CTF-writeups

Some CTF writeups


Project maintained by Qyn-CTF

authy - 178 solves (150 points)

Description

Check out this new storage application that your government has started! It's supposed to be pretty secure since everything is authenticated...

curl crypto.chal.csaw.io:5003

First looks

Were given the source of the running application handout.py.
We can quickly see that this is just some API, hashing our input and handing it back to us. With another option allowing us to view/verify that the signature is correct.

Crypto

When we post some data to /new, the note is converted from JSON to a representation of form data:

info = {"admin": "False", "access_sensitive": "False" }
info.update(payload)
info["entrynum"] = 783

infostr = ""
for pos, (key, val) in enumerate(info.items()):
    infostr += "{}={}".format(key, val)
    if pos != (len(info) - 1):
        infostr += "&"

There’s already a bug here, since the this: infostr += "{}={}".format(key, val) doesn’t URL encode our payload, so we can set our note to something like: note&admin=True, which would work, but we can’t modify the entrynum since it’s set afterwards and I don’t think there is a way to escape it somehow.
After this conversion, the actual vulnerable part happens:

infostr = infostr.encode()

identifier = base64.b64encode(infostr).decode()

hasher = hashlib.sha1()
hasher.update(SECRET + infostr)
return "Successfully added {}:{}\n".format(identifier, hasher.hexdigest())

Here we can see that the sha1 hash is taken of SECRET|infostr, where infostr is the form like data (sha1(SECRET || infostr)). This is just a simple length extension attack (Not all hash types are vulnerable against this).
Continuing to the /view endpoint, we see it takes the id (identifier) and the hash in order to verify it was right:

identifier = base64.b64decode(info["id"]).decode()
checksum = info["integrity"]

params = identifier.replace('&', ' ').split(" ")
note_dict = { param.split("=")[0]: param.split("=")[1]  for param in params }

encode = base64.b64decode(info["id"]).decode('unicode-escape').encode('ISO-8859-1')
hasher = hashlib.sha1()
hasher.update(SECRET + encode)
gen_checksum = hasher.hexdigest()

Here we see that if you have a string such as: admin=False&access_sensitive=False&note=someNote&admin=True&access_sensitive=True&author=me&entrynum=783 that the latest entry is chosen, so instead of admin=False, the later admin=True is used.
And if the entrynum, previously set to 783 is equal to 7, the admin is set to True and the access_sensitive is set to True, we get the flag:

entrynum = int(note_dict["entrynum"])
if 0 <= entrynum <= 10:

    if (note_dict["admin"] not in [True, "True"]):
        return ">:(\n"
    if (note_dict["access_sensitive"] not in [True, "True"]):
        return ">:(\n"

    if (entrynum == 7):
        return "\nAuthor: admin\nNote: You disobeyed our rules, but here's the note: " + FLAG + "\n\n"
    else:
        return "Hmmmmm...."

else:
    return """\nAuthor: {}
Note: {}\n\n""".format(note_dict["author"], note_dict["note"])

We previously found out we can just set the admin and access_sensitive to True, but we now need to set the entrynum to 7 as well.
For this, I’m going to use some tool called HashPump since it has Python bindings for it. Such a length extension attack allows us to append data to the given data, so let’s say for example from /new we get:
admin=False&access_sensitive=False&note=someNote&admin=True&access_sensitive=True&author=me&entrynum=783, would result in:
admin=False&access_sensitive=False&note=someNote&admin=True&access_sensitive=True&author=me&entrynum=783{RANDOMDATA}{CONTROLLED DATA}
While still giving a correct hash. We can quickly implement something like this:

import hashpumpy
inputId = "admin=False&access_sensitive=False&note=someNote&admin=True&access_sensitive=True&author=me&entrynum=783"
inputHash = "2dff57726cd588741d73c554d0eb4e1ebb0d9b32"

result = hashpumpy.hashpump(inputHash, inputId, '&admin=True&entrynum=7&access_sensitive=True', 13) #Derrived length of just trying
print(result[0]) # df20c11894172bbc5bcc0a3da2c5e723a35c553d
print(result[1]) #b'admin=False&access_sensitive=False&note=someNote&admin=True&access_sensitive=True&author=me&entrynum=783\x80\x00\x00\x00\x00\x00\x00\x00\x00\x03\xa8&admin=True&entrynum=7&access_sensitive=True'

NOTE that the 13 is the key length, which we can just try until we succeed Also, I obviously don’t need the extra admin=True in the appended data, but if you didn’t catch the first bug, this would work too.
However, just base64 encoding this and sending it to the /view doesn’t work as it first runs .decode() on it and the 0x80 character is not a valid UTF8 character.
But we can also see that it doesn’t use that value to call the sha1(SECRET || identifier) on it:

identifier = base64.b64decode(info["id"]).decode()
checksum = info["integrity"]
...

encode = base64.b64decode(info["id"]).decode('unicode-escape').encode('ISO-8859-1')
hasher = hashlib.sha1()
hasher.update(SECRET + encode)

Instead it takes the info["id"], decodes it as base64, decodes it again with unicode-escape and then encodes it with ISO-8859-1 and then uses that to do the sha1 operation.
It turns out, if we do that in reverse on our previous script that it generates normal UTF8 characters (because of the unicode-escape) ->

...
result = hashpumpy.hashpump(inputHash, inputId, '&admin=True&entrynum=7&access_sensitive=True', 13) #Derrived length of just trying
...
print(result[1].decode("ISO-8859-1").encode('unicode-escape')) #b'admin=False&access_sensitive=False&note=someNote&admin=True&access_sensitive=True&author=me&entrynum=783\\x80\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x03\\xa8&admin=True&entrynum=7&access_sensitive=True'

And base64 encoding this and sending it to /view together with the new hash generated by HashPump, we get the flag.
Final script:

import hashpumpy
from base64 import b64encode
import requests

inputId = "admin=False&access_sensitive=False&note=someNote&admin=True&access_sensitive=True&author=me&entrynum=783"
inputHash = "2dff57726cd588741d73c554d0eb4e1ebb0d9b32"

for i in range(2, 20):
    try:
        print(f"Doing {i}/19")
        result = hashpumpy.hashpump(
            inputHash, inputId, '&admin=True&entrynum=7&access_sensitive=True', i)
            
        resultIdentifier = b64encode(result[1].decode(
            "ISO-8859-1").encode('unicode-escape')).decode()

        r = requests.post('http://crypto.chal.csaw.io:5003/view',
                          data={'id': resultIdentifier,
                                'integrity': result[0]},
                          headers={"Content-Type": "application/x-www-form-urlencoded"})

        if "disobeyed our rules" in r.text:
            print(r.text)
            break
    except:
        pass

flag{h4ck_th3_h4sh}

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