30th August 2019

SSLsplit - transparent SSL/TLS interception

Copyright 2009–2019 Daniel Roethlisberger and contributors. All rights reserved.
Licensed under a two-clause BSD license.


Latest release: sslsplit-0.5.5.tar.bz2 (.asc)
Documentation: sslsplit(1)sslsplit.conf(5)README.mdNEWS.md
Development: droe/sslsplitgitter — follow @droethlisberger for news


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.


% sslsplit -h
Usage: sslsplit [-D] [-f conffile] [-o opt=val] [options...] [proxyspecs...]
  -f conffile use conffile to load configuration from
  -o opt=val  override conffile option opt with value val
  -c pemfile  use CA cert (and key) from pemfile to sign forged certs
  -k pemfile  use CA key (and cert) from pemfile to sign forged certs
  -C pemfile  use CA chain from pemfile (intermediate and root CA certs)
  -K pemfile  use key from pemfile for leaf certs (default: generate)
  -q crlurl   use URL as CRL distribution point for all forged certs
  -t certdir  use cert+chain+key PEM files from certdir to target all sites
              matching the common names (non-matching: -T or generate if CA)
  -A pemfile  use cert+chain+key PEM file as fallback leaf cert when none of
              those given by -t match, instead of generating one on the fly
  -w gendir   write leaf key and only generated certificates to gendir
  -W gendir   write leaf key and all certificates to gendir
  -O          deny all OCSP requests on all proxyspecs
  -P          passthrough SSL connections if they cannot be split because of
              client cert auth or no matching cert and no CA (default: drop)
  -a pemfile  use cert from pemfile when destination requests client certs
  -b pemfile  use key from pemfile when destination requests client certs
  -g pemfile  use DH group params from pemfile (default: keyfiles or auto)
  -G curve    use ECDH named curve (default: prime256v1)
  -Z          disable SSL/TLS compression on all connections
  -r proto    only support one of ssl3 tls10 tls11 tls12 (default: all)
  -R proto    disable one of ssl3 tls10 tls11 tls12 (default: none)
  -s ciphers  use the given OpenSSL cipher suite spec (default: ALL:-aNULL)
  -x engine   load OpenSSL engine with the given identifier
  -e engine   specify default NAT engine to use (default: pf)
  -E          list available NAT engines and exit
  -u user     drop privileges to user (default if run as root: nobody)
  -m group    when using -u, override group (default: primary group of user)
  -j jaildir  chroot() to jaildir (impacts sni proxyspecs, see manual page)
  -p pidfile  write pid to pidfile (default: no pid file)
  -l logfile  connect log: log one line summary per connection to logfile
  -L logfile  content log: full data to file or named pipe (excludes -S/-F)
  -S logdir   content log: full data to separate files in dir (excludes -L/-F)
  -F pathspec content log: full data to sep files with % subst (excl. -L/-S):
              %T - initial connection time as an ISO 8601 UTC timestamp
              %d - destination host and port
              %D - destination host
              %p - destination port
              %s - source host and port
              %S - source host
              %q - source port
              %x - base name of local process        (requires -i)
              %X - full path to local process        (requires -i)
              %u - user name or id of local process  (requires -i)
              %g - group name or id of local process (requires -i)
              %% - literal '%'
      e.g.    "/var/log/sslsplit/%X/%u-%s-%d-%T.log"
  -X pcapfile pcap log: packets to pcapfile (excludes -Y/-y)
  -Y pcapdir  pcap log: packets to separate files in dir (excludes -X/-y)
  -y pathspec pcap log: packets to sep files with % subst (excl. -X/-Y):
              see option -F for pathspec format
  -I if       mirror packets to interface
  -T addr     mirror packets to target address (used with -I)
  -M logfile  log master keys to logfile in SSLKEYLOGFILE format
  -i          look up local process owning each connection for logging
  -d          daemon mode: run in background, log error messages to syslog
  -D          debug mode: run in foreground, log debug messages on stderr
  -V          print version information and exit
  -h          print usage information and exit
  proxyspec = type listenaddr+port [natengine|targetaddr+port|"sni"+port]
      e.g.    http 8080 www.roe.ch 80  # http/4; static hostname dst
              https ::1 8443 2001:db8::1 443   # https/6; static address dst
              https 9443 sni 443     # https/4; SNI DNS lookups
              tcp 10025              # tcp/4; default NAT engine
              ssl 2001:db8::2 9999 pf          # ssl/6; NAT engine 'pf'
              autossl ::1 10025                # autossl/6; STARTTLS et al
  sslsplit -k ca.key -c ca.pem -P  https 8443  https ::1 8443

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 engines:

  • 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: ipfw fwd and pf rdr

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.


SSLsplit is available as a package or port on the following systems, with varying up-to-dateness:

To install from source:

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.


SSLsplit is being developed on Github. For bug reports, please use the Github issue tracker. For patch submissions, please send me pull requests.


Build Status

New releases are announced on my Twitter feed @droethlisberger and by pushing release tags to Github.

Further Reading and Success Stories

These links are provided in the hope that they may be useful, but without implying endorsement of any kind.

Foreign language original content:

Videos featuring SSLsplit:

SSLsplit is also mentioned in the following books:

  • «Bulletproof SSL and TLS» by Ivan Ristić, the very recommended modern book on SSL/TLS; if you have an early release under the category “SSL Stripping” instead of “MITM Certificate”, which Ivan later fixed in the digital version of the book
  • «Web Penetration Testing with Kali Linux» by Juned Ahmed Ansari

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