For the “trivia” assignment: 11:59pm, Monday, October 5.
For the main assignment: 11:59pm, Monday, October 12.
If you want to use slip days you have to tell us before the deadline by emailing Shuang Zhai.
This assignment involves generating a total of five attacks on two programs having different security vulnerabilities. Outcomes you will gain from this lab include:
GDB
and OBJDUMP
.Note: In this lab, you will gain firsthand experience with methods used to exploit security weaknesses in operating systems and network servers. Our purpose is to help you learn about the runtime operation of programs and to understand the nature of these security weaknesses so that you can avoid them when you write system code. We do not condone the use of any other form of attack to gain unauthorized access to any system resources.
In a manner similar to assignment 2, each group can obtain theie files from
http://cycle3.csug.rochester.edu:15513
which is only accessible on the csug network.
Since you are accessing the csug machines remotely, one way to access the webpage is to ssh into any of the CSUG cycle machines and use a text-based browser such as lynx.
On the command line, run "lynx <url>" to open the specified URL in the web browser. For instance, to request the bomb, you will type: "lynx http://cycle3.csug.rochester.edu:15513". Hit enter and the corresponding page will be loaded. Fill in your netID and email address in the corresponding fields. You can use the arrow keys to move your cursor around. Move your cursor to "submit". Press "enter/return" and the browser will prompt you to download targetk.tar. Accept the download by pressing "D". In the download options, select "Save to disk" and then press enter again when prompted to enter the filename. Press 'Q' twice to exit lynx.
Fill out the HTML form with the email address and NetID of just one of the
team members, and then submit the form by clicking the “Submit”
button. The server will build your files and return them
to your browser in a tar
file called
targetk.tar
, where k is the unique number of your
target programs.
Save the targetk.tar
file to a (protected) directory in which you plan to do your
work. Then use the command: tar -xvf targetk.tar
.
This will create a directory called targetk
with the following files:
README
:
A file describing the contents of the directory.
ctarget
:
An executable program vulnerable to code-injection attacks.
rtarget
:
An executable program vulnerable to return-oriented-programming attacks.
cookie.txt
:
An 8-digit hex code that you will use as a unique identifier in your attacks.
farm.c
:
The source code of your target’s “gadget farm”, which you will use in generating
return-oriented programming attacks.
hex2raw
:
A utility to generate attack strings.
Here is a summary of some important rules regarding valid solutions for this lab. These points will not make much sense when you read this document for the first time. They are presented here as a central reference of rules once you get started.
touch1
, touch2
, or
touch3
.
rtarget
with addresses
ranging between those for functions start_farm
and end_farm
.
Both CTARGET
and RTARGET
read strings from standard input.
They do so with the function getbuf
defined below:
unsigned getbuf() { char buf[BUFFER_SIZE]; Gets(buf); return 1; }
The function Gets
is similar to the standard library function
gets
. It reads a string from standard input (terminated by
‘\n’ or end-of-file) and stores it (along with a null
terminator) at the specified destination. In this code, the
destination is an array buf
, declared as having BUFFER_SIZE
bytes. At the time your targets were generated, BUFFER_SIZE
was
a compile-time constant specific to your version of the programs.
Functions Gets()
and gets()
have no way to determine
whether their destination buffers are large enough to store the string they
read. They simply copy sequences of bytes, possibly overrunning the bounds
of the storage allocated at the destinations.
If the string typed by the user and read by getbuf
is sufficiently short,
it is clear that getbuf
will return 1, as shown by the following execution
example:
Cookie: 0x1a7dd803 Type string: Keep it short! No exploit. Getbuf returned 0x1 Normal return
Typically an error occurs if you type a long string:
unix> ./ctarget Cookie: 0x1a7dd803 Type string: This is not a very interesting string, but it has the property ... Ouch!: You caused a segmentation fault! Better luck next time
(Note that the value of the cookie shown will differ from yours.) Program
RTARGET
will have the same behavior. As the error message indicates,
overrunning the buffer typically causes the program state to be corrupted, leading to a
memory access error. Your task is to be more clever with the strings you feed
CTARGET
and RTARGET
so that they do more interesting
things. These are called exploit strings.
Both CTARGET
and RTARGET
take several different command line
arguments:
HEX2RAW
will enable
you to generate these raw strings. See Appendix A for more information
on how to use HEX2RAW
.
Important points:
0x0a
at any intermediate
position, since this is the ASCII code for newline (‘\n
’). When Gets
encounters this byte, it will assume you intended to terminate the string.
HEX2RAW
expects two-digit hex values separated by one or more white
spaces. So if you want to create a byte with a hex value of 0, you need to write
it as 00
. To create the word 0xdeadbeef you should pass
“ef be ad de
” to HEX2RAW
(note the reversal required for
little-endian byte ordering).
When you have correctly solved one of the levels, your target program will automatically send a notification to the grading server. For example:
unix> ./hex2raw < ctarget.l2.txt | ./ctarget Cookie: 0x1a7dd803 Type string:Touch2!: You called touch2(0x1a7dd803) Valid solution for level 2 with target ctarget PASSED: Sent exploit string to server to be validated. NICE JOB!
The server will test your exploit string to make sure it really works, and it will update the Attacklab scoreboard page indicating that your userid (listed by your target number for anonymity) has completed this phase.
You can view the scoreboard by pointing your Web browser at
http://cycle3.csug.rochester.edu:15513/scoreboard
Unlike the Bomb Lab, there is no penalty for making mistakes in this lab. Feel free
to fire away at CTARGET
and RTARGET
with any strings you like.
IMPORTANT NOTE: You can work on your solution on any Linux machine, but in order to submit your solution, you will need to be running on one of the following machines:
cycle[1-3].csug.rochester.edu
Figure 1 summarizes the five phases of the lab. As can be seen, the first three involve
code-injection (CI) attacks on CTARGET
, while the last two involve
return-oriented-programming (ROP) attacks on RTARGET
.
Before midnight, Monday, October 5, submit answers on blackboard to the following questions.
HEX2RAW
HEX2RAW
takes as input a hex-formatted string. In this
format, each byte value is represented by two hex digits. For example, the
string “012345
” could be entered in hex format as
“30 31 32 33 34 35 00
”. (Recall that the ASCII code
for decimal digit x
is 0x3x
, and that the end of a
string is indicated by a null byte.)
The hex characters you pass to HEX2RAW
should be separated by
whitespace (blanks or newlines). We recommend separating different parts of
your exploit string with newlines while you’re working on it.
HEX2RAW
supports C-style block comments, so you can mark off
sections of your exploit string. For example:
48 c7 c1 f0 11 40 00 /* mov $0x40011f0,%rcx */
Be sure to leave space around both the starting and ending comment strings
(“/*
”, “*/
”), so that the
comments will be properly ignored.
If you generate a hex-formatted exploit string in the file
exploit.txt
, you can apply the raw string to
CTARGET
or RTARGET
in several different ways:
HEX2RAW
.unix> ./hex2raw < exploit.txt > exploit-raw.txt unix> ./ctarget < exploit-raw.txtThis approach can also be used when running from within
GDB
:
unix> gdb ctarget (gdb) run < exploit-raw.txt
unix> ./hex2raw < exploit.txt > exploit-raw.txt unix> ./ctarget -i exploit-raw.txtThis approach can also be used when running from within
GDB
.
Using GCC
as an assembler and OBJDUMP
as a disassembler
makes it convenient to generate the byte codes for instruction sequences. For
example, suppose you write a file example.s
containing the following
assembly code:
# Example of hand-generated assembly code pushq $0xabcdef # Push value onto stack addq $17,%rax # Add 17 to %rax movl %eax,%edx # Copy lower 32 bits to %edx
The code can contain a mixture of instructions and data. Anything to the right
of a ‘#
’ character is a comment.
You can now assemble and disassemble this file:
unix> gcc -c example.s unix> objdump -d example.o > example.d
The generated file example.d
contains the following:
example.o: file format elf64-x86-64 Disassembly of section .text: 0000000000000000 <.text>: 0: 68 ef cd ab 00 pushq $0xabcdef 5: 48 83 c0 11 add $0x11,%rax 9: 89 c2 mov %eax,%edx
The lines at the bottom show the machine code generated from the assembly
language instructions. Each line has a hexadecimal number on the left indicating
the instruction’s starting address (starting with 0), while the hex digits
after the ‘:
’ character indicate the byte codes for the
instruction. Thus, we can see that the instruction push $0xABCDEF
has
hex-formatted byte code 68 ef cd ab 00
.
From this file, you can get the byte sequence for the code:
68 ef cd ab 00 48 83 c0 11 89 c2
This string can then be passed through HEX2RAW
to generate an input
string for the target programs. Alternatively, you can edit example.d to omit
extraneous values and to contain C-style comments for readability, yielding:
68 ef cd ab 00 /* pushq $0xabcdef */ 48 83 c0 11 /* add $0x11,%rax */ 89 c2 /* mov %eax,%edx */
This is also a valid input you can pass through HEX2RAW
before sending
to one of the target programs.
There is no explicit turn-in. You can keep track of how you (and the other groups) are doing by looking at http://cycle3.csug.rochester.edu:15513/scoreboard .
This web page is updated continuously to show the progress of each group.