Payload generation failed unsupported buffer format


  • Qt Documentation
  • Generating Payloads in Metasploit
  • Explanation of Drop code and Module-ID values in Packet Capture output (SonicOS 6.2.4.2-20n)
  • Generating Payloads in Metasploit Generate a Payload for Metasploit During exploit development, you will most certainly need to generate shellcode to use in your exploit. In Metasploit, payloads can be generated from within the msfconsole.

    When you use a certain payload, Metasploit adds the generate, pry, and reload commands. Generate will be the primary focus of this section in learning how to use Metasploit.

    More often than not, bad characters and specific types of encoders will be used depending on the targeted machine. Granted some exploits allow us to use it but not many. To accomplish this, we issue the generate command followed by the -b switch with accompanying bytes we wish to be disallowed during the generation process.

    Thus giving us a null byte free payload. We also see other significant differences as well, due to the change we enforced during generation. In our previous iteration the size was bytes, this new shellcode is 27 bytes larger.

    Another significant change is the added use of an encoder. By default Metasploit will select the best encoder to accomplish the task at hand. The encoder is responsible for removing unwanted characters amongst other things entered when using the -b switch.

    When specifying bad characters the framework will use the best encoder for the job. If we add a few more bad characters a different encoder may be used to accomplish the same task. Lets add several more bytes to the list and see what happens. We see a different encoder was used in order to successfully remove our unwanted bytes. Payload Generation Failed Having the ability to generate shellcode without the use of certain characters is one of the great features offered by this framework.

    If too many restricted bytes are given no encoder may be up for the task. At which point Metasploit will display the following message. Using an Encoder During Payload Generation As mentioned previously the framework will choose the best encoder possible when generating our payload. However there are times when one needs to use a specific type, regardless of what Metasploit thinks.

    Imagine an exploit that will only successfully execute provided it only contains non-alphanumeric characters. If everything went according to plan, our payload will not contain any alphanumeric characters.

    But we must be careful when using a different encoder other than the default. As it tends to give us a larger payload. For instance, this one is much larger than our previous examples. Our next option on the list is the -f switch. This gives us the ability to save our generated payload to a file instead of displaying it on the screen.

    As always it follows the generate command with file path. By using the cat command the same way we would from the command shell, we can see our payload was successfully saved to our file.

    As we can see it is also possible to use more than one option when generating our shellcode. Generating Payloads with Multiple Passes Next on our list of options is the iteration switch -i. In a nutshell, this tells the framework how many encoding passes it must do before producing the final payload. One reason for doing this would be stealth, or anti-virus evasion. Anti-virus evasion is covered in greater detail in another section of MSFU.

    Comparing the two outputs we see the obvious effect the second iteration had on our payload. First of all, the byte size is larger than the first. The more iterations one does the larger our payload will be. Secondly comparing the first few bytes of the highlighted code, we also see they are no longer the same. This is due to the second iteration, or second encoding pass. It encoded our payload once, than took that payload and encoded it again.

    Lets look at our shellcode and see how much of a difference 5 iterations would make. The change is significant when comparing to all previous outputs. Which would, in theory, make this version of our payload less prone to detection. In the case of a bind shell the default listening port is Often this must be changed. We can accomplish this by using the -o switch followed by the value we wish to change. Although the ruby language is extremely powerful and popular, not everyone codes in it.

    We have the capacity to tell the framework to give our payload in different coding formats such as Perl, C and Java for example. Adding a NOP sled at the beginning is also possible when generating our shellcode. Like all the other options all that needs to be done is type in the switch followed by the format name as displayed in the help menu. Looking at the output for the different programming languages, we see that each output adheres to their respective language syntax.

    Looking at all three outputs, the arrays are properly declared for the language format selected. Making it ready to be copied and pasted into your script. This will add the sled at the beginning of our payload. Keep in mind the larger the sled the larger the shellcode will be. So adding a 10 NOPs will add 10 bytes to the total size.

    Comparing the next 3 lines with the shellcode just above, we see they are exactly the same. Total bytes, as expected, grew by exactly 14 bytes.

    As such, there is no one perfect interface to use with the Metasploit console, although the MSFConsole is the only supported way to access most Metasploit commands. It is still beneficial, however, to be comfortable with all Metasploit interfaces. This allows you to easily add Metasploit exploits into any scripts you may create.

    One way to obtain similar functionality through msfconsole is by using the -x option. Note: Although Zsh is often available, please be aware it isn't usually installed by default. Benefits of the MSFcli Interface Supports the launching of exploits and auxiliary modules Useful for specific tasks Convenient to use when testing or developing a new exploit Good tool for one-off exploitation Excellent if you know exactly which exploit and options you need Wonderful for use in scripts and basic automation The only real drawback of msfcli is that it is not supported quite as well as msfconsole and it can only handle one shell at a time, making it rather impractical for client-side attacks.

    This, however is not required. Just as you can in commercial routers, you can switch modules from within other modules. As a reminder, variables will only carry over if they are set globally. By issuing the connect command with an IP address and port number, you can connect to a remote host from within msfconsole the same as you would with Netcat or Telnet. By default, this will open the current module in Vim. It matches a given pattern from the output of another msfconsole command.

    Help menu banner Display an awesome metasploit banner cd Change the current working directory color Toggle color connect Communicate with a host Be sure to always read the module description prior to using it as some may have un-desired effects.

    Windows Vista without SP1 does not seem affected by this flaw. This feature is also very useful for understanding the internals of the Framework.

    The jobs command provides the ability to list and terminate these jobs. Resource files may also contain ruby code between tags. See also: makerc Some attacks, such as Karmetasploit, use resource files to run a set of commands in a karma. Later, we will discuss how, outside of Karmetasploit, that can be very useful. Batch files can greatly speed up testing and development times as well as allow the user to automate many tasks. Besides loading a batch file from within msfconsole, they can also be passed at startup using the -r flag.

    The simple example below creates a batch file to display the Metasploit version number at startup. To add a route, you pass the target subnet and network mask followed by the session comm number.

    If you have a general idea of what you are looking for, you can search for it via search. The search function will locate this string within the module names, descriptions, references, etc.

    Note the naming convention for Metasploit modules uses underscores versus hyphens. The sessions can be shells, Meterpreter sessions, VNC, etc. For example: sessions -s checkvm -i 1, or sessions -k ,5,6 To list any active sessions, pass the -l options to sessions.

    You can remove all assigned variables with unset all. You can do this with the setg command. Once these have been set, you can use them in as many exploits and auxiliary modules as you like. You can also save them for use the next time you start msfconsole. However, the pitfall is forgetting you have saved globals, so always check your options before you run or exploit.

    Conversely, you can use the unsetg command to unset a global variable. In the examples that follow, variables are entered in all-caps ie: LHOST , but Metasploit is case-insensitive so it is not necessary to do so. With your settings saved, they will be automatically loaded on startup, which saves you from having to set everything again.

    There are a number of show commands you can use but the ones you will use most frequently are show auxiliary, show exploits, show payloads, show encoders, and show nops. As mentioned earlier, auxiliary modules include scanners, denial of service modules, fuzzers, and more. Run show exploits to get a listing of all exploits contained in the framework. Fortunately, when you are in the context of a particular exploit, running show payloads will only display the payloads that are compatible with that particular exploit.

    For instance, if it is a Windows exploit, you will not be shown the Linux payloads. The use command changes your context to a specific module, exposing type-specific commands. Notice in the output below that any global variables that were previously set are already configured.

    Granted some exploits allow us to use it but not many. To accomplish this, we issue the generate command followed by the -b switch with accompanying bytes we wish to be disallowed during the generation process. Thus giving us a null byte free payload. We also see other significant differences as well, due to the change we enforced during generation.

    Qt Documentation

    In our previous iteration the size was bytes, this new shellcode is 27 bytes larger. Another significant change is the added use of an encoder.

    By default Metasploit will select the best encoder to accomplish the task at hand. The encoder is responsible for removing unwanted characters amongst other things entered when using the -b switch. When specifying bad characters the framework will use the best encoder for the job. If we add a few more bad characters a different encoder may be used to accomplish the same task.

    Lets add several more bytes to the list and see what happens. We see a different encoder was used in order to successfully remove our unwanted bytes. Payload Generation Failed Having the ability to generate shellcode without the use of certain characters is one of the great features offered by this framework. If too many restricted bytes are given no encoder may be up for the task. At which point Metasploit will display the following message.

    Using an Encoder During Payload Generation As mentioned previously the framework will choose the best encoder possible when generating our payload. However there are times when one needs to use a specific type, regardless of what Metasploit thinks. Imagine an exploit that will only successfully execute provided it only contains non-alphanumeric characters.

    If everything went according to plan, our payload will not contain any alphanumeric characters. But we must be careful when using a different encoder other than the default. As such, there is no one perfect interface to use with the Metasploit console, although the MSFConsole is the only supported way to access most Metasploit commands.

    It is still beneficial, however, to be comfortable with all Metasploit interfaces. This allows you to easily add Metasploit exploits into any scripts you may create.

    Generating Payloads in Metasploit

    One way to obtain similar functionality through msfconsole is by using the -x option. Note: Although Zsh is often available, please be aware it isn't usually installed by default. Benefits of the MSFcli Interface Supports the launching of exploits and auxiliary modules Useful for specific tasks Convenient to use when testing or developing a new exploit Good tool for one-off exploitation Excellent if you know exactly which exploit and options you need Wonderful for use in scripts and basic automation The only real drawback of msfcli is that it is not supported quite as well as msfconsole and it can only handle one shell at a time, making it rather impractical for client-side attacks.

    This, however is not required. Just as you can in commercial routers, you can switch modules from within other modules. As a reminder, variables will only carry over if they are set globally. By issuing the connect command with an IP address and port number, you can connect to a remote host from within msfconsole the same as you would with Netcat or Telnet.

    By default, this will open the current module in Vim. It matches a given pattern from the output of another msfconsole command. Help menu banner Display an awesome metasploit banner cd Change the current working directory color Toggle color connect Communicate with a host Be sure to always read the module description prior to using it as some may have un-desired effects.

    To limit the size of the read buffer, call setReadBufferSize.

    Explanation of Drop code and Module-ID values in Packet Capture output (SonicOS 6.2.4.2-20n)

    To close the socket, call disconnectFromHost. After all pending data has been written to the socket, QAbstractSocket actually closes the socket, enters QAbstractSocket::UnconnectedStateand emits disconnected. If you want to abort a connection immediately, discarding all pending data, call abort instead. The port and address of the connected peer is fetched by calling peerPort and peerAddress. QAbstractSocket provides a set of functions that suspend the calling thread until certain signals are emitted.

    These functions can be used to implement blocking sockets: waitForConnected blocks until a connection has been established. Programming with a blocking socket is radically different from programming with a non-blocking socket. A blocking socket doesn't require an event loop and typically leads to simpler code. However, in a GUI application, blocking sockets should only be used in non-GUI threads, to avoid freezing the user interface.


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