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The reason for this is that filters are tied to a specific domain, type, and protocol, meaning that every possible socket configuration must be specified if we want to filter every possible type of network connection (which Little Snitch is designed to do).ДЇor example, in AppleвЂ™s tcplognke sample code we can find two filters, one for IPv4 and another for IPv6, and a specific socket type as shown in the code comments: There is a loop (not visible in this disassembly) installing many filters. It then configures all the available callbacks, and finally registers the filter. Little Snitch is interested in filtering every new socket created after it is installed using the SFLT_GLOBAL option. Using the Little Snitch kernel extensionвЂ™s import table to locate the sflt_register() function, we can easily find out what kind of functionality it implements by looking at the installed callbacks. The sf_connect_in and sf_connect_out callbacks allow us to filter the creation of incoming and outgoing connections. The sf_data_in and sf_data_out callbacks are triggered on incoming and outgoing data, allowing us to filter data in transit. This cookie will be available to all the subsequent callbacks (the first argument to all callbacks that have access to it). In this callback we can create a cookie to store user-defined data related to the socket, for example the process PID that created the socket. Depending on configuration, this happens to every new created socket (after the socket filter is installed) or only to specific sockets (using AppleвЂ™s custom SO_NKE socket option). The sf_attach callback will execute when the filter is attached to a socket. The first argument to this function is a structure where we configure the callbacks we want.

REMOVE LITTLE SNITCH INSTALL

To install a new socket filter, we call the sflt_register() function in the associated kernel extension. Both are less interesting for an application like Little Snitch and filtering at those levels is probably better achieved with the operating systemвЂ™s вЂњpfвЂќ firewall. Two other filters are available, IP and Interface, which allow filtering traffic at the IP and interface levels. Parent-process information is available making it very easy to implement, for example, an OSI-layer-7 sniffer application, or an application firewall like Little Snitch. The following diagram from this document describes its implementation in the networking stack:Д®ssentially these filters allow us to access information about incoming and outgoing network connections and make a decision to allow/block the connection.

A complete description and implementation guide to socket filters is in AppleвЂ™s Network Kernel Extensions Programming Guide. The OS X feature that makes Little Snitch possible is called socket filters. It is widely popular: I personally make sure itвЂ™s the first thing I install when configuring new OS X images. It is a super-useful addition to OS X because you directly observe and control the network traffic on your Mac, expected and unexpected. Little Snitch is an application firewall able to detect applications that try to connect to the Internet or other networks, and then prompt the user to decide if they want to allow or block those connection attempts. You are reading this because the answer is yes! What is Little Snitch? (Hopefully Little SnitchвЂ™s developers will revise this policy and be more clear about the vulnerabilities they address, so users can better understand their threat posture.) Are there any more interesting security issues remaining in version 3.6.3 (current at the time of research) for us to find? Little Snitch version 3.6.2, released in January 2016, fixes a kernel heap overflow vulnerability despite not being mentioned in the release notes вЂ“ just a вЂњFixed a rare issue that could cause a kernel panicвЂќ. The upcoming DEF CON presentation on Little Snitch re-sparked my curiosity last week and it was finally time to give the firewall a closer look. In the past I reported some weaknesses related to their licensing scheme but I never audited their kernel code since I am not a fan of I-O Kit reversing.

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Little Snitch was among the first software packages I tried to reverse and crack when I started using Macs. Shut up snitch! вЂ“ reverse engineering and exploiting a critical Little Snitch vulnerability

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