Veritasium recently released an excellent video about the almost-backdoor into OpenSSH in 2024. The attackers (Russia? China?) would have had a secret backdoor into the majority of servers worldwide. The Internet came within several weeks of this disaster.
The video provides lots of context, in addition to the almost-backdoor, including: history of the "open source" movement; background on Linux; remote access encryption & OpenSSH; and how compression works.
If you're already familiar with all that, you can skip 20 minutes of this hour long video. Start at 9:12. When it gets to about 12:00 (about encryption), skip ahead to 23:26 and watch the rest of the video.
If you want to learn more, start with "XZ Utils backdoor" on Wikipedia.