Bug: *BSD libc (strfmon) Multiple vulnerabilities ( Ascii Version )

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*BSD libc (strfmon) Multiple vulnerabilities

Published
Credit
Risk
2008.03.25
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

[ *BSD libc (strfmon) Multiple vulnerabilities ]

Author: Maksymilian Arciemowicz
Date:
- - Written: 10.03.2008
- - Public: 25.03.2008

CVE: CVE-2008-1391
Risk: High

Affected Software:
FreeBSD lines: 6,7
NetBSD 4
another systems what use this functions.
Standard C Library (libc, -lc) for BSD
probably some MacOS version

Vendor: http://www.php.net

- --- 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. /usr/src/lib/libc/stdlib/strfmon.c - Integer Overflow ---
The main problem and vulnerability exist in strfmon() function. When we use this function in example program:

- ---example-start--
#include <stdio.h>
#include <monetary.h>

int main(int argc, char* argv[]){
char buff[51];
char *bux=buff;
int res;

res=strfmon(bux, 50, argv[1], "0");
return 0;
}
- ---example-end--

and compile it, we can manipulate format string.

Let's try to run example:
cxib# ./pln %99999999999999999999n
Segmentation fault (core dumped)

What is wrong? Let's see

cxib# gdb -q pln
(no debugging symbols found)...(gdb) r %99999999999999999999n
Starting program: /cxib/C/pln %99999999999999999999n
(no debugging symbols found)...(no debugging symbols found)...
Program received signal SIGSEGV, Segmentation fault.
0x2814e0e6 in memmove () from /lib/libc.so.7
(gdb)

memmove() will bad reallocation memory.

cxib# gdb -q pln
(no debugging symbols found)...(gdb) r %.9999999999n
Starting program: /cxib/C/pln %.9999999999n
(no debugging symbols found)...(no debugging symbols found)...
Program received signal SIGSEGV, Segmentation fault.
0x2814f093 in abort () from /lib/libc.so.7


Next example is :

cxib# ./pln %#99999999999999999999n

Long execution time. Let's try check this process :
- --------------------------
cxib# ps -aux | grep pln
cxib 1843 89.1 13.2 140320 119588 p2 R+ 4:29PM 0:09.68 ./pln %#99999999999999999999n
cxib# ps -aux | grep pln
cxib 1843 94.7 48.4 482336 438236 p2 R+ 4:29PM 1:54.07 ./pln %#99999999999999999999n

1 VSZ=140320
2 VSZ=482336

- ----------------------------

Why? pln will allocate more memory that we have. PHP use strfmon() in money_format() function. When we use mod_php5 in
apache, we can create example exploit.. result will be :

- ---apache-child-die---
swap_pager: out of swap space
swap_pager_getswapspace(16): failed
Mar 15 21:03:23 cxib kernel: pid 1210 (httpd), uid 80, was killed: out of swap space
- ---apache-child-die---

Difference between %99999999999999999999n and (%#99999999999999999999n or %.9999999999n) is "#" or
"."

o A `#' sign followed by a decimal number specifying the maximum
expected number of digits after the radix character.
o A `.' character followed by a decimal number specifying the number
the number of digits after the radix character.

Let's see the source of strfmon() function :

- ---strfmon()-start---
ssize_t
strfmon(char * __restrict s, size_t maxsize, const char * __restrict format,
...)
{
va_list ap;
char *dst; /* output destination pointer */
const char *fmt; /* current format poistion pointer */
struct lconv *lc; /* pointer to lconv structure */
char *asciivalue; /* formatted double pointer */

int flags; /* formatting options */
int pad_char; /* padding character */
int pad_size; /* pad size */
int width; /* field width */
int left_prec; /* left precision */
int right_prec; /* right precision */
double value; /* just value */
char space_char = ' '; /* space after currency */

char cs_precedes, /* values gathered from struct lconv */
sep_by_space,
sign_posn,
*signstr,
*currency_symbol;

char *tmpptr; /* temporary vars */
int sverrno;

va_start(ap, format);

lc = localeconv();
dst = s;
fmt = format;
asciivalue = NULL;
currency_symbol = NULL;
pad_size = 0;

while (*fmt) {
/* pass nonformating characters AS IS */
if (*fmt != '%')
goto literal;

/* '%' found ! */

/* "%%" mean just '%' */
if (*(fmt+1) == '%') {
fmt++;
literal:
PRINT(*fmt++);
continue;
}

/* set up initial values */
flags = (NEED_GROUPING|LOCALE_POSN);
pad_char = ' '; /* padding character is "space" */
left_prec = -1; /* no left precision specified */
right_prec = -1; /* no right precision specified */
width = -1; /* no width specified */
value = 0; /* we have no value to print now */

/* Flags */
while (1) {
switch (*++fmt) {
case '=': /* fill character */
pad_char = *++fmt;
if (pad_char == '\0')
goto format_error;
continue;
case '^': /* not group currency */
flags &= ~(NEED_GROUPING);
continue;
case '+': /* use locale defined signs */
if (flags & SIGN_POSN_USED)
goto format_error;
flags |= (SIGN_POSN_USED|LOCALE_POSN);
continue;
case '(': /* enclose negatives with () */
if (flags & SIGN_POSN_USED)
goto format_error;
flags |= (SIGN_POSN_USED|PARENTH_POSN);
continue;
case '!': /* suppress currency symbol */
flags |= SUPRESS_CURR_SYMBOL;
continue;
case '-': /* alignment (left) */
flags |= LEFT_JUSTIFY;
continue;
default:
break;
}
break;
}

/* field Width */
if (isdigit((unsigned char)*fmt)) {
GET_NUMBER(width);
/* Do we have enough space to put number with
* required width ?
*/
if (dst + width >= s + maxsize)
goto e2big_error;
}

/* Left precision */
if (*fmt == '#') {
if (!isdigit((unsigned char)*++fmt))
goto format_error;
GET_NUMBER(left_prec);
}

/* Right precision */
if (*fmt == '.') {
if (!isdigit((unsigned char)*++fmt))
goto format_error;
GET_NUMBER(right_prec);
}

/* Conversion Characters */
switch (*fmt++) {
case 'i': /* use internaltion currency format */
flags |= USE_INTL_CURRENCY;
break;
case 'n': /* use national currency format */
flags &= ~(USE_INTL_CURRENCY);
break;
default: /* required character is missing or
premature EOS */
goto format_error;
}

if (flags & USE_INTL_CURRENCY) {
currency_symbol = strdup(lc->int_curr_symbol);
if (currency_symbol != NULL)
space_char = *(currency_symbol+3);
} else
currency_symbol = strdup(lc->currency_symbol);

if (currency_symbol == NULL)
goto end_error; /* ENOMEM. */

/* value itself */
value = va_arg(ap, double);

/* detect sign */
if (value < 0) {
flags |= IS_NEGATIVE;
value = -value;
}

/* fill left_prec with amount of padding chars */
if (left_prec >= 0) {
pad_size = __calc_left_pad((flags ^ IS_NEGATIVE),
currency_symbol) -
__calc_left_pad(flags, currency_symbol);
if (pad_size < 0)
pad_size = 0;
}

asciivalue = __format_grouped_double(value, &flags,
left_prec, right_prec, pad_char);
if (asciivalue == NULL)
goto end_error; /* errno already set */
/* to ENOMEM by malloc() */

/* set some variables for later use */
__setup_vars(flags, &cs_precedes, &sep_by_space,
&sign_posn, &signstr);

/*
* Description of some LC_MONETARY's values:
*
* p_cs_precedes & n_cs_precedes
*
* = 1 - $currency_symbol precedes the value
* for a monetary quantity with a non-negative value
* = 0 - symbol succeeds the value
*
* p_sep_by_space & n_sep_by_space
*
* = 0 - no space separates $currency_symbol
* from the value for a monetary quantity with a
* non-negative value
* = 1 - space separates the symbol from the value
* = 2 - space separates the symbol and the sign string,
* if adjacent.
*
* p_sign_posn & n_sign_posn
*
* = 0 - parentheses enclose the quantity and the
* $currency_symbol
* = 1 - the sign string precedes the quantity and the
* $currency_symbol
* = 2 - the sign string succeeds the quantity and the
* $currency_symbol
* = 3 - the sign string precedes the $currency_symbol
* = 4 - the sign string succeeds the $currency_symbol
*
*/

tmpptr = dst;

while (pad_size-- > 0)
PRINT(' ');

if (sign_posn == 0 && (flags & IS_NEGATIVE))
PRINT('(');

if (cs_precedes == 1) {
if (sign_posn == 1 || sign_posn == 3) {
PRINTS(signstr);
if (sep_by_space == 2) /* XXX: ? */
PRINT(' ');
}

if (!(flags & SUPRESS_CURR_SYMBOL)) {
PRINTS(currency_symbol);

if (sign_posn == 4) {
if (sep_by_space == 2)
PRINT(space_char);
PRINTS(signstr);
if (sep_by_space == 1)
PRINT(' ');
} else if (sep_by_space == 1)
PRINT(space_char);
}
} else if (sign_posn == 1)
PRINTS(signstr);

PRINTS(asciivalue);

if (cs_precedes == 0) {
if (sign_posn == 3) {
if (sep_by_space == 1)
PRINT(' ');
PRINTS(signstr);
}

if (!(flags & SUPRESS_CURR_SYMBOL)) {
if ((sign_posn == 3 && sep_by_space == 2)
|| (sep_by_space == 1
&& (sign_posn == 0
|| sign_posn == 1
|| sign_posn == 2
|| sign_posn == 4)))
PRINT(space_char);
PRINTS(currency_symbol); /* XXX: len */
if (sign_posn == 4) {
if (sep_by_space == 2)
PRINT(' ');
PRINTS(signstr);
}
}
}

if (sign_posn == 2) {
if (sep_by_space == 2)
PRINT(' ');
PRINTS(signstr);
}

if (sign_posn == 0 && (flags & IS_NEGATIVE))
PRINT(')');

if (dst - tmpptr < width) {
if (flags & LEFT_JUSTIFY) {
while (dst - tmpptr < width)
PRINT(' ');
} else {
pad_size = dst-tmpptr;
memmove(tmpptr + width-pad_size, tmpptr,
pad_size);
memset(tmpptr, ' ', width-pad_size);
dst += width-pad_size;
}
}
}

PRINT('\0');
va_end(ap);
free(asciivalue);
free(currency_symbol);
return (dst - s - 1); /* return size of put data except trailing '\0' */

e2big_error:
errno = E2BIG;
goto end_error;

format_error:
errno = EINVAL;

end_error:
sverrno = errno;
if (asciivalue != NULL)
free(asciivalue);
if (currency_symbol != NULL)
free(currency_symbol);
errno = sverrno;
va_end(ap);
return (-1);
}
- ---strfmon()-end---

As we can see locks are corrected, but function GET_NUMBER()

- ---GET_NUMBER()-start---
#define GET_NUMBER(VAR) do { \
VAR = 0; \
while (isdigit((unsigned char)*fmt)) { \
VAR *= 10; \
VAR += *fmt - '0'; \
fmt++; \
} \
} while (0)
- ---GET_NUMBER()-end---

fmt=2147483647n => GET_NUMBER(2147483647)
fmt=2147483648n => GET_NUMBER(-2147483648)
fmt=2147483649n => GET_NUMBER(-2147483647)
fmt=4294967296n => GET_NUMBER(0)
fmt=4294967297n => GET_NUMBER(1)

We have integer overflow.

Next problem is with int left_prec and right_prec. Sum of this int's isn't checked.


Problem exist also in printf() function.

Example code will show Integer Overflow .

- ---example-start--
#include <stdio.h>

int
main(int argc, char *argv[])
{
printf("%1410065408.1410065407f\n", 2);
return 0;
}
- ---example-end--

cxib# gcc -o pln pln.c && ./pln
Segmentation fault (core dumped)

What is wrong? the same problem that was in strfmon() function.

- ---
/* convert to string */
snprintf(fmt, sizeof(fmt), "%%%d.%df", left_prec + right_prec + 1,
right_prec);
avalue_size = asprintf(&avalue, fmt, value);
- ---

fmt is here 32 chars table. So for format like

strfmon(bux, 50, "%.10n", "1.1");

fmt will have %11.10f

for
res=strfmon(bux, 50, "%.1410065407n", "1.1");

will be crash here
avalue_size = asprintf(&avalue, fmt, value);

fmt=%1410065408.1410065407f
value=1.1

it is possible? asprintf(&avalue, "%.1410065407f", "1.1");
and the question is why? Let's see to gdb

cxib# gdb -q pln
(no debugging symbols found)...(gdb) r
Starting program: /cxib/C/pln
(no debugging symbols found)...(no debugging symbols found)...
Program received signal SIGSEGV, Segmentation fault.
0x2814f0a3 in abort () from /lib/libc.so.7

SecurityReason will only alarm all BSDs developers to check libc. A lot of programs (with strfmon function) should be
recompiled after libc updated. We have informed NetBSD and FreeBSD teams, but we don't know how exactly use this
functions. The priority of strfmon() issues are very High.

- --- 2. Exploit ---
N/A

- --- 3. How to fix ---
We have informed NetBSD teams about it. strfmon() function is fixed in

http://cvsweb.netbsd.org/bsdweb.cgi/src/lib/libc/stdlib/strfmon.c

but we are not sure of it is correct.

- --- 4. Contact ---
Author: Maksymilian Arciemowicz

References:

http://www.us-cert.gov/cas/techalerts/TA08-350A.html
http://www.vupen.com/english/advisories/2008/3444
http://www.debian.org/security/2010/dsa-2058
http://support.apple.com/kb/HT3338

See this note in TXT Version

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