url.JoinPath resolves path-traversal segments like '..' and '.',
which silently drops the OIDC subject from the identifier. For
example, Iss='https://example.com' with Sub='..' produces
'https://example.com' — the subject is lost entirely. This causes
distinct OIDC users to receive colliding identifiers.
Replace url.JoinPath with simple string concatenation using a slash
separator. This preserves the subject literally regardless of its
content. url.PathEscape does not help because dots are valid URL
path characters and are not escaped.
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