)

* remote: validate foreign layer URLs to prevent SSRF

Foreign layer descriptors in OCI/Docker manifests may carry arbitrary
URLs in the descriptor's "urls" field.  When the registry blob endpoint
returns 404, the client fetches these URLs directly with no validation,
allowing a malicious registry to point them at internal services (e.g.
the cloud instance-metadata endpoint 169.254.169.254).

Add validateForeignURL, which applies the same scheme and private-IP
checks as transport.validateRealmURL to every foreign layer URL before
making a network request.  HTTP is only permitted when the registry
itself was reached over HTTP (insecure=true).  DNS-based SSRF remains
out of scope, consistent with the design decision in validateRealmURL.

Fixes #2259.

* remote: block SSRF redirect bypass for foreign layer URLs

The previous fix validates each foreign layer URL with validateForeignURL
before adding it to the fetch list.  However, http.Client follows
redirects by default: an attacker can host a foreign layer URL on a
public domain that passes the initial check, then redirect the client to
http://169.254.169.254/... (AWS/GCP instance-metadata), leaking cloud
credentials.

Add fetchForeignBlobURL (on *fetcher) that reuses the existing transport
but sets a CheckRedirect hook calling validateForeignURL on every redirect
hop.  Compressed() now routes foreign layer fetches through this method
instead of the shared fetchBlobURL so that the validation happens at both
the initial URL and each redirect destination.

New tests:
- TestPullingForeignLayerSSRFViaRedirect: attacker httptest server issues
  a 302 to a loopback victim; confirms Compressed() returns an error
  before any credentials are returned.

* remote: move foreign layer SSRF helpers to fetcher.go per review

Move validateForeignURL and fetchForeignBlobURL (plus their tests) next
to the fetcher receiver, trim verbose function comments, and rework
Compressed() to call fetchBlobURL on the single registry URL with
foreign URLs collected and iterated separately.

* remote: rename unused params to satisfy revive linter

… (google#2293)

* remote: validate foreign layer URLs to prevent SSRF

Foreign layer descriptors in OCI/Docker manifests may carry arbitrary
URLs in the descriptor's "urls" field.  When the registry blob endpoint
returns 404, the client fetches these URLs directly with no validation,
allowing a malicious registry to point them at internal services (e.g.
the cloud instance-metadata endpoint 169.254.169.254).

Add validateForeignURL, which applies the same scheme and private-IP
checks as transport.validateRealmURL to every foreign layer URL before
making a network request.  HTTP is only permitted when the registry
itself was reached over HTTP (insecure=true).  DNS-based SSRF remains
out of scope, consistent with the design decision in validateRealmURL.

Fixes google#2259.

* remote: block SSRF redirect bypass for foreign layer URLs

The previous fix validates each foreign layer URL with validateForeignURL
before adding it to the fetch list.  However, http.Client follows
redirects by default: an attacker can host a foreign layer URL on a
public domain that passes the initial check, then redirect the client to
http://169.254.169.254/... (AWS/GCP instance-metadata), leaking cloud
credentials.

Add fetchForeignBlobURL (on *fetcher) that reuses the existing transport
but sets a CheckRedirect hook calling validateForeignURL on every redirect
hop.  Compressed() now routes foreign layer fetches through this method
instead of the shared fetchBlobURL so that the validation happens at both
the initial URL and each redirect destination.

New tests:
- TestPullingForeignLayerSSRFViaRedirect: attacker httptest server issues
  a 302 to a loopback victim; confirms Compressed() returns an error
  before any credentials are returned.

* remote: move foreign layer SSRF helpers to fetcher.go per review

Move validateForeignURL and fetchForeignBlobURL (plus their tests) next
to the fetcher receiver, trim verbose function comments, and rework
Compressed() to call fetchBlobURL on the single registry URL with
foreign URLs collected and iterated separately.

* remote: rename unused params to satisfy revive linter