Depending on configuration, this happens to every new created socket (after the socket filter is installed) or only to specific sockets (using Apple’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. 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 system’s “pf” 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:Įssentially 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 Apple’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 it’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 Snitch’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 – just a “Fixed a rare issue that could cause a kernel panic”. 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. Little Snitch was among the first software packages I tried to reverse and crack when I started using Macs. The application (version 4) received a positive 4.5/5 review from Macworld.Shut up snitch! – reverse engineering and exploiting a critical Little Snitch vulnerability Little Snitch's integral network monitor shows ongoing traffic in real time with domain names and traffic direction displayed. The dialog also allows the user to restrict the parameters of the connection, restricting it to a specific port, protocol, or domain. For that, a dialog is presented to the user, which allows one to deny or permit the connection on a one-time, time limited, or permanent basis. If an application or process attempts to establish a network connection, Little Snitch prevents the connection, if a rule for that connection has been set by the user. Until Little Snitch 4, it controlled network traffic by registering kernel extensions through the standard application programming interface (API) provided by Apple, but for its 5th release it switched to using Apple's Network Extensions due to the deprecation of Kernel Extensions on macOS Catalina. Unlike a stateful firewall, which is designed primarily to protect a system from external attacks by restricting inbound traffic, Little Snitch is designed to protect privacy by limiting outbound traffic. It is produced and maintained by the Austrian firm Objective Development Software GmbH. It can be used to monitor applications, preventing or permitting them to connect to attached networks through advanced rules. Little Snitch is a host-based application firewall for macOS.
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