tree. Then developers gave all files a ``.php'' extension - which meant that the contents weren't seen, but now files never meant to be entry points became entry points and were sometimes exploitable. As mentioned earlier, the usual security advice is the best: place only the proected entry points (files) in the document tree, and place other code (e.g., libraries) outside the document tree. There shouldn't be any ``.inc'' files in the document tree at all.
Avoid the session mechanism. The ``session'' mechanism is handy for storing persistent data, but its current implementation has many problems. First, by default sessions store information in temporary files - so if you're on a multi-hosted system, you open yourself up to many attacks and revelations. Even those who aren't currently multi-hosted may find themselves multi-hosted later! You can "tie" this information into a database instead of the filesystem, but if others on a multi-hosted database can access that database with the same permissions, the problem is the same. There are also ambiguities if you're not careful (``is this the session value or an attacker's value''?) and this is another case where an attacker can force a file or key to reside on the server with content of their choosing - a dangerous situation - and the attacker can even control to some extent the name of the file or key where this data will be placed.
For all inputs, check that they match a pattern for acceptability (as with any language), and then use type casting to coerce non-string data into the type it should have. Develop ``helper'' functions to easily check and import a selected list of (expected) inputs. PHP is loosely typed, and this can cause trouble. For example, if an input datum has the value "000", it won't be equal to "0" nor is it empty(). This is particularly important for associative arrays, because their indexes are strings; this means that $data["000"] is different than $data["0"]. For example, to make sure $bar has type double (after making sure it only has the format legal for a double):
$bar = (double) $bar;
Be especially careful of risky functions. This includes those that perform PHP code execution (e.g., require(), include(), eval(), preg_replace()), command execution (e.g., exec(), passthru(), the backtick operator, system(), and popen()), and open files (e.g., fopen(), readfile(), and file()). This is not an exhaustive list!
Use magic_quotes_gpc() where appropriate - this eliminates many kinds of attacks.
Avoid file uploads, and consider modifying the php.ini file to disable them (file_uploads = Off). File uploads have had security holes in the past, so on older PHP's this is a necessity, and until more experience shows that they're safe this isn't a bad thing to remove. Remember, in general, to secure a system you should disable or remove anything you don't need.