SSLsplit - transparent SSL/TLS interception
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.