Block SSRF via backchannel_logout_uri

backchannel_logout_uri was validated only for scheme/HTTPS, so an admin (or a
compromised admin account) could point it at internal infrastructure — cloud
metadata (169.254.169.254), loopback, or RFC1918 hosts — and every user logout
would fire a server-side POST there.

Add PrivateAddressCheck (app/lib) and apply it as defense-in-depth:
- Application validation rejects URIs whose host is, or is a literal, internal
  address (loopback / private / link-local / 0.0.0.0 / localhost / metadata
  hostnames). Fast, DNS-free, immediate admin feedback.
- BackchannelLogoutJob re-checks at request time WITH DNS resolution and aborts
  (no retry) if the host resolves to a non-public address — covering URIs that
  predate the validation and public hostnames pointed at internal IPs.

Tests cover the address classification, the model validation, and updates an
existing test that used a localhost logout URI.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

app/jobs/backchannel_logout_job.rb

@@ -28,6 +28,14 @@ class BackchannelLogoutJob < ApplicationJob
     # Send HTTP POST to the application's backchannel logout URI
     uri = URI.parse(application.backchannel_logout_uri)
 
+    # SSRF guard: re-check at request time (with DNS resolution) in case the URI
+    # predates the validation, or a public hostname now resolves to an internal
+    # address. Abort without retrying — retries would not change the outcome.
+    if PrivateAddressCheck.internal_host?(uri.host) || PrivateAddressCheck.resolves_to_internal?(uri.host)
+      Rails.logger.error "BackchannelLogout: Refusing to send logout to #{application.name} - #{uri.host} is or resolves to a non-public address (SSRF guard)"
+      return
+    end
+
     begin
       response = Net::HTTP.start(uri.host, uri.port, use_ssl: uri.scheme == "https", open_timeout: 5, read_timeout: 5) do |http|
         request = Net::HTTP::Post.new(uri.path.presence || "/")

app/lib/private_address_check.rb

@@ -0,0 +1,57 @@
+require "ipaddr"
+require "resolv"
+
+# SSRF guard for outbound requests to admin-configured URLs (currently the OIDC
+# backchannel logout endpoint). Blocks hosts that are, or resolve to, private,
+# loopback, link-local (incl. the cloud metadata address 169.254.169.254) or
+# otherwise non-public address space.
+module PrivateAddressCheck
+  module_function
+
+  # Hostnames that are internal by definition and must never be dialled.
+  BLOCKED_HOSTNAMES = %w[localhost metadata.google.internal].freeze
+
+  # Fast, DNS-free check: catches IP literals and well-known internal hostnames.
+  # Suitable for model validation (deterministic, immediate admin feedback).
+  def internal_host?(host)
+    host = host.to_s.downcase
+    return true if host.blank?
+    return true if BLOCKED_HOSTNAMES.include?(host)
+    return true if host.end_with?(".localhost")
+
+    ip = parse_ip(host)
+    ip ? internal_ip?(ip) : false
+  end
+
+  # Authoritative check: resolves the hostname and blocks if ANY address is
+  # internal. Suitable for request time — also defeats a public hostname that
+  # has been pointed at an internal IP (DNS rebinding to internal space).
+  def resolves_to_internal?(host)
+    addresses(host).any? { |ip| internal_ip?(ip) }
+  end
+
+  def addresses(host)
+    ip = parse_ip(host)
+    return [ip] if ip
+
+    Resolv.getaddresses(host.to_s).filter_map { |a| parse_ip(a) }
+  rescue StandardError
+    # Resolution failure: surface no addresses. Callers treat "can't resolve" as
+    # not-provably-internal; the dial itself will then fail safely.
+    []
+  end
+
+  def internal_ip?(ip)
+    ip.loopback? || ip.private? || ip.link_local? || unspecified?(ip)
+  end
+
+  def parse_ip(str)
+    IPAddr.new(str.to_s)
+  rescue IPAddr::Error
+    nil
+  end
+
+  def unspecified?(ip)
+    ip == IPAddr.new("0.0.0.0") || ip == IPAddr.new("::")
+  end
+end

app/models/application.rb

@@ -56,6 +56,7 @@ class Application < ApplicationRecord
     message: "must be a valid HTTP or HTTPS URL"
   }
   validate :backchannel_logout_uri_must_be_https_in_production, if: -> { backchannel_logout_uri.present? }
+  validate :backchannel_logout_uri_not_internal, if: -> { backchannel_logout_uri.present? }
 
   # Icon validation using ActiveStorage validators
   validate :icon_validation
@@ -390,4 +391,17 @@ class Application < ApplicationRecord
       # Let the format validator handle invalid URIs
     end
   end
+
+  # SSRF guard: the backchannel logout URI is dialled server-side on every user
+  # logout, so it must not target internal infrastructure (loopback, private
+  # ranges, or the link-local cloud metadata endpoint). This is the fast,
+  # config-time check; BackchannelLogoutJob re-checks with DNS resolution.
+  def backchannel_logout_uri_not_internal
+    uri = URI.parse(backchannel_logout_uri)
+    if uri.host.present? && PrivateAddressCheck.internal_host?(uri.host)
+      errors.add(:backchannel_logout_uri, "must not point to a private, loopback, or link-local address")
+    end
+  rescue URI::InvalidURIError
+    # Let the format validator handle invalid URIs
+  end
 end