PHP Tricks
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If == is used in PHP, then there are unexpected cases where the comparison doesn't behave as expected. This is because "==" only compare values transformed to the same type, if you also want to compare that the type of the compared data is the same you need to use ===.
PHP comparison tables: https://www.php.net/manual/en/types.comparisons.php
"string" == 0 -> True A string which doesn't start with a number is equals to a number
"0xAAAA" == "43690" -> True Strings composed by numbers in dec or hex format can be compare to other numbers/strings with True as result if the numbers were the same (numbers in a string are interpreted as numbers)
"0e3264578" == 0 --> True A string starting with "0e" and followed by anything will be equals to 0
"0X3264578" == 0X --> True A string starting with "0" and followed by any letter (X can be any letter) and followed by anything will be equals to 0
"0e12334" == "0" --> True This is very interesting because in some cases you can control the string input of "0" and some content that is being hashed and compared to it. Therefore, if you can provide a value that will create a hash starting with "0e" and without any letter, you could bypass the comparison. You can find already hashed strings with this format here: https://github.com/spaze/hashes
"X" == 0 --> True Any letter in a string is equals to int 0
More info in https://medium.com/swlh/php-type-juggling-vulnerabilities-3e28c4ed5c09
Type Juggling also affects to the in_array() function by default (you need to set to true the third argument to make an strict comparison):
If this function is used for any authentication check (like checking the password) and the user controls one side of the comparison, he can send an empty array instead of a string as the value of the password (https://example.com/login.php/?username=admin&password[]=) and bypass this check:
The same error occurs with strcasecmp()
Even if === is being used there could be errors that makes the comparison vulnerable to type juggling. For example, if the comparison is converting the data to a different type of object before comparing:
preg_match() could be used to validate user input (it checks if any word/regex from a blacklist is present on the user input and if it's not, the code can continue it's execution).
However, when delimiting the start of the regexppreg_match() only checks the first line of the user input, then if somehow you can send the input in several lines, you could be able to bypass this check. Example:
To bypass this check you could send the value with new-lines urlencoded (%0A) or if you can send JSON data, send it in several lines:
Find an example here: https://ramadistra.dev/fbctf-2019-rceservice
(This bypass was tried apparently on PHP 5.2.5 and I couldn't make it work on PHP 7.3.15)
If you can send to preg_match() a valid very large input, it won't be able to process it and you will be able to bypass the check. For example, if it is blacklisting a JSON you could send:
From: https://medium.com/bugbountywriteup/solving-each-and-every-fb-ctf-challenge-part-1-4bce03e2ecb0
Trick from: https://simones-organization-4.gitbook.io/hackbook-of-a-hacker/ctf-writeups/intigriti-challenges/1223 and https://mizu.re/post/pong
In short the problem happens because the preg_* functions in PHP builds upon the PCRE library. In PCRE certain regular expressions are matched by using a lot of recursive calls, which uses up a lot of stack space. It is possible to set a limit on the amount of recursions allowed, but in PHP this limit defaults to 100.000 which is more than fits in the stack.
This Stackoverflow thread was also linked in the post where it is talked more in depth about this issue. Our task was now clear:
Send an input that would make the regex do 100_000+ recursions, causing SIGSEGV, making the preg_match() function return false thus making the application think that our input is not malicious, throwing the surprise at the end of the payload something like {system(<verybadcommand>)} to get SSTI --> RCE --> flag :).
Well, in regex terms, we're not actually doing 100k "recursions", but instead we're counting "backtracking steps", which as the PHP documentation states it defaults to 1_000_000 (1M) in the pcre.backtrack_limit variable.
To reach that, 'X'*500_001 will result in 1 million backtracking steps (500k forward and 500k backwards):
If PHP is redirecting to another page but no die or exit function is called after the header Location is set, the PHP continues executing and appending the data to the body:
Check:
register_globals: In PHP < 4.1.1.1 or if misconfigured, register_globals may be active (or their behavior is being mimicked). This implies that in global variables like $_GET if they have a value e.g. $_GET["param"]="1234", you can access it via $param. Therefore, by sending HTTP parameters you can overwrite variables that are used within the code.
The PHPSESSION cookies of the same domain are stored in the same place, therefore if within a domain different cookies are used in different paths you can make that a path accesses the cookie of the path setting the value of the other path cookie. This way if both paths access a variable with the same name you can make the value of that variable in path1 apply to path2. And then path2 will take as valid the variables of path1 (by giving the cookie the name that corresponds to it in path2).
When you have the usernames of the users of the machine. Check the address: /~<USERNAME> to see if the php directories are activated.
This functions are typically used in PHP to generate hashes from passwords and to to check if a password is correct compared with a hash.
The supported algorithms are: PASSWORD_DEFAULT and PASSWORD_BCRYPT (starts with $2y$). Note that PASSWORD_DEFAULT is frequently the same as PASSWORD_BCRYPT. And currently, PASSWORD_BCRYPT has a size limitation in the input of 72bytes. Therefore, when you try to hash something larger than 72bytes with this algorithm only the first 72B will be used:
From this twitter thread you can see that sending more than 1000 GET params or 1000 POST params or 20 files, PHOP is not going to be setting headers in the response.
Allowing to bypass for example CSP headers being set in codes like:
If a PHP page is printing errors and echoing back some input provided by the user, the user can make the PHP server print back some content long enough so when it tries to add the headers into the response the server will throw and error. In the following scenario the attacker made the server throw some big errors, and as you can see in the screen when php tried to modify the header information, it couldn't (so for example the CSP header wasn't sent to the user):
Check ther page:
system("ls"); &#xNAN;`ls`; shell_exec("ls");
Check this for more useful PHP functions
To execute the code in the "replace" argument is needed at least one match. This option of preg_replace has been deprecated as of PHP 5.5.0.
This function within php allows you to execute code that is written in a string in order to return true or false (and depending on this alter the execution). Usually the user variable will be inserted in the middle of a string. For example:
assert("strpos($_GET['page']),'..') === false") --> In this case to get RCE you could do:
You will need to break the code syntax, add your payload, and then fix it again. You can use logic operations such as "and" or "%26%26" or "|". Note that "or", "||" doesn't work because if the first condition is true our payload won't get executed. The same way ";" doesn't work as our payload won't be executed.
Other option is to add to the string the execution of the command: '.highlight_file('.passwd').'
Other option (if you have the internal code) is to modify some variable to alter the execution: $file = "hola"
This function is used to sort an array of items using an specific function. To abuse this function:
You can also use // to comment the rest of the code.
To discover the number of parenthesis that you need to close:
?order=id;}//: we get an error message (Parse error: syntax error, unexpected ';'). We are probably missing one or more brackets.
?order=id);}//: we get a warning. That seems about right.
?order=id));}//: we get an error message (Parse error: syntax error, unexpected ')' i). We probably have too many closing brackets.
If you can upload a .htaccess, then you can configure several things and even execute code (configuring that files with extension .htaccess can be executed).
Different .htaccess shells can be found here
If you find a vulnerability that allows you to modify env variables in PHP (and another one to upload files, although with more research maybe this can be bypassed), you could abuse this behaviour to get RCE.
LD_PRELOAD: This env variable allows you load arbitrary libraries when executing other binaries (although in this case it might not work).
PHPRC : Instructs PHP on where to locate its configuration file, usually called php.ini. If you can upload your own config file, then, use PHPRC to point PHP at it. Add an auto_prepend_file entry specifying a second uploaded file. This second file contains normal PHP code, which is then executed by the PHP runtime before any other code.
Upload a PHP file containing our shellcode
Upload a second file, containing an auto_prepend_file directive instructing the PHP preprocessor to execute the file we uploaded in step 1
Set the PHPRC variable to the file we uploaded in step 2.
Get more info on how to execute this chain from the original report.
PHPRC - another option
If you cannot upload files, you could use in FreeBSD the "file" /dev/fd/0 which contains the stdin, being the body of the request sent to the stdin:
curl "http://10.12.72.1/?PHPRC=/dev/fd/0" --data-binary 'auto_prepend_file="/etc/passwd"'
Or to get RCE, enable allow_url_include and prepend a file with base64 PHP code:
curl "http://10.12.72.1/?PHPRC=/dev/fd/0" --data-binary $'allow_url_include=1\nauto_prepend_file="data://text/plain;base64,PD8KICAgcGhwaW5mbygpOwo/Pg=="'
Technique from this report.
The webserver parses HTTP requests and passes them to a PHP script executing a request such as as http://host/cgi.php?foo=bar as php.exe cgi.php foo=bar, which allows a parameter injection. This would allow to inject the following parameters to load the PHP code from the body:
Moreover, it's possible to inject the "-" param using the 0xAD character due to later normalization of PHP. Check. the exploit example from this post:
In this post it's possible to find great ideas to generate a brain fuck PHP code with very few chars being allowed. Moreover it's also proposed an interesting way to execute functions that allowed them to bypass several checks:
Look if you can insert code in calls to these functions (from here):
If yo are debugging a PHP application you can globally enable error printing in/etc/php5/apache2/php.ini adding display_errors = On and restart apache : sudo systemctl restart apache2
You can use the web www.unphp.net to deobfuscate php code.
PHP Wrappers ad protocols could allow you to bypass write and read protections in a system and compromise it. For more information check this page.
If you see that Xdebug is enabled in a phpconfig() output you should try to get RCE via https://github.com/nqxcode/xdebug-exploit
If in a page you can create a new object of an arbitrary class you might be able to obtain RCE, check the following page to learn how:
https://securityonline.info/bypass-waf-php-webshell-without-numbers-letters/
According to this writeup the following it's possible to generate an easy shellcode this way:
So, if you can execute arbitrary PHP without numbers and letters you can send a request like the following abusing that payload to execute arbitrary PHP:
For a more in depth explanation check https://ctf-wiki.org/web/php/php/#preg_match
Get a hacker's perspective on your web apps, network, and cloud
Find and report critical, exploitable vulnerabilities with real business impact. Use our 20+ custom tools to map the attack surface, find security issues that let you escalate privileges, and use automated exploits to collect essential evidence, turning your hard work into persuasive reports.
Learn & practice AWS Hacking:HackTricks Training AWS Red Team Expert (ARTE) Learn & practice GCP Hacking: HackTricks Training GCP Red Team Expert (GRTE)
