Serve sstate-cache with basic auth

I'm trying to serve a Yocto cache to the internet and I want to password-protect the share with basic authentication.

This works:

$ curl http://foo:bla@localhost:8081/build/old-sstate-cache/ # OK!
$ curl -u foo:bla http://localhost:8081/build/old-sstate-cache/ # OK!

However, when building Yocto, it doesn't. Here is my local.conf:

SSTATE_MIRRORS ?= "file://.* http://localhost:8081/build/old-sstate-cache/PATH;user=foo:bla"

Looking at the server logs it says 401 (Unauthorized).

Is the syntax for basic auth in the url correct for building?