A double-eviction in the Incognito mode cache that lead to a user-after-free in cache in Google Chrome prior to 66.0.3359.139 allowed a remote attacker who had compromised the renderer process to execute arbitrary code via a crafted HTML page.
The product reuses or references memory after it has been freed. At some point afterward, the memory may be allocated again and saved in another pointer, while the original pointer references a location somewhere within the new allocation. Any operations using the original pointer are no longer valid because the memory "belongs" to the code that operates on the new pointer.