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tlsfuzzer is a test suite for SSLv2, SSLv3, TLS 1.0, TLS 1.1, TLS 1.2, and TLS 1.3 implementations. It's in early stages of development, so there are no API stability guarantees. While it uses fuzzing techniques for testing (randomisation of passed in inputs), the scripts are generally written in a way that verifies correct error handling: unlike typical fuzzers it doesn't check only that the system under test didn't crash, it checks that it returned correct error messages.

You can find ready-to-use scripts testing for many vulnerabilities ( ROBOT, DROWN, etc.) and general standards conformity (RFC 5246, RFC 7627, RFC 7905, etc.) in the scripts/ directory.


You'll need:

  • Python 2.6 or later or Python 3.6 or later
  • tlslite-ng 0.8.0-beta1 or later (note that tlslite will not work and they conflict with each other)
  • ecdsa python module (dependency of tlslite-ng, should get installed automatically with it), use at least version 0.15 for optimal performance

Optionally, to make cryptographic calculations significantly faster, you may want to install the following libraries (see tlslite-ng and python-ecdsa README files for details):

  • m2crypto
  • gmpy

To get pip (if your python installation doesn't already have it) download and run (or see for alternative configuration that does not require installation of packages):


Then install tlslite-ng:

pip install --pre tlslite-ng

(Use --upgrade --pre if you did install it before)

Download the tlsfuzzer:

git clone


After all dependencies are installed, make sure:

  • you're in the directory of the project (after git clone just cd tlsfuzzer)
  • the server you want to test is running on the same computer (localhost)
  • the server is listening on port 4433
  • and the server will answer with data to HTTP queries (answer with valid HTTP responses is optional)

Then you can run one of the tests in scripts directory, like so:

PYTHONPATH=. python scripts/

If test has additional requirements, it will output them to console. No errors printed means that all expecations were met (so for tests with bad data the server rejected our messages).

All scripts also accept --help to print the help message (specification of all the options given script supports), -h to specify the hostname or IP address of the server-to-be-tested and -p to specify the port of the service to be tested.

See for more info and how to interpret errors and failures reported by scripts.

You can find mode detailed documentation for the project at

Using tlsfuzzer to test for timing side-channel attacks (Lucky13, padding oracle attacks and timing-based Bleichenbacher oracle) is described in the document.

Server under test configuration

In general, the server under test requires just a RSA certificate, you can create it using the following OpenSSL command:

openssl req -x509 -newkey rsa -keyout localhost.key -out localhost.crt -subj \
/CN=localhost -nodes -batch

Note: tlsfuzzer verifies only TLS level behaviour, it does not perform any checks on the certificate (like hostname validation, CA signatures or key usage). It does however verify if the signatures made on TLS message by the server (like in Server Key Exchange or Certificiate Verify message) match the certificate sent by the server.

More detailed instructions, including how to build the different frameworks from source, are available in the Server setup wiki page.

Example server configurations:


To test OpenSSL, it's sufficient to pass an extra -www option to a typical s_server command line:

openssl s_server -key localhost.key -cert localhost.crt -www


To test GnuTLS server, you need to tell it to behave as an HTTP server and additionally, to not ask for client certificates:

gnutls-serv --http -p 4433 --x509keyfile localhost.key --x509certfile \
localhost.crt --disable-client-cert


To test the Mozilla NSS library server, you first need to create a database with server certificate:

mkdir nssdb
certutil -N -d sql:nssdb --empty-password
openssl pkcs12 -export -passout pass: -out localhost.p12 -inkey localhost.key \
-in localhost.crt -name localhost
pk12util -i localhost.p12 -d sql:nssdb -W ''

Finally, start the server with support for TLSv1.0 and later protocols, DHE ciphers and with the above certificate:

selfserv -d sql:./nssdb -p 4433 -V tls1.0: -H 1 -n localhost

Advanced configuration

More advanced and complex configurations as well as description how to compile the above servers from source is available on the wiki page Server setup.


See the document for description how to set up your development environment, sanity check the changes and requirements the changes need to follow.

You may also want to read the to learn more about the planned scope of the project.

Contributors are expected to follow the project's CODE OF CONDUCT when interacting with other members of the community.