How to attack common vulnerabilities in PHP programs (bottom)

How to attack common vulnerabilities in PHP programs (bottom)
Translation: Analysist (analyst)
Source: http://www.china4lert.org

How to attack common vulnerabilities in PHP programs (bottom)

Original: Shaun Clowes
Translation: Analysist

[Library file]
As we discussed earlier, the include () and require () are primarily designed to support code libraries, because we typically put some frequently used functions into a separate file, which is the code base, and when you need to use one of the functions, We just have to include this codebase in the current file.

Initially, when people developed and published PHP programs, in order to distinguish between the code base and the main program code, the code base file is usually set up a ". Inc" extension, but they quickly found that this is a mistake, because such a file can not be correctly parsed by the PHP interpreter into PHP code. If we directly request such a file on the server, we will get the source code of the file, because when using PHP as an Apache module, the PHP interpreter is based on the file extension to determine whether to parse into PHP code. The extension is specified by the site administrator, typically ". php", ". PhP3" and ". PhP4". If important configuration data is included in a PHP file that does not have the appropriate extension, it is easy for a remote attacker to get that information.

The simplest solution is to specify a php file extension for each file, which can be a good way to prevent the problem of leaking source code, but also create a new problem, by requesting this file, the attacker may be running in the context of the code to run independently, which could lead to all the attacks discussed earlier.

The following is an obvious example:

In main.php:
$libDir = "/libdir";
$langDir = "$libdir/languages";

...

Include ("$libdir/loadlanguage.php":
?>

In libdir/loadlanguage.php:
...

Include ("$langDir/$userLang");
?>

It is fairly secure when "libdir/loadlanguage.php" is called by "main.php", but because "libdir/loadlanguage" has an extension of ". PHP", remote attackers can request this file directly, You can optionally specify values for $langDir and $userLang.
[Session file]
PHP 4 or later provides support for sessions, whose main purpose is to store state information between pages and pages in a PHP program. For example, when a user logs on to the site, he logs in to the fact and whoever logs into the site is saved in the session, and all PHP code can get the status information when he is browsing around the site.

In fact, when a session is started (which is actually set in the configuration file to start automatically on the first request), a random "session ID" is generated, if the remote browser always submits the "session ID" when sending the request, The session will always be maintained. This can be done easily through cookies, or by submitting a form variable (containing "session ID") on each page. The PHP program can register a special variable with the session, and its value will be in the session file at the end of each PHP script, and will be loaded into the variable before each PHP script starts. The following is a simple example:

Session_destroy (); Kill any data currently in the session
$session _auth = "Shaun";
Session_register ("Session_auth"); Register $session _auth as a session variable
?>

The new version of PHP will automatically set the "$session _auth" value to "Shaun", if they are modified, the future script will automatically accept the modified value, which is a very good tool for the stateless web, but we should also be careful.

One obvious problem is to make sure that the variable really comes from the session, for example, given the code above, if the following script is:

if (!empty ($session _auth))
Grant access to Site here
?>

The code above assumes that if "$session _auth" is set, it is set from the session, not from the user input, and if the attacker is set up by Form input, he can gain access to the site. Note that an attacker must use this attack method before the session registers the variable, and once the variable is placed in the session, it will overwrite any form input.

Session data is generally stored in the file (the location is configurable, usually "/tmp"), the file name is generally similar to the "Sess_ "In the form of this file contains variable names, variable types, variable values, and some other data. In a multi-host system, because the file is stored as the user who runs the Web server (typically nobody), a malicious site owner can create a session file to gain access to other sites and even check for sensitive information in the session file.

The session mechanism also provides another convenient place for attackers to save their input to files in the remote system, and for the example above, an attacker would need to place a file containing PHP code in the remote system, if it is not possible to do so with a file upload. He usually uses the session to assign a value to a variable at his own will, then guesses the location of the session file, and he knows the filename is "php , so just guess the directory, and the directory is usually "/tmp".

Alternatively, an attacker could arbitrarily specify the session ID (for example, "Hello") and then create a session file (such as "/tmp/sess_hello") with this session ID, but the session ID can only be a combination of letters and numbers.

[Data type]
PHP has loosely typed data types, and the types of variables depend on the context in which they are located. For example: "$hello" begins as a string variable with a value of "", but when evaluated, it becomes the shaping variable "0", which can sometimes lead to unexpected results. If the value of "$hello" is different for "000" or "0", the result returned by empty () will not be true.

An array in PHP is an associative array, that is, the index of an array is of a string type. This means that "$hello [" 000 "]" and "$hello [0]" are also different.

The development of the program should carefully consider the above problem, for example, we should not be in one place to test whether a variable is "0", and in another place using empty () to verify.

[ERROR-prone function]
When we analyze bugs in PHP programs, if we can get the source code, then an error-prone list of functions is what we really need. If we can change the parameters of these functions remotely, then we are likely to find the holes in them. The following is a more detailed list of error-prone functions:


Require (): reads the contents of the specified file and interprets it as PHP code
Include (): Ibid.
Eval (): Executes the given string as PHP code
Preg_replace (): When used with the "/E" switch, the replacement string is interpreted as PHP code

<命令执行>
EXEC (): Executes the specified command, returning the last line of the execution result
PassThru (): Executes the specified command, returning all results to the client browser
": Executes the specified command, returning all results to an array
System (): Same as PassThru (), but does not process binary data
Popen (): Executes the specified command to connect the input or output to the PHP file descriptor

<文件泄露>
fopen (): Open the file and correspond to a PHP file descriptor
ReadFile (): reads the contents of the file and then outputs it to the customer's browser
File (): reads the entire contents into an array

Note: In fact, this list is not very full, such as "Mail ()" and other commands may also execute commands, so you need to add a bit.
[How to enhance the security of PHP]
All of the attacks I've described above are good implementations for the default installation of PHP 4, but I've repeated it many times and the PHP configuration is very flexible, and by configuring some PHP options, we can be completely resistant to some of these attacks. Here are some of the configurations that I have categorized according to the difficulty of implementation:

* Low Difficulty
* * Medium and low difficulty
Medium to High difficulty
High Difficulty

The above classification is only personal, but I can guarantee that if you use all the options provided by PHP, then your PHP will be very safe, even for third-party code, because many of these features are no longer available.

Set "Register_globals" to "off"
This option prevents PHP from creating global variables for user input, meaning that if the user submits the form variable "Hello", PHP will not create "$ hello" and will only create "http_get/post_vars[' Hello '". This is an extremely important option in PHP, and turning off this option can cause great inconvenience to programming.

Set "Safe_mode" to "on"
Turning this option on will add the following restrictions:
1. Restrict which command can be executed
2. Restrict which function can be used
3. File access restrictions based on script ownership and target file ownership
4. Disable File Upload feature
This is a great option for ISPs as well as a great way to improve PHP security.

* * Set "Open_basedir"
This option disables file operations outside of the specified directory, effectively eliminating local files or remote files being used by include (), but still requires attention to file uploads and session file attacks.

* * Set "display_errors" to "off" and set "Log_errors" to "on"
This option prevents the error message from being displayed in the Web page, but is recorded in a log file, which effectively resists the attacker's ability to detect functions in the target script.

* Set "Allow_url_fopen" to "off"
This option can prohibit remote file function, highly recommended!

Well, the article ends here, if you want to know some other relevant information, please refer to the original http://www.securereality.com.au/studyinscarlet.txt.

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