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github: WithAuthToken leaks bearer token on cross-host redirects #4190

@evilgensec

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@evilgensec

WithAuthToken wraps the Transport with a RoundTripper that injects
Authorization: Bearer TOKEN on every RoundTrip call. When http.Client
follows a redirect to a different host, Go strips the Authorization
header — but the wrapped Transport immediately re-adds it, so the token
is forwarded to the redirect destination.

The GitHub API redirects several endpoints cross-host:

  • Repository archive downloads redirect to codeload.github.com
  • Release asset downloads redirect to objects.githubusercontent.com

Any application using WithAuthToken that follows such redirects, or that
accepts user-provided URLs, will leak its GitHub token to the redirect
destination.

Fix tracked in #4189.

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