httpTokenCacheKey uses path.Base to extract the expected HTTP-01 token value to lookup in the DirCache implementation. On Windows, 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.