httpTokenCacheKey uses path.Base to extract the expected HTTP-01 token value to lookup in the DirCache implementation. On Window path.Base acts differently to filepath.Base since Windows uses a different path separator (\ vs. /), allowing a user to provide a relative path, i.e. .well-known/acme-challenge/..\..\asd becomes ..\..\asd. The extracted path is then suffixed with +http-01, joined with the cache directory, and opened.

Since the controlled path is suffixed with +http-01 before opening, the impact of this is significantly limited, since it only allows reading arbitrary files on the system if and only if they have this suffix.

Thanks to Juho Nurminen of Mattermost for reporting this.

This is CVE-2022-30636.