fix: validate path parameters and prevent traversal/injection in REST transcoder

gapic-common/lib/gapic/rest/grpc_transcoder.rb

@@ -111,19 +111,61 @@ def transcode request
       # @return [Hash{String, String}]
       #   Name to value hash of the variables for the uri template expansion.
       #   The values are percent-escaped with slashes potentially preserved.
+      # @raise [Gapic::Common::Error] If any parameter value fails path traversal or injection validation.
       def bind_uri_values! http_binding, request_hash
         http_binding.field_bindings.to_h do |field_binding|
           field_path_camel = field_binding.field_path.split(".").map { |part| camel_name_for part }.join(".")
           field_value = extract_scalar_value! request_hash, field_path_camel, field_binding.regex
 
           if field_value
+            validate_field_binding! field_binding, field_value
             field_value = field_value.split("/").map { |segment| percent_escape segment }.join("/")
           end
 
           [field_binding.field_path, field_value]
         end
       end
 
+      # Validates a user-supplied parameter value bound to a standard (*) or path (**) URI template variable
+      # to prevent directory traversal and parameter injection exploits.
+      #
+      # @param field_binding [HttpBinding::FieldBinding] The field binding template metadata.
+      # @param field_value [String] The parameter value to validate.
+      # @raise [Gapic::Common::Error] If validation fails.
+      def validate_field_binding! field_binding, field_value
+        validate_path_binding! field_binding, field_value
+      end
+
+      # Validates standard (*) and path (**) parameters by ensuring that no segment in the parameter
+      # value is a directory traversal segment (. or ..).
+      #
+      # Validation Mechanism:
+      # 1. URL-decodes the parameter value to ensure all encoded dot (`%2e` / `%2E`)
+      #    and slash (`%2f` / `%2F`) segments are expanded.
+      # 2. Splits the decoded parameter value by slash (`/`) using `-1` limit to preserve all segments.
+      # 3. Checks each segment. If any segment matches `.` or `..`, it immediately raises
+      #    a `Gapic::Common::Error`, aborting the request.
+      # 4. Empty segments (e.g. duplicate slashes `//` or trailing slashes `/`) are allowed
+      #    by this linter and passed to the server, which handles normalization or returns 400.
+      #
+      # @param field_binding [HttpBinding::FieldBinding] The field binding template metadata.
+      # @param field_value [String] The parameter value to validate.
+      # @raise [Gapic::Common::Error] If validation fails.
+      def validate_path_binding! field_binding, field_value
+        unescaped_value = CGI.unescape field_value
+        segments = unescaped_value.split("/", -1)
+        segments.each do |segment|
+          next unless segment == "." || segment == ".."
+          if field_binding.preserve_slashes
+            raise ::Gapic::Common::Error,
+                  "Value for #{field_binding.field_path} must not contain segments that are exactly . or .."
+          else
+            raise ::Gapic::Common::Error,
+                  "Invalid value #{segment} for #{field_binding.field_path}"
+          end
+        end
+      end
+
       # Percent-escapes a string.
       # @param str [String] String to escape.
       # @return [String] Escaped string.

gapic-common/test/gapic/rest/grpc_transcoder_test.rb

@@ -299,6 +299,136 @@ def test_last_one_wins
     assert_transcoding_matches transcoder, test_cases
   end
 
+  def test_transcode_validation_parameter_injection
+    # 1. Parameter Injection (Rejection check)
+    # Proto: post: "/v3/{name=projects/*/locations/*/agents/*/sessions/*}:detectIntent"
+    # Template: v3/{name}:detectIntent (representing Dialogflow session method)
+    transcoder_inj = Gapic::Rest::GrpcTranscoder.new.with_bindings(
+      uri_method: :post,
+      uri_template: "/v3/{name}:detectIntent",
+      matches: [["name", %r{^projects/[^/]+/locations/[^/]+/agents/[^/]+/sessions/[^/]+$}, false]]
+    )
+
+    # Valid payload should pass
+    transcoder_inj.transcode example_request(name: "projects/p/locations/l/agents/a/sessions/s1")
+
+    # Payload with query injection should succeed and escape the ? character
+    _uri_method, uri, _query_params, _body =
+      transcoder_inj.transcode example_request(name: "projects/p/locations/l/agents/a/sessions/s1?key=val")
+    assert_equal "/v3/projects/p/locations/l/agents/a/sessions/s1%3Fkey%3Dval:detectIntent", uri
+
+    # Payload with fragment injection should succeed and escape the # character
+    _uri_method, uri, _query_params, _body =
+      transcoder_inj.transcode example_request(name: "projects/p/locations/l/agents/a/sessions/s1#frag")
+    assert_equal "/v3/projects/p/locations/l/agents/a/sessions/s1%23frag:detectIntent", uri
+  end
+
+  def test_transcode_validation_standard_wildcard
+    # 2. Standard Single-Wildcard Matchers (*)
+    # Proto: delete: "/v3/projects/{name}/webhooks/{sub_request.name}"
+    # Template: v3/projects/{name}/webhooks/{sub_request.name}
+    transcoder_std = Gapic::Rest::GrpcTranscoder.new.with_bindings(
+      uri_method: :delete,
+      uri_template: "/v3/projects/{name}/webhooks/{sub_request.name}",
+      matches: [
+        ["name", %r{^[^/]+$}, false],
+        ["sub_request.name", %r{^[^/]+$}, false]
+      ]
+    )
+
+    # Valid payload should pass
+    transcoder_std.transcode example_request(name: "p1", sub_name: "w1")
+
+    # Traversal segment '..' in standard parameter should fail
+    err = assert_raises ::Gapic::Common::Error do
+      transcoder_std.transcode example_request(name: "p1", sub_name: "..")
+    end
+    assert_equal "Invalid value .. for sub_request.name", err.message
+
+    # Traversal segment '.' in standard parameter should fail
+    err = assert_raises ::Gapic::Common::Error do
+      transcoder_std.transcode example_request(name: "p1", sub_name: ".")
+    end
+    assert_equal "Invalid value . for sub_request.name", err.message
+
+    # URL-encoded traversal segment '%2e%2e' in standard parameter should fail
+    err = assert_raises ::Gapic::Common::Error do
+      transcoder_std.transcode example_request(name: "p1", sub_name: "%2e%2e")
+    end
+    assert_equal "Invalid value .. for sub_request.name", err.message
+
+    # URL-encoded traversal segment '%2e' in standard parameter should fail
+    err = assert_raises ::Gapic::Common::Error do
+      transcoder_std.transcode example_request(name: "p1", sub_name: "%2e")
+    end
+    assert_equal "Invalid value . for sub_request.name", err.message
+
+    # Slashes in standard parameter should fail matching (regex rejects slashes)
+    err = assert_raises ::Gapic::Common::Error do
+      transcoder_std.transcode example_request(name: "p1", sub_name: "w1/w2")
+    end
+    assert err.message.include?("does not match any transcoding template")
+  end
+
+  def test_transcode_validation_path_wildcard
+    # 3. Path/Double-Wildcard Matchers (**)
+    # Proto: post: "/v1/{name=projects/*/databases/*/documents/*/**}/{sub_request.name}"
+    # Template: v1/{name}/{sub_request.name}
+    transcoder_wild = Gapic::Rest::GrpcTranscoder.new.with_bindings(
+      uri_method: :post,
+      uri_template: "/v1/{name}/{sub_request.name}",
+      matches: [
+        ["name", %r{^projects/[^/]+/databases/[^/]+/documents/[^/]+(?:/(?<__wildcard__>.*))?$}, true],
+        ["sub_request.name", %r{^[^/]+$}, false]
+      ]
+    )
+
+    # Valid path should pass
+    transcoder_wild.transcode example_request(name: "projects/p/databases/d/documents/doc/a/b/c", sub_name: "col")
+
+    # Segment '..' anywhere in parameter should fail
+    err = assert_raises ::Gapic::Common::Error do
+      transcoder_wild.transcode example_request(name: "projects/p/databases/d/documents/doc/../../../../doc2", sub_name: "col")
+    end
+    assert_equal "Value for name must not contain segments that are exactly . or ..", err.message
+
+    # Segment '.' anywhere in parameter should fail
+    err = assert_raises ::Gapic::Common::Error do
+      transcoder_wild.transcode example_request(name: "projects/p/databases/d/documents/doc/./a", sub_name: "col")
+    end
+    assert_equal "Value for name must not contain segments that are exactly . or ..", err.message
+
+    # Prefix traversal in the parameter should fail
+    err = assert_raises ::Gapic::Common::Error do
+      transcoder_wild.transcode example_request(name: "projects/p/databases/../documents/doc/a/b", sub_name: "col")
+    end
+    assert_equal "Value for name must not contain segments that are exactly . or ..", err.message
+
+    # URL-encoded segment '%2e%2e' in path parameter should fail
+    err = assert_raises ::Gapic::Common::Error do
+      transcoder_wild.transcode example_request(name: "projects/p/databases/d/documents/doc/%2e%2e/doc2", sub_name: "col")
+    end
+    assert_equal "Value for name must not contain segments that are exactly . or ..", err.message
+
+    # URL-encoded segment '%2e' in path parameter should fail
+    err = assert_raises ::Gapic::Common::Error do
+      transcoder_wild.transcode example_request(name: "projects/p/databases/d/documents/doc/%2e/a", sub_name: "col")
+    end
+    assert_equal "Value for name must not contain segments that are exactly . or ..", err.message
+
+    # URL-encoded traversal slashes '..%2f..%2f' in path parameter should fail
+    err = assert_raises ::Gapic::Common::Error do
+      transcoder_wild.transcode example_request(name: "projects/p/databases/d/documents/doc/..%2f..%2fescape-db", sub_name: "col")
+    end
+    assert_equal "Value for name must not contain segments that are exactly . or ..", err.message
+
+    # Mixed URL-encoded dots and slashes '%2e%2e%2f%2e%2e%2f' in path parameter should fail
+    err = assert_raises ::Gapic::Common::Error do
+      transcoder_wild.transcode example_request(name: "projects/p/databases/d/documents/doc/%2e%2e%2f%2e%2e%2fescape-db", sub_name: "col")
+    end
+    assert_equal "Value for name must not contain segments that are exactly . or ..", err.message
+  end
+
   private
 
   def assert_transcoding_matches transcoder, test_cases