tlspuffin

TLS Protocol Under FuzzINg

A symbolic-model-guided fuzzer for TLS

Master Thesis | Thesis Presentation | Documentation

Disclaimer: The term "symbolic-model-guided" should not be confused with symbolic execution or concolic fuzzing.

Description

Fuzzing implementations of cryptographic protocols is challenging. In contrast to traditional fuzzing of file formats, cryptographic protocols require a specific flow of cryptographic and mutually dependent messages to reach deep protocol states. The specification of the TLS protocol describes sound flows of messages and cryptographic operations.

Although the specification has been formally verified multiple times with significant results, a gap has emerged from the fact that implementations of the same protocol have not undergone the same logical analysis. Because the development of cryptographic protocols is error-prone, multiple security vulnerabilities have already been discovered in implementations in TLS which are not present in its specification.

Inspired by symbolic protocol verification, we present a reference implementation of a fuzzer named tlspuffin which employs a concrete semantic to execute TLS 1.2 and 1.3 symbolic traces. In fact attacks which mix \TLS versions are in scope of this implementation. This method allows us to utilize a genetic fuzzing algorithm to fuzz protocol flows, which is described by the following three stages.

• By mutating traces we can deviate from the specification to test logical flaws.
• Selection of interesting protocol flows advance the fuzzing procedure.
• A security violation oracle supervises executions for the absence of vulnerabilities.

The novel approach allows rediscovering known vulnerabilities, which are out-of-scope for classical bit-level fuzzers. This proves that it is capable of reaching critical protocol states. In contrast to the promising methodology no new vulnerabilities were found by tlspuffin. This can can be explained by the fact that the implementation effort of TLS protocol primitives and extensions is high and not all features of the specification have been implemented. Nonetheless, the innovating approach is promising in terms of quickly reaching high edge coverage, expressiveness of executable protocol traces and stable and extensible implementation.

Features

• Uses the LibAFL fuzzing framework
• Fuzzer which is inspired by the Dolev-Yao symbolic model used in protocol verification
• Domain specific mutators for Protocol Fuzzing!
• Supported Libraries Under Test: OpenSSL 1.0.1f, 1.0.2u, 1.1.1k and LibreSSL 3.3.3
• Reproducible for each LUT. We use Git submodules to link to forks this are in the tlspuffin organisation
• 70% Test Coverage
• Writtin in Rust!

Dependencies

• build-essential (make, gcc)
• clang
• graphviz

OpenSSL 1.0:

• makedepend from `xutils-dev package

WolfSSL:

• autoconf
• libtool

For the python tlspuffin-analyzer:

• libyajl-dev
• wheel from Python pip

Building

Build the project:

git clone git@github.com/tlspuffin/tlspuffin
git submodule update --init --recursive
cargo build

Running

Fuzz using three clients:

cargo run --bin tlspuffin -- --cores 0-3

Note: After switching the Library Under Test or its version do a clean rebuild (cargo clean). For example when switching from OpenSSL 1.0.1 to 1.1.1.

Testing

cargo test

Command-line Interface

The syntax for the command-line of is:

      tlspuffin [⟨options] [⟨sub-commands⟩]

Global Options

Before we explain each sub-command, we first go over the options in the following.

• -c, --cores ⟨spec⟩

  This option specifies on which cores the fuzzer should assign its worker processes. It can either be specified as a list by using commas "0,1,2,7" or as a range "0-7". By default, it runs just on core 0.

• -i, --max-iters ⟨i⟩

  This option allows to bound the amount of iterations the fuzzer does. If omitted, then infinite iterations are done.

• -p, --port ⟨n⟩

  As specified in [sec:design-multiprocessing] the initial communication between the fuzzer broker and workers happens over TCP/IP. Therefore, the broker requires a port allocation. The default port is 1337.

• -s, --seed ⟨n⟩

  Defines an initial seed for the prng used for mutations. Note that this does not make the fuzzing deterministic, because of randomness introduced by the multiprocessing (see [sec:design-multiprocessing]).

Sub-commands

Now we will go over the sub-commands execute, plot, experiment, and seed.

• execute ⟨input⟩

  This sub-command executes a single trace persisted in a file. The path to the file is provided by the ⟨input⟩ argument.

• plot ⟨input⟩ ⟨format⟩ ⟨output_prefix⟩

  This sub-command plots the trace stored at ⟨input⟩ in the format specified by ⟨format⟩. The created graphics are stored at a path provided by ⟨output_prefix⟩. The option --multiple can be provided to create for each step in the trace a separate file. If the option --tree is given, then only a single graphic which contains all steps is produced.

• experiment

  This sub-command initiates an experiment. Experiments are stored in a directory named experiments/ in the current working directory. An experiment consists of a directory which contains . The title and description of the experiment can be specified with --title ⟨t⟩ and --description ⟨d⟩ respectively. Both strings are persisted in the metadata of the experiment, together with the current commit hash of , the version and the current date and time.

• seed

  This sub-command serializes the default seed corpus in a directory named corpus/ in the current working directory. The default corpus is defined in the source code of using the trace dsl.

Rust Setup

Install rustup.

The toolchain will be automatically downloaded when building this project. See ./rust-toolchain.toml for more details about the toolchain.

Make sure that you have the clang compiler installed. Optionally, also install llvm to have additional tools like sancov available. Also make sure that you have the usual tools for building it like make, gcc etc. installed. They may be needed to build OpenSSL.

Advanced Features

Running with ASAN

ASAN_OPTIONS=abort_on_error=1 \
    cargo run --bin tlspuffin --features asan -- --cores 0-3

It is important to enable abort_on_error, else the fuzzer workers fail to restart on crashes.

Generate Corpus Seeds

cargo run --bin tlspuffin -- seed

Plot Symbolic Traces

To plot SVGs do the following:

cargo run --bin tlspuffin -- plot corpus/seed_client_attacker12.trace svg ./plots/seed_client_attacker12

Note: This requires that the dot binary is in on your path. Note: The utility tools/plot-corpus.sh plots a whole directory

Execute a Symbolic Trace (with ASAN)

To analyze crashes you can also execute a trace which crashes the testing harness using ASAN:

cargo run --bin tlspuffin -- execute test.trace

To do the same with ASAN enabled:

ASAN_OPTIONS=detect_leaks=0 \
      cargo run --bin tlspuffin --features asan -- execute test.trace

Crash Deduplication

Creates log files for each crash and parses ASAN crashes to group crashes together.

tools/analyze-crashes.sh

Benchmarking

There is a benchmark which compares the execution of the dynamic functions to directly executing them in benchmark.rs. You can run them using:

cargo bench
xdg-open target/criterion/report/index.html

Documentation

This generates the documentation for this crate and opens the browser. This also includes the documentation of every dependency like LibAFL or rustls.

cargo doc --open

You can also view the up-to-date documentation here.