SSL TLS interception: SSLsplit
SSLsplit is a tool for man-in-the-middle attacks against SSL/TLS encrypted network connections. Connections are transparently intercepted through a network address translation engine and redirected to SSLsplit. SSLsplit terminates SSL/TLS and initiates a new SSL/TLS connection to the original destination address, while logging all data transmitted. SSLsplit is intended to be useful for network forensics and penetration testing.
Transparent and scalable SSL TLS interception
SSLsplit works quite similar to other transparent SSL proxy tools: It acts as a middle man between the client and the actual server. Provided that traffic is being redirected to the server on which SSLsplit is running, SSLsplit picks up SSL connections and pretends to be the server the client is connecting to. To do so, it dynamically generates a certificate and signs it with a the private key of a CA certificate that the client must trust.
If, for example, a client wants to send an e-mail using the secure Gmail SMTP server (smtp.gmail.com on port 465), SSLsplit creates a certificate for “smtp.gmail.com” and thereby pretends to be the Gmail mail server towards the client. In the upstream direction (towards the actual Gmail mail server), SSLsplit connects to the server just like a normal client — forwarding all the traffic the actual client writes on the SSL socket.
So in order for attack to be successful attacker has to be placed somewhere in between the victim machine and the server. This can be done in many different ways:
- Use ARP spoofing to redirect the traffic of the victim by publishing false mappings from the standard gateway MAC address to the attacker’s IP address. You do not need physical access to the victim’s device to do that. Check out the arpspoof tool.
- Change the default gateway address in the victim’s network settings. This is the easiest method if you have access to the victim’s device.
- Forging DNS entries with a DNS server that returns the attacker’s IP address for certain (or all) domains.
- Redirect traffic for individual domains by modifying entries in the /etc/hosts file of the victim’s machine.
The easiest way is to change the default gateway address of your victim’s device to the attacker’s IP address. That makes sure that all the traffic goes through your machine.
- SSLsplit supports plain TCP, plain SSL, HTTP and HTTPS connections over both IPv4 and IPv6.
- For SSL and HTTPS connections, SSLsplit generates and signs forged X509v3 certificates on-the-fly, based on the original server certificate subject DN and subjectAltName extension.
- SSLsplit fully supports Server Name Indication (SNI) and is able to work with RSA, DSA and ECDSA keys and DHE and ECDHE cipher suites.
- SSLsplit can also use existing certificates of which the private key is available, instead of generating forged ones.
- SSLsplit supports NULL-prefix CN certificates and can deny OCSP requests in a generic way.
- SSLsplit removes HPKP response headers in order to prevent public key pinning.
SSLsplit depends on the OpenSSL and libevent 2.x libraries. The build depends on GNU make and a POSIX.2 environment in
PATH. 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 (experimental)
SSLsplit is or will be available as a package or port on the following systems:
- Arch Linux AUR:
To install from source:
make make test # optional unit tests make install # optional install
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% sslsplit -h Usage: sslsplit [options...] [proxyspecs...] -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) -t certdir use cert+chain+key PEM files from certdir to target all sites matching the common names (non-matching: generate if CA) -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) -g pemfile use DH group params from pemfile (default: keyfiles or auto) -G curve use ECDH named curve (default: secp160r2 for non-RSA leafkey) -Z disable SSL/TLS compression on all connections -s ciphers use the given OpenSSL cipher suite spec (default: ALL:-aNULL) -e engine specify default NAT engine to use (default: ipfw) -E list available NAT engines and exit -u user drop privileges to user (default if run as root: nobody) -j jaildir chroot() to jaildir (default if run as root: /var/empty) -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) -S logdir content log: full data to separate files in dir (excludes -L) -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 0.0.0.0 8080 www.roe.ch 80 # http/4; static hostname dst https ::1 8443 2001:db8::1 443 # https/6; static address dst https 127.0.0.1 9443 sni 443 # https/4; SNI DNS lookups tcp 127.0.0.1 10025 # tcp/4; default NAT engine ssl 2001:db8::2 9999 pf # ssl/6; NAT engine 'pf' Example: sslsplit -k ca.key -c ca.pem -P https 127.0.0.1 8443 https ::1 8443
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ALL GLORY TO THE HYPNOTOAD !
If you are not already inside the network perimeter it can be problematic to position yourself between your target and the server. In those cases most penetration testers will first try to use wireless connection and drop a rough dhcp server into the network environment. Keep in mind that SSLsplit is a network proxy and if your network speed is not blazing fast, the target will most definitely notice a network delay - [ie] jumping from 200ms to 2s