In last week's update to macOS 15.4, Apple replaced the rsync utility with an alternative implementation called openrsync, developed by the OpenBSD project. The executable file /usr/bin/rsync in macOS now points to openrsync. It is believed that the reason for the replacement was recently discovered security issues in rsync, efforts to reduce components under copyleft licenses, and the desire to eliminate the outdated version of rsync which cannot be updated due to licensing policy.
The key features of openrsync include its initial development focused on ensuring high security, the provision of code under a permissive ISC license (BSD family) instead of the copyleft GPL license used by rsync, and the use of a different internal architecture (a single process for receiving and sending data that uses an event processing loop). At the protocol level, openrsync is compatible with rsync but does not support all command-line options, only the most requested for file synchronization and backup. The lack of support for secondary functions helps keep the openrsync codebase size within 10,000 lines of code to reduce the attack surface. In comparison, the latest version of rsync has 62,000 lines in C.
Considering that macOS had been shipping with the outdated version rsync 2.6.9, released in November 2006, the replacement will not lead to any significant reduction in functionality. The version rsync 2.6.9 was chosen as the last one distributed under the GPLv2+ license. The next release was rsync 3.0, which switched to the GPLv3 license that prohibits tethering (binding software to hardware).
Source: opennet.ru
