chore: make plugin library path traversal check cross-platform

Signed-off-by: Leonardo Grasso <me@leonardograsso.com>

CMakeLists.txt

@@ -1,6 +1,6 @@
 # SPDX-License-Identifier: Apache-2.0
 #
-# Copyright (C) 2023 The Falco Authors.
+# Copyright (C) 2026 The Falco Authors.
 #
 # Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except
 # in compliance with the License. You may obtain a copy of the License at
@@ -134,10 +134,17 @@ if(NOT DEFINED FALCO_COMPONENT_NAME)
 endif()
 
 if(CMAKE_INSTALL_PREFIX_INITIALIZED_TO_DEFAULT)
-	set(CMAKE_INSTALL_PREFIX
-		/usr
-		CACHE PATH "Default install path" FORCE
-	)
+	if(WIN32)
+		set(CMAKE_INSTALL_PREFIX
+			"C:/Program Files/${CMAKE_PROJECT_NAME}"
+			CACHE PATH "Default install path" FORCE
+		)
+	else()
+		set(CMAKE_INSTALL_PREFIX
+			/usr
+			CACHE PATH "Default install path" FORCE
+		)
+	endif()
 endif()
 
 set(CMD_MAKE make)

unit_tests/falco/test_configuration_schema.cpp

@@ -1,6 +1,6 @@
 // SPDX-License-Identifier: Apache-2.0
 /*
-Copyright (C) 2023 The Falco Authors.
+Copyright (C) 2026 The Falco Authors.
 
 Licensed under the Apache License, Version 2.0 (the "License");
 you may not use this file except in compliance with the License.
@@ -146,6 +146,57 @@ plugins:
 	EXPECT_EQ(falco_config.m_plugins[0].m_init_config, "");
 }
 
+TEST(Configuration, plugin_library_path_traversal) {
+	falco_configuration falco_config;
+	config_loaded_res res;
+
+	// A relative path that stays within the plugins dir should succeed.
+	std::string config = R"(
+plugins:
+  - name: myplugin
+    library_path: libmyplugin.so
+)";
+	EXPECT_NO_THROW(res = falco_config.init_from_content(config, {}));
+	EXPECT_VALIDATION_STATUS(res, yaml_helper::validation_ok);
+
+	// A relative path with ".." that escapes the plugins dir must be rejected.
+	config = R"(
+plugins:
+  - name: evil
+    library_path: ../../tmp/evil.so
+)";
+	EXPECT_THROW(falco_config.init_from_content(config, {}), std::exception);
+
+	// Traversal via "./" prefix followed by ".." must also be rejected.
+	config = R"(
+plugins:
+  - name: evil
+    library_path: ./../../tmp/evil.so
+)";
+	EXPECT_THROW(falco_config.init_from_content(config, {}), std::exception);
+
+	// Nested traversal that descends then escapes must be rejected.
+	config = R"(
+plugins:
+  - name: evil
+    library_path: subdir/../../../tmp/evil.so
+)";
+	EXPECT_THROW(falco_config.init_from_content(config, {}), std::exception);
+
+#ifndef _WIN32
+	// Absolute paths bypass the prefix logic and are allowed as-is.
+	// This test uses a Unix absolute path syntax.
+	config = R"(
+plugins:
+  - name: myplugin
+    library_path: /opt/falco/plugins/libmyplugin.so
+)";
+	EXPECT_NO_THROW(res = falco_config.init_from_content(config, {}));
+	EXPECT_VALIDATION_STATUS(res, yaml_helper::validation_ok);
+	EXPECT_EQ(falco_config.m_plugins[0].m_library_path, "/opt/falco/plugins/libmyplugin.so");
+#endif
+}
+
 TEST(Configuration, schema_yaml_helper_validator) {
 	yaml_helper conf;
 	falco_configuration falco_config;

userspace/falco/configuration.h

@@ -1,6 +1,6 @@
 // SPDX-License-Identifier: Apache-2.0
 /*
-Copyright (C) 2025 The Falco Authors.
+Copyright (C) 2026 The Falco Authors.
 
 Licensed under the Apache License, Version 2.0 (the "License");
 you may not use this file except in compliance with the License.
@@ -31,6 +31,7 @@ limitations under the License.
 #include <set>
 #include <iostream>
 #include <fstream>
+#include <filesystem>
 
 #include "config_falco.h"
 #include "yaml_helper.h"
@@ -400,9 +401,35 @@ struct convert<falco_configuration::plugin_config> {
 			return false;
 		}
 		rhs.m_library_path = node["library_path"].as<std::string>();
-		if(!rhs.m_library_path.empty() && rhs.m_library_path.at(0) != '/') {
-			// prepend share dir if path is not absolute
-			rhs.m_library_path = std::string(FALCO_ENGINE_PLUGINS_DIR) + rhs.m_library_path;
+		if(!rhs.m_library_path.empty() &&
+		   !std::filesystem::path(rhs.m_library_path).is_absolute() &&
+		   rhs.m_library_path.at(0) != '/') {
+			// Relative path: resolve against the plugins directory
+			// and verify the result stays within it.
+			auto full_path = std::filesystem::path(FALCO_ENGINE_PLUGINS_DIR) / rhs.m_library_path;
+			// lexically_normal resolves . and .. purely lexically,
+			// without filesystem access (unlike weakly_canonical which
+			// leaves .. unresolved for non-existent path components).
+			auto normalized = full_path.lexically_normal();
+			auto plugins_dir = std::filesystem::path(FALCO_ENGINE_PLUGINS_DIR).lexically_normal();
+			auto rel = normalized.lexically_relative(plugins_dir);
+			if(rel.empty()) {
+				throw YAML::Exception(node["library_path"].Mark(),
+				                      "plugin library_path '" +
+				                              node["library_path"].as<std::string>() +
+				                              "' resolves outside the plugins directory (" +
+				                              std::string(FALCO_ENGINE_PLUGINS_DIR) + ")");
+			}
+			for(const auto& component : rel) {
+				if(component == "..") {
+					throw YAML::Exception(node["library_path"].Mark(),
+					                      "plugin library_path '" +
+					                              node["library_path"].as<std::string>() +
+					                              "' resolves outside the plugins directory (" +
+					                              std::string(FALCO_ENGINE_PLUGINS_DIR) + ")");
+				}
+			}
+			rhs.m_library_path = normalized.string();
 		}
 
 		if(node["init_config"] && !node["init_config"].IsNull()) {