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Input Validation Attacks ======================== Written by R a v e N for BSRF (http://blacksun.box.sk) 17/7/2000 "Input Validation Attacks". Some of you may be startled by this term. But, to tell you the truth, it's quite simple, and fun too. Here, let me explain. What's an IVA? -------------- IVA stands for an Input Validation Attack. I'll try to make this as simple as possible, so even the ones of you who don't have any programming experience would still understand. Suppose you have a program that receives input. That could be practically anything. In fact, almost every application that you know receives some sort of input. When you tell your browser where to go, you're giving it input. When you play a computer game and you tell your figure to move to the left, that's input. When you type in your password, that's input too. So, what happens when a program doesn't validate the input that you give it correctly? Suppose, for example, that if you typed your input, and then a certain character and afterwards a command and it will be executed? Or maybe, if you type a password that's too long, the program will go amok and let you in without the password. Or maybe it'll let you in if you won't type anything at all. The program didn't validate the input correctly - it didn't make sure that the user is typing what he is supposed to type. So let's make a long story short - there is a bug or a hole in the program or it's implementation that involves certain input, and the program doesn't make sure that such input is not given, thus allowing us to exploit the hole. Now for a few examples, to clear things up and to show you how this can be done and exploited. Examples -------- The best possible real example that I can think of is the PHF hole. Yes, PHF! Some of you may already recognize this hole. Yeah, yeah, we know it's dated back in 1996, but let's forget about it for a second and concentrate on how this works. Let's imagine we're back in 1996. PHF is a CGI script that comes standard with the Apache Web Server - the world's most popular web server (and it still is until this very day). Everything is doing just fine, until Jennifer Myers found out that the PHF script will accept the newline character and issue commands to the command line with the webserver's privileges. This means that if httpd (the HTTP Daemon, i.e. the program that listens on port 80 (by default) and waits for HTTP connections. The term daemon isn't limited to Internet-related issues, and is better explained in other BSRF tutorials) is running as root (which is a very stupid thing to do. Web servers should run from a very restricted account), every command can be executed with root privileges. Basically, to get the password file, all you had to do is to type this: http://www.some-vulnerable-webserver.com/cgi-bin/phf?Qalias=x&0a/bin/cat&20/etc/passwd Then you will get the password file, as if you had console access to a root terminal and typed in cat /etc/passwd (if the file is shadowed, you can also get /etc/shadow. After all, you have root permissions). From this point, all you have to do is to run a password cracker and wait. Smarter crackers would issue different commands. For example, they could create an .rhosts file on root's home directory and add their hostname and username, and then use rlogin to remotely login to that system (if such a service is running and is not firewalled. But then again, an admin that is stupid enough to run httpd as root would probably also have it running unrestrictedly as well). Refer to rlogin's manual page for further instructions. Analyzing the attack -------------------- http://www.some-vulnerable-webserver.com/cgi-bin/phf?Qalias=x&0a/bin/cat&20/etc/passwd Hmm... That's awfully long, isn't it? Let's take it piece by piece. http://www.some-vulnerable-webserver.com There's not much to explain here - this part tells your web browser who to contact. /cgi-bin/phf This part tells your browser to request for the file called phf, under the cgi-bin directory, which is under the root directory (it's the main directory, similar to c:\ in the DOS / Windows world). ? Passes input to phf, the cgi script. Qalias=x So far this is normal input, which the program (and the programmer) is expecting. &0a This is the fun part. &0a is the new line metacharacter. It tells PHF to start a completely new command line. /bin/cat&20/etc/passwd The command to execute. Tells PHF to run the following command: /bin/cat /etc/passwd The cat program is a standard Unix program that dumps the contents of a file to the "standard output" (stdout). This usually means your computer monitor, unless the output is redirected (to the printer, to a file, or in this case, through a TCP/IP socket and straight down to our browser). &20 is another metacharacter, which stands for a blank space, which is also called a "white space" (it is used instead of real spaces because httpd cannot accept spaces in URL requests). How to prevent such attacks --------------------------- First of all, make sure that everything runs with privileges that are as restricted as possible, and can only access files that it should and has to have access to (and if so, minimize access. For example: a web server needs to be able to read html, gif, jpg, cgi scripts or other files that belong to the web site, but does not require writing access to them). That way you can minimize or completely eliminate any possible damage. Also, as a developer, you must take security into mind when you code your programs, and also test them under unexpectable conditions. In other words, when you test a car, you test it for all sorts of strange situations and wild crashes, and drive it through different kinds of terrains. You can't just make sure that it can drive and then send it out to the market, otherwise people will realize that although the car works just fine in normal conditions, it doesn't handle unexpected ones properly, and will eventually switch to another manufacturer. Thank you for reading. You can catch up with other BSRF tutorials at blacksun.box.sk - just find the tutorials page. And remember, if you have a question, we have an excellent message board, and you can post your question there.