De-socketing for Fuzzing

When fuzzing network applications the fuzzers provide their inputs over stdin although the applications expect their input over network connections. This library redirects all network communication to stdin and stdout such that network applications can be traditionally fuzzed with AFL++.

This has the advantage of

1. Improving performance by reducing kernel interactions because libdesock operates 100% in user-space
2. Reducing the amount of manual effort necessary to create a harness

For an in-depth explanation of de-socketing see our blog post.

Building

Libdesock uses meson and ninja as its build system.

meson setup ./build
cd ./build

You can configure the build using

meson configure -D <optname>=<optvalue>

You can get an overview over all options with

meson configure

The following options are specific to libdesock:

Option	Description	Default
arch	The CPU architecture for which you are compiling libdesock.so	x86_64
debug_desock	If this is true, calls to functions in libdesock.so get logged to stderr.	false
desock_client	If this is true, calls to connect() get hooked. This enables the desocketing of clients.	false
desock_server	If this is true, calls to bind() get hooked. This enables the desocketing of servers.	true
fd_table_size	Only fds < fd_table_size can be desocked.	128
interpreter	Path to ld.so (will be determined dynamically if not set)

If configuration is done compile with

meson compile

This creates a shared library ./build/libdesock.so and a static library ./build/libdesock.a.

Usage

Prepend

LD_PRELOAD=libdesock.so

to the invocation of any network application or set the environment variable

AFL_PRELOAD=libdesock.so

when using AFL++.

Examples

If you are using libdesock and AFL for fuzzing, the programs under test usually require a special setup to work with AFL. Checkout our examples directory for some examples on how to properly setup network applications for fuzzing.

Known Bugs

• TCP servers using libuv cannot be de-socket-ed (yet). De-socketing of libuv currently only works with UDP servers. It only takes a small change to fix this though, if anyone needs this create an issue.
• ioctl() is not supported. Make sure your target does not rely on ioctl requests

Copyright Notice

Libdesock is based upon musl libc and parts of the musl source code can be found in libdesock.