SSLsplit - transparent SSL/TLS interception
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.
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
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.
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
CHECK_BASE to the respective prefixes.
See the manual pages
sslsplit.conf(5) for user
NEWS.md for release notes listing significant
changes between releases and
SECURITY.md for information on
security vulnerability disclosure.
SSLsplit is provided under a 2-clause BSD license.
SSLsplit contains components licensed under the MIT and APSL licenses.
LICENSE.third as well as the respective source file headers
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.