Audit F3: validateURL checked scheme/host/userinfo but never resolved the
host, so an operator-supplied webhook_url could point at the canary's own
Redis (redis:6379), Postgres, link-local IMDS (169.254.169.254), or any
RFC1918 host on the docker network or VPS subnet — a classic confused-deputy
SSRF triggered by self-triggering a token after creation.
validateURL now resolves the hostname (or parses a literal IP) and rejects
loopback, RFC1918, CGNAT, link-local, multicast, unspecified, IMDS, and
IPv6 unique-local. The default HTTP client also installs a DialContext that
re-checks the dialed IP for defense-in-depth against DNS rebinding. The
Config gains an AllowPrivateHosts flag (default false) that test code opts
into when targeting httptest.NewServer.
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