Skip to content
Branch: master
Find file Copy path
Find file Copy path
Fetching contributors…
Cannot retrieve contributors at this time
152 lines (101 sloc) 5.7 KB

Using afl fuzzer against masscan

AFL (American-Fuzzy-Lop) is an automated fuzzer. It takes existing input to a program, then morphs that input in order to test new code paths through the code. It's extremely successful at finding bugs as well as vulnerabilities.

There are two inputs to masscan. One is the files it reads, which come in various formats. The second is input from the network, in response to network probes, which consist of various network protocols.

For AFL, there is also the issue of how masscan crashes. It tries to print a backtrace. This causes AFL to falsely mark this as a "hang" rather than a "crash". To fix this, run masscan with the --nobacktrace option.

Fuzzing file formats

The masscan program reads the following files. You can set the fuzzer at each one of these to fuzz how it parses the contents.

-c <filename.conf>

Masscan can read its configuration either from the command-line or from a file. To create a file, run masscan as normal, then hit . This will save all it's settings, even default values, into a file. It's a good starting point for fuzzing.

--readscan <filename.mass>

One of the possible outputs of masscan is in a proprietary binary format. You can then run masscan to convert to any other output format.

In other words, you can run masscan to output XML like:

masscan <blah blah blah> -oX scan.xml

Or, in a two step process:

masscan <blah blah blah> -ob scan.mass

masscan --readscan scan.mass -oX scan.xml

--exclude-file <filename.ranges>

Masscan can scan large ragnes, like (the entire Internet). You may want to exclude specific addresses or ranges. These are configured in the "exclude file".

You can also read ranges using --include-file <filename.ranges> or -iL <filename.ranges>, but as far as fuzzing, I'm pretty sure it'll be the same results.


This file is read, then dumped blindly across a TCP connection, in order to say "hello" to a service. Since there's no parsing here, I'm not sure you'll find anything fuzzing this.


This file specifies the default payloads for UDP. It's in the same file format as for nmap. There's some juicy parsing here that may lead to bugs.

--pcap-payloads <filename.pcap>

This is the same as --nmap-payloads, but reads the UDP payloads from a libpcap file.

Fuzzing network protocols

Masscan parses several network protocols. It also must reassemble fragmented responses over TCP for any application protocol. Remember: masscan has it's own stack, so must parse everything that comes over the network.

AFL has no ability to read from the network at this time. Moreover, even then it wouldn't work easily, since masscan has a network stack rather than just an application layer to deal with.

The trick is to first use masscan on a target that responds back on a protocol, then save just the response side of the TCP connection to a file. Then, when running masscan under AFL, read in that file using the option --adapter file:<filename.pcap>. Then, and this is critical, you must hard code all the TCP stack values to match those of the original connection.

I generated the file masscan/data/afl-http.pcap as an example file to read for fuzzing the parsing of HTTP. The command-line parameters to use are:

bin/masscan --nobacktrace --adapter file:data/afl-http.pcap --source-ip --source-port 6000 --source-mac 00-11-22-33-44-55 --router-mac c0-c1-c0-a0-9b-9d --seed 0 --banners -p80 --nostatus

The explanation are:

*--nobacktrace*: so that AFL correctly marks crashes as crashes
and not as hangs.

*--adapter file:*: This option normally specifies the adapter,
like *eth0* or *en1*. By putting the *file:* prefix on an adapter name,
it'll use a file (in *libpcap* format) to use instead. In this case,
transmits are dropped, and any packets are read from a file.

*--source-ip*, *--source-port*, *--source-mac*, *--router-mac*:
These are the hard-coded TCP/IP stack settings that must match the packets
in the file.

*--seed*: This must match the randomization seed in the packet file. Since
everything else is hardcoded, I think the only thing this will control
will be the sequence number of the TCP connection.

*--banners*: this tell the scanner to not simply find open ports, but also
establish a TCP connection and interact with the protocol, and report on
the results.

*-p<port>*: The destination IP address to connect to.

*<ip-address>*: The IP address to connect to

This should produce an output like the following. If you get the banner back, then you know you've successfully done everything correctly. Conversely, if you set --seed 1, then it won't work, because it'll reject responses that match the wrong seed.

Starting masscan 1.0.3 ( at 2016-06-06 05:19:03 GMT
-- forced options: -sS -Pn -n --randomize-hosts -v --send-eth
Initiating SYN Stealth Scan
Scanning 1 hosts [1 port/host]
Discovered open port 80/tcp on
Banner on port 80/tcp on [title] Google
Banner on port 80/tcp on [http] HTTP/1.0 200 OK\x0d\x0a...

(Additional output is truncated -- you get the idea).

The problem with this is that masscan will take about 10 seconds before it produces the results. When it establishes a connection, it waits a few seconds for the other side to transmit, then sends it's "hello", then waits many seconds for all output to be received. I don't know if this messes AFL up, whether I need to add additional options to truncate any waiting.

You can’t perform that action at this time.