Chrome100 - Downgrade your Chrome OS

Chrome100 is a website which enables you to download old versions of Chrome OS for your Chromebook.

Old versions may have vulnerabilities which are now patched; thus, these old Chrome OS images are essential for many exploits!

Update: It is now better to use https://chromiumdash.appspot.com/serving-builds?deviceCategory=ChromeOS To get older versions.

After you download your image of choice you need to flash it with a USB using this extension:
Chromebook Recovery Utility