validateRealmURL() in pkg/v1/remote/transport/bearer.go checks IsLoopback, IsPrivate, IsLinkLocalUnicast, and IsLinkLocalMulticast but not IsUnspecified.

0.0.0.0 and :: pass all four checks, but on Linux and macOS they resolve to 127.0.0.1. A malicious container registry can redirect auth requests to the victim's localhost by returning:

WWW-Authenticate: Bearer realm="https://0.0.0.0:9999/token",service="registry"

The client then sends Basic auth credentials to localhost:9999 — credential theft.

Confirmed with a PoC:

• net.ParseIP("0.0.0.0").IsLoopback() = false
• net.ParseIP("0.0.0.0").IsPrivate() = false
• net.ParseIP("0.0.0.0").IsUnspecified() = true (not checked)
• net.Dial("tcp", "0.0.0.0:PORT") connects to 127.0.0.1

I'll submit a PR adding ip.IsUnspecified() to the existing check.