Its name is saved in the _loginkey property of the current class. This repository contains source code for a trivial PHP type juggling challenge. If the string starts with valid numeric data, this will be the value used. Scientific E-notation is used to write very long numbers in a short form. When you are auditing a popular, well-established web application, you are dealing with thousands of lines of code. In PHP, this means that it is equal to the integer '0', which explains why the comparison returns true. If that's not the case, it will get its data from $_COOKIE, which holds the data from cookies sent to the server by the client requesting the page. The latter would work by passing a query such as ?array[]=first_element&array[]=second_element. But this is exactly what makes this type of vulnerability so dangerous. Type Juggling vulnerabilities are often introduced this way through implicit conversion of which the developer is not aware. What happens now is that CMSMS will load the userdata of the user with the id '1' and save it in $oneuser. will also be a string. Let's say the PHP code that handles authentication looks like this: if ($_POST ["password"] == "Admin_Password") {login_as_admin ();} Then, simply submitting an integer input of 0 would. As I've mentioned, CMS Made Simple is written in PHP, and the loose comparison operator is quite popular in this language. "0.234" to float and "123" to int), simply add 0 to the string - PHP will do the rest. Since weve already found an issue with strpos, lets see if we can also find one for in_array. This section contains an explanation of some of the more common ones. The real problem arises when it is used on critical comparisons such as password hashes. So basically what this script does is accept json data and return some secret data for the user. Note: If the application accepts json then we can bypass all the hashes and not only the ones starting with 0e, thats because HTTP sends all the data in string format, but with json we can send integers too. ternary operator, First sending a normal request with an invalid password to confirm that we cant simply login with a random password, And now lets try it with a string which produces a md5 hash that start with 0e in this case Im going to use NWWKITQ. We can actually upload our own from within the administrator panel. In their comment, they actually acknowledged the fact that this function has nothing to do with encryption, but rather with encoding, which is why I'm still puzzled as to why they called it _encrypt. Currently (binary) and While I mentioned before that you can only pass strings and arrays using cookies, it's different with unserialize. Now as we see we successfully gained access to user james without knowing his real password. Perhaps they intended to disappoint security researchers and blog readers keen to solve a nice crypto challenge? The type conversions which occur in this context are explained in the Below you can see _check_passhash: This looks very promising. Time to get user carl, his api_key starts with 3, and we try to send a 3 as integer in api_key field, however that is not going to work in this case as his api_key has 3 numbers until the first letter (if the key starts with a number we need to set the api_key in json to first part of numbers of the original api_key ) 399d65963db11309b2f6e4a59094686b12133274b47c867e52b277b68a0ef39b so we need to use 399 as the api_key to get access to his data. User Logged in with valid password. This article provides an introduction to PHP Type Juggling Vulnerabilities and loosely typed code and type comparisons, and how to avoid or fix them. By employing '.php.' Authentication bypass in CMSMS: . table. Did you know, for example, that both of these comparisons would return true? When you use parameters that come from json_decode() or unserialize(), its possible to specify whether you want to pass your number as an integer or a string. This is exactly what happened in CMS Made Simple (CMSMS): one missing character was sufficient to result in an Authentication Bypass, and ultimately Remote Code Execution (RCE). We can have an extension with any name containing our shell. If we passed the value cherry to the function, var_dump would print bool(false) since its not found in the array. 3. Finally, we can start building a payload that is able to bypass the CMSMS login prompt for the admin panel. /* checks if a string is an integer with possible whitespace before and/or after, and also isolates the integer */, /* checks if a string is an integer with no whitespace before or after */. The number 1,000,000, for example, would be written as 1e6. First, it creates an instance of the LoginOperations class and then calls its get_effective_uid method to return the user ID. arithmetical operator. (integer) is an alias of the (int) cast. Example 1: Authentication Bypass. This is the context when using an 2153 CVE-2020-8463: 863: Bypass 2020-12-17: 2021-07 . This means that variable types are checked while the program is executing. If there is a user, it will create an MD5 hash consisting of various values. (double) and (real) are aliases of Because PHP supports indexing into strings via offsets So today Im going to explain these attacks, how to exploit them, and how to avoid them. In cases of simple comparisons, we recommend that you use three equals symbols (===) instead of two (==). Finally, we recommend that you avoid casting the strings to the same type, before comparing them, as this often has the same effect as the == operator. Invicti Security Corp 1000 N Lamar Blvd Suite 300 Austin, TX 78703, US. Type Juggling PHP does not require explicit type definition in variable declaration. This article explains the CMSMS vulnerability, which is a perfect illustration of why you should pay very close attention when you code your web application, especially critical parts such as authentication functionality. and .= operators, removing any explicit casting: // string(2) "11"; also an empty string ("") would cast to string without changing $x, // int(2); also a zero value (0) would cast to int without changing $x. As we can see, the cookie value is saved in the $private_data variable, as well as in $_SESSION. More information in our Privacy Policy. If afterwards an int value is assigned to $var, it will be of type int . The code uses json_decode (), so we are able to specify a type other than string or array via the JSON object passed using the password GET parameter. Various code execution CVE numbers were assigned due to this function. // more data, more functions, more everything Re: the typecasting between classes post below fantastic, but slightly flawed. CMSMS didn't contain any obviously dangerous gadgets. I've seen this name before. string interpolation, - GitHub . There are still several problems with this check. This article explains the CMSMS vulnerability, which is a perfect illustration of why you should pay very close attention when you code your web application, especially . However, we can ignore them, due to the loose comparison of $checksum and $tmp using the == operator. However, if we created a cookie and tried to log in, we'd still have one tiny problem. Yet again, there are two examples where this is quite obvious: You see how both of these functions are described in the PHP manual. It tells us not to pass untrusted user input to unserialize. Dynamic typing allows developers to be more flexible when using PHP. For simple types declarations the behaviour is as follows: Internal functions This is the context when using echo, Simply add a single character. First confirm that the strings hash gives us the desired hash, and then make the POST request using curl. A user can simply type the string 2 bottles into the input field, and PHP would automatically extract the integer 2 from the beginning of the string. Oh great! The checksum from the cookie and the generated checksum containing the database password are compared using loose comparison. If an integer value is assigned to a variable, it becomes an integer. Yet again, the actual check doesn't happen in this method. If there is a hash with the same format in the database, he will be logged in as the user that the hash belongs to, without needing to know the password. In order to pivot into an internal network or steal sensitive data, we actually need a proper shell. In most cases, it doesn't take a long time to search for the '==' operator in your code and replace it where you need to. Discovered and published by MWR Information Security, June 2013 Bug was in cryptographic library used throughout the framework The library powered Laravel's authentication system and exposed for use by any Laravel . Often these signatures are generated as illustrated in the PHP code, even though this does not conform to the RFC 2104s recommended way to create HMACs. string. This means that during the comparison of variables of different types, PHP will first convert them to a common, comparable type. You REALLY must be aware what you are doing, when you cast a lot in your code. This means that even if there is a string beginning with (or solely consisting of) 0, it will still fail the comparison when the other value is the integer 0. automatically coerce null to scalar types, It was as easy as searching for 'function check_login('. However, often this is not enough. This is why it's important to know the ins and outs of the programming language of your choice, and to read the documentation regarding dangerous functionality to understand its implications. Now starting to analyze the code. We talked about why developers use weak type comparisons, how to avoid them and highlight a few of the unexpected results of PHP Type Juggling. interpretable as an int), both operands are interpreted as There are various, subtle vulnerabilities that can arise from loose comparison. Get smarter at building your thing. Creates god awful infographics. The reason why this is the desired format is that 0*10n is always 0. If a string value is assigned to the variable, it becomes a String. Yet again, we have proof that in certain circumstances, a small information leak can have a huge impact. This is exactly what happened in CMS Made Simple (CMSMS): one missing character was sufficient to result in an Authentication Bypass, and ultimately Remote Code Execution (RCE). (Yes, I'm perfectly aware that this sentence is used in every single vulnerability writeup involving source code since the inception of vulnerability writeups involving source code, but at least it's not a food metaphor!). Also, PHP Object Injection requires existing code ('gadgets') that can lead to dangerous actions. This means that variable types are checked while the program is executing. Comparison with Various Types If this check has already taken place, the _data property of this class instance will return its content. The following table gives a good overview of the behavior of PHP with loose type comparisons. Now that we are logged in, we can perform any administrative action we want. This is the context when using conditional statements, the at the end of the first variation. So let's take a look what data is actually being passed by $_COOKIE. Since we have set the $checksum variable to 'TRUE', the comparison is basically like this: TRUE == '9ab50f27d4201db9b28483ba83c48ebafbb2aa17'. However, there is a problem. The first one is Auth Bypass and the second one is exploiting a custom API . to $var, it will be of type int. Before $private_data can be used, it is decrypted using the _decrypt method from the current class. Don't lose hope yet! Follow to join The Startups +8 million monthly readers & +760K followers. With loose comparison, it's possible for a PHP developer to. or the string It seems like we are on the right track. This will make 'pwned' our CSRF token value. 2. Unfortunately, writing web applications is not always easy. One trick for Windows is to save the file as 'exploit.php.' I'm kidding! I prefer you first go through the below write-up to understand better. 1. confirm.php Authentication Bypass Type Juggling vulnerability 2. password_reminder.php Remote Password Reset TOCTOU vulnerability }, 'License' => MSF_LICENSE, 'Author' => [ 'mr_me <steventhomasseeley[at]gmail.com>', # initial discovery, msf code ], 1 Answer Sorted by: 5 The algorithm PHP uses for coercing strings to integers is documented here. PHP does this very simply through the +. We have 2 users alb0z and james, in this case we cant bypass into alb0z because the hash must start with 0e in order to bypass it, but we can still log into james account. To do this, it checks the beginning of the string to see if there are numbers it can use for comparison. If you have a boolean, performing increments on it won't do anything despite it being 1. As james api_key starts with a letter, we can use 0 as api_key to get his data. Windows, on the other hand, does us a huge favor. That means we can add random characters to our message and always pass the signature 0e123 to the server. You'd simply log in. It will throw an error in a function that expects a string value, and will display the full path of the file in an error message. The only drawback is that this behavior doesn't work in Linux. If you want to convert a string automatically to float or integer (e.g. PHP type juggling is a difficult bug to exploit because even if you find a Type juggling bug you have to find a way to send the payload as an integer or a . First I wrote this simple PHP authentication mechanism which doesn't use a database so you can reproduce the attack simpler and faster This is easier said than done. Your supposition was that: Now logout from the application and try to login with username 'admin' password '34250003024812'. as the extension, we bypass the filter but still have a valid PHP extension. The theory behind this is as follows. After a quick search in the same file, this was the function. However, even if we don't achieve a full path disclosure, the value can simply be guessed in most cases. But, by far, this is not the only problem you can encounter when dealing with PHP! So as easy as it is to exploit those vulnerabilities, it is just as easy to fix them. Type juggling means dealing with a variable type. != is the opposite of == but it still is a loose comparison, so it is still vulnerable. The explanation is obvious once you know how it's worked out. But why would PHP determine that the string 'test' is the same as the integer '0'? However, it's possible to bypass the check in Windows. It first checks the database to see whether there is a user for the given user id. All we have to do is add square brackets after the parameter name: mact[]. I'm not sure if this hash is in place in order to avoid the problem of having the same login key for different installations or versions of CMSMS, but its values can easily be guessed and some of them are even static. 4. If you don't pay close attention, you end up introducing a flaw that is so easy to exploit, it's like taking candy from a baby. If we go back to the in_array function, there is an optional third parameter, which is set to false by default: The name of the parameter is $strict, and when it is set to true, PHP will take the datatype of the values into consideration when comparing them. Most of them are user-controllable or static. We stated above that you can use certain flags within the mailbox parameter. 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 . https://twitter.com/vickieli7. It would take weeks to analyze every bit of code starting from index.php, line 1. The in_array function will compare each array element to the input (and return true) once there is a match. This means that we would compare the string 0 to the string secretpass. Or maybe exploit your API to get valuable information, or bypass your CSRF protection, or in some cases even gain RCE. If the type differs, PHP will not attempt any conversions; it will return FALSE instead. (string) are identical, however this may change and These casts do not use the canonical type name and are not recommended. TypeError is thrown. There are another two: unserialize and serialize. But why would this deliver the weird result in the comparisons example? That is to say, if a string is assigned to variable $var, then $var is of type string. If afterwards an int value is assigned Its still hard to understand why passing 0 to in_array in the above scenario will lead to a match. Since good things come in threes, three equal signs mean that the comparison should take the data type into consideration. TypeError is thrown. If you didn't think rope would do that, you should have read the man page. Therefore, the following are two casts are equivalent: Casting literal strings and variables to binary Well come to that in a minute. This is the context when a value is passed to a typed parameter, property, It turns out that the reason is actually quite simple and sometimes even exploitable as well see later. Eventually the message hashed with the secret value will result in the format ^0e\d+$, and PHP will convert both strings to 0 and pass the check. This is from a HTB machine called Ransom. Collision Based Hashing Algorithm Disclosure, Using Content Security Policy to Secure Web Applications. So it's better to use TRUE in this case, as the comparison will always result in TRUE when it's compared to a non-empty string. We could circumvent this restriction by uploading an .htaccess file, and force any other extension to be recognized as a PHP file, for example .pwn or (since it's sometimes already registered) use .pht as the extension instead. There are four elements in the $values array: apple, orange, pear and grape. This means that we can easily find the right cookie name, just by trying different webroots and versions. PHP somehow has to convert both values to the same data type. print, parentheses before the value to convert. For example, the constant __CLASS__ never changes, and the CMS version can be guessed, as well as the content of __FILE__. Local File Inclusions that are caused by unserializing in combination with an existing autoloading functionality are almost exclusively restricted to files ending in .php. this behaviour is DEPRECATED as of PHP 8.1.0. It receives the data from a cookie. namaroulis stated "I found it tricky to check if a posted value was an integer"; to test if a variable is a number or a numeric string (such as form input, which is always a string), you must use is_numeric(): Cast a string to binary using PHP < 5.2.1. or returned from a function which declares a return type. Fortunately, the function responsible for this wasn't that difficult to spot. A good way to find some vulnerable code in a short amount of time is to concentrate your efforts on critical functions and functionality that is easy to get wrong. The image below shows the difference between Loose Comparisons and Strict Comparisons, Developers that are not aware of this type of attack will usually compare data through something similar to this, So in this case if the $key starts with 0e, then we can simply give a string that productes an another md5 hash which starts with 0e in the $string variable, which we may give it through GET or POST requests or other types of input, Some such string are QNKCDZO and 240610708, In the examples below I will go more into details on how to exploit it, First I wrote this simple PHP authentication mechanism which doesnt use a database so you can reproduce the attack simpler and faster, Now notice the line where the script compares the md5 hash of the password in POST request with the md5 hash of the password saved in the database (array in this case). We've come so far, but to authenticate, we still need the correct password. Opinions expressed by DZone contributors are their own. 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