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SSLsplit - transparent SSL/TLS interception

https://www.roe.ch/SSLsplit

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Overview

SSLsplit is a tool for man-in-the-middle attacks against SSL/TLS encrypted network connections. It is intended to be useful for network forensics, application security analysis and penetration testing.

SSLsplit is designed to transparently terminate connections that are redirected to it using a network address translation engine. SSLsplit then terminates SSL/TLS and initiates a new SSL/TLS connection to the original destination address, while logging all data transmitted. Besides NAT based operation, SSLsplit also supports static destinations and using the server name indicated by SNI as upstream destination. SSLsplit is purely a transparent proxy and cannot act as a HTTP or SOCKS proxy configured in a browser.

SSLsplit supports plain TCP, plain SSL, HTTP and HTTPS connections over both IPv4 and IPv6. It also has the ability to dynamically upgrade plain TCP to SSL in order to generically support SMTP STARTTLS and similar upgrade mechanisms. SSLsplit fully supports Server Name Indication (SNI) and is able to work with RSA, DSA and ECDSA keys and DHE and ECDHE cipher suites. Depending on the version of OpenSSL built against, SSLsplit supports SSL 3.0, TLS 1.0, TLS 1.1 and TLS 1.2, and optionally SSL 2.0 as well.

For SSL and HTTPS connections, SSLsplit generates and signs forged X509v3 certificates on-the-fly, mimicking the original server certificate's subject DN, subjectAltName extension and other characteristics. SSLsplit has the ability to use existing certificates of which the private key is available, instead of generating forged ones. SSLsplit supports NULL-prefix CN certificates but otherwise does not implement exploits against specific certificate verification vulnerabilities in SSL/TLS stacks.

SSLsplit implements a number of defences against mechanisms which would normally prevent MitM attacks or make them more difficult. SSLsplit can deny OCSP requests in a generic way. For HTTP and HTTPS connections, SSLsplit mangles headers to prevent server-instructed public key pinning (HPKP), avoid strict transport security restrictions (HSTS), avoid Certificate Transparency enforcement (Expect-CT) and prevent switching to QUIC/SPDY, HTTP/2 or WebSockets (Upgrade, Alternate Protocols). HTTP compression, encodings and keep-alive are disabled to make the logs more readable.

Logging options include traditional SSLsplit connect and content log files as well as PCAP files and mirroring decrypted traffic to a network interface. Additionally, certificates, master secrets and local process information can be logged.

See the manual page sslsplit(1) for details on using SSLsplit and setting up the various NAT engines.

Requirements

SSLsplit depends on the OpenSSL, libevent 2.x, libpcap and libnet 1.1.x libraries by default; libpcap and libnet are not needed if the mirroring feature is omitted. The build depends on GNU make and a POSIX.2 environment in PATH. If available, pkg-config is used to locate and configure the dependencies. The optional unit tests depend on the check library.

SSLsplit currently supports the following operating systems and NAT mechanisms:

  • FreeBSD: pf rdr and divert-to, ipfw fwd, ipfilter rdr
  • OpenBSD: pf rdr-to and divert-to
  • Linux: netfilter REDIRECT and TPROXY
  • Mac OS X: pf rdr and ipfw fwd

Support for local process information (-i) is currently available on Mac OS X and FreeBSD.

SSL/TLS features and compatibility greatly depend on the version of OpenSSL linked against. For optimal results, use a recent release of OpenSSL or LibreSSL.

Installation

With the requirements above available, run:

make
make test       # optional unit tests
make sudotest   # optional unit tests requiring privileges
make install    # optional install

Dependencies are autoconfigured using pkg-config. If dependencies are not picked up and fixing PKG_CONFIG_PATH does not help, you can specify their respective locations manually by setting OPENSSL_BASE, LIBEVENT_BASE, LIBPCAP_BASE, LIBNET_BASE and/or CHECK_BASE to the respective prefixes.

You can override the default install prefix (/usr/local) by setting PREFIX. For more build options and build-time defaults see GNUmakefile and defaults.h.

Documentation

See the manual pages sslsplit(1) and sslsplit.conf(5) for user documentation. See NEWS.md for release notes listing significant changes between releases and SECURITY.md for information on security vulnerability disclosure.

License

SSLsplit is provided under a 2-clause BSD license. SSLsplit contains components licensed under the MIT and APSL licenses. See LICENSE, LICENSE.contrib and LICENSE.third as well as the respective source file headers for details.

Credits

See AUTHORS.md for the list of contributors.

SSLsplit was inspired by mitm-ssl by Claes M. Nyberg and sslsniff by Moxie Marlinspike, but shares no source code with them.