Prevent silent attestation downgrade on gem push

lib/rubygems/commands/push_command.rb

@@ -46,7 +46,8 @@ def initialize
     end
 
     add_option("--attestation FILE",
-                "Push with sigstore attestations") do |value, options|
+                "Push with sigstore attestations",
+                "  (FILE must be a JSON sigstore bundle)") do |value, options|
       options[:attestations] << value
     end
 
@@ -94,7 +95,7 @@ def send_gem(name)
   def send_push_request(name, args)
     # Always honor explicit --attestation option
     # Auto-attestation is only supported on rubygems.org with GitHub Actions (not JRuby)
-    if options[:attestations].any? || (RUBY_ENGINE != "jruby" && attestation_supported_host? && ENV["GITHUB_ACTIONS"])
+    if options[:attestations].any? || (RUBY_ENGINE != "jruby" && attestation_supported_host? && ENV["GITHUB_ACTIONS"] == "true")
       send_push_request_with_attestation(name, args)
     else
       send_push_request_without_attestation(name, args)
@@ -115,14 +116,23 @@ def send_push_request_without_attestation(name, args)
   def send_push_request_with_attestation(name, args)
     attestations = if options[:attestations].any?
       options[:attestations].map do |attestation|
-        Gem.read_binary(attestation)
+        load_attestation(attestation)
       end
     else
-      bundle_path = attest!(name)
+      # Only the opportunistic signing step falls back. The request below stays
+      # outside this rescue because once the server may have seen the attested
+      # push, a network error must not trigger an unattested retry.
       begin
-        [Gem.read_binary(bundle_path)]
-      ensure
-        File.unlink(bundle_path) if bundle_path && File.exist?(bundle_path)
+        [attest!(name)]
+      rescue StandardError => e
+        message = "Failed to create an attestation, pushing without one.\n"
+        message += if Gem.configuration.really_verbose
+          e.full_message
+        else
+          e.message
+        end
+        alert_warning message
+        return send_push_request_without_attestation(name, args)
       end
     end
     bundles = "[" + attestations.join(",") + "]"
@@ -134,38 +144,52 @@ def send_push_request_with_attestation(name, args)
       ], "multipart/form-data")
       request.add_field "Authorization", api_key
     end
-  rescue StandardError => e
-    message = "Failed to push with attestation, retrying without attestation.\n"
-    message += if Gem.configuration.really_verbose
-      e.full_message
-    else
-      e.message
+  end
+
+  def load_attestation(file)
+    data = begin
+      Gem.read_binary(file)
+    rescue SystemCallError, IOError, ArgumentError => e
+      raise Gem::Exception, "Failed to read attestation #{file}: #{e.message}"
+    end
+    validate_attestation_json(data, file)
+  end
+
+  def validate_attestation_json(data, source)
+    require "json"
+
+    parsed = begin
+      JSON.parse(data)
+    rescue JSON::ParserError => e
+      raise Gem::Exception, "Attestation #{source} is not valid JSON: #{e.message}"
     end
-    alert_warning message
-    send_push_request_without_attestation(name, args)
+    raise Gem::Exception, "Attestation #{source} is not a JSON object" unless parsed.is_a?(Hash)
+    data
   end
 
   def attest!(name)
     require "open3"
     require "shellwords"
     require "tempfile"
 
-    tempfile = Tempfile.new([File.basename(name, ".*"), ".sigstore.json"])
-    bundle = tempfile.path
-    tempfile.close(false)
-
     env = defined?(Bundler.unbundled_env) ? Bundler.unbundled_env : ENV.to_h
-    # Gem.ruby is quoted if it contains whitespace, so split it into argv
-    # elements to keep the quotes out of the spawned command.
-    out, st = Open3.capture2e(
-      env,
-      *Shellwords.split(Gem.ruby), "-S", "gem", "exec", "--conservative",
-      "sigstore-cli", "sign", name, "--bundle", bundle,
-      unsetenv_others: true
-    )
-    raise Gem::Exception, "Failed to sign gem:\n\n#{out}" unless st.success?
-
-    bundle
+
+    Tempfile.create([File.basename(name, ".*"), ".sigstore.json"]) do |tempfile|
+      tempfile.close
+      bundle = tempfile.path
+
+      # Gem.ruby is quoted if it contains whitespace, so split it into argv
+      # elements to keep the quotes out of the spawned command.
+      out, st = Open3.capture2e(
+        env,
+        *Shellwords.split(Gem.ruby), "-S", "gem", "exec", "--conservative",
+        "sigstore-cli", "sign", name, "--bundle", bundle,
+        unsetenv_others: true
+      )
+      raise Gem::Exception, "Failed to sign gem:\n\n#{out}" unless st.success?
+
+      validate_attestation_json(Gem.read_binary(bundle), "generated by sigstore-cli")
+    end
   end
 
   def get_hosts_for(name)

test/rubygems/helper.rb

@@ -388,6 +388,7 @@ def setup
     ENV["XDG_STATE_HOME"] = nil
     ENV["MAKEFLAGS"] = nil
     ENV["SOURCE_DATE_EPOCH"] = nil
+    ENV["GITHUB_ACTIONS"] = nil
     ENV["BUNDLER_VERSION"] = nil
     ENV["BUNDLE_CONFIG"] = nil
     ENV["BUNDLE_USER_CONFIG"] = nil