An issuer identifier written with a terminating slash, like
`https://auth.example.com/`, makes authorization server metadata
discovery miss every endpoint. `GetAuthServerMetadata` probes three URLs
that no conformant server serves, finds nothing, and the caller falls
back to the guessed 2025-03-26 endpoints, which are themselves malformed
because they are built by appending to the same slash-terminated issuer.
`authorizationServerMetadataURLs` in `auth/shared.go` decides whether
the issuer has a path component with `baseURL.Path == ""` (line 47), and
a trailing-slash issuer has `Path == "/"`, so it takes the
path-insertion branch. That branch inserts
`strings.TrimLeft(originalPath, "/")` (lines 59 and 62), which strips
the leading slash but keeps the trailing one. For
`https://auth.example.com/` the probes come out as
`/.well-known/oauth-authorization-server/`,
`/.well-known/openid-configuration/` and
`//.well-known/openid-configuration`. For
`https://auth.example.com/tenant1/` the two path-insertion probes come
out with a trailing slash. Only the path-appending form was already
right, because it uses `strings.Trim`.
RFC 8414 section 3.1 settles it: "If the issuer identifier value
contains a path component, any terminating "/" MUST be removed before
inserting "/.well-known/" and the well-known URI suffix between the host
component and the path component." So `https://auth.example.com/` has no
path component and gets the two root forms, which is the list the MCP
authorization spec gives for issuers without a path component.
The change trims slashes off both ends of the issuer path once, uses
that to pick between the root and path-insertion branches, and reuses it
for all three URLs. An issuer without a terminating slash produces
exactly the same list as before.
`TestAuthorizationServerMetadataURLs` pins the URL list for all four
issuer shapes. `TestGetAuthServerMetadataTrailingSlashIssuer` drives the
whole lookup against the existing fake authorization server, serving
metadata at the root well-known path. On unmodified main the first fails
on both trailing-slash cases and the second fails with
`GetAuthServerMetadata() error = Get
"/.well-known/openid-configuration": redirect into loopback address
"127.0.0.1"`, because `//.well-known/openid-configuration` makes
`net/http.ServeMux` issue a canonicalizing redirect that the discovery
client's `CheckRedirect` refuses. Both pass with the change.
Verification on go1.26.4: `gofmt -l .` clean, `go vet ./...` clean,
`staticcheck ./...` clean at v0.6.1 (the version the Test workflow
pins), `go test ./auth/...` ok, `go test ./...` ok.
I did not file an issue first. The fix is self-contained, confined to
one unexported helper, and every issuer shape other than the
slash-terminated ones is byte-identical to before.
Co-authored-by: Guglielmo Colombo <guglielmoc@google.com>
5bc078a795