Version: 0.3.1

Policy Reference

jGuard uses a declarative policy language to define what code is permitted to do. Policies are defined in module-info.jguard files and compiled to binary format for runtime enforcement.

Core Concepts

Capabilities, Not Permissions

Unlike traditional permission systems, jGuard uses capabilities:

Permission: "Does this code have permission X?"
Capability: "This code can do X, Y, and Z"

Capabilities are explicit grants. If a capability isn't granted, the operation is denied.

Deny by Default

jGuard operates on a deny-by-default model:

✗ Not entitled → BLOCKED
✓ Entitled     → ALLOWED

This is the safest default posture. You must explicitly grant every capability your code needs.

Modules as Principals

jGuard uses JPMS module identity as the security boundary:

Each module has its own policy
Packages within a module can have different entitlements
Module identity is verified through signed JARs in production

Policy File Format

Policies are defined in module-info.jguard:

security module com.example.myapp {
    entitle <subject> to <capability>;
    entitle <subject> to <capability>;
    ...
}

Subject Patterns

Pattern	Matches
`module`	Entire module
`com.example`	Exact package
`com.example.*`	Direct subpackages only
`com.example..`	Package and all descendants

Example Policy

security module com.example.webapp {
    // Module-wide system property access
    entitle module to system.property.read;

    // Package-specific network access
    entitle com.example.webapp.http.. to network.outbound;

    // Server can bind ports
    entitle com.example.webapp.server to network.listen("8080-8443");

    // Workers can create threads
    entitle com.example.webapp.worker.. to threads.create;
}

Supported Capabilities

jGuard v0.3.0 supports 14 capabilities across 9 categories.

Filesystem

fs.read(root, glob)

Read files matching a glob pattern under a root directory.

entitle module to fs.read("config", "**");        // Any file under config/
entitle module to fs.read("/data", "*.json");     // JSON files in /data
entitle module to fs.read(".", "*.txt");          // Text files in current dir

Instrumented APIs:

java.nio.file.Files: newInputStream, newBufferedReader, readAllBytes, readAllLines, readString, lines, list, walk, find
java.io.FileInputStream, java.io.FileReader
java.io.RandomAccessFile (read mode)
java.nio.channels.FileChannel.open()

fs.write(root, glob)

Write files matching a glob pattern under a root directory.

entitle module to fs.write("build", "**");        // Any file under build/
entitle module to fs.write("logs", "*.log");      // Log files only

Instrumented APIs:

java.nio.file.Files: newOutputStream, newBufferedWriter, write, writeString, copy, move, createFile, createDirectory, createDirectories, delete, deleteIfExists
java.io.FileOutputStream, java.io.FileWriter

fs.hardlink(root, glob)

Create hard links to files under a root directory.

entitle module to fs.hardlink("data/indices", "**");  // Allow hard links in indices

Security Note

Hard links can bypass filesystem boundaries by creating alternative paths to files. Only grant this capability when truly necessary.

Instrumented APIs:

java.nio.file.Files.createLink()

Network

network.outbound(hostPattern?, portSpec?)

Make outbound network connections.

// Any host, any port
entitle module to network.outbound;

// Host pattern, any port
entitle module to network.outbound("*.example.com");

// Any host, specific port
entitle module to network.outbound("*", 443);

// Host pattern + port range
entitle module to network.outbound("*.example.com", "80-443");

Host Patterns:

Pattern	Matches	Example
`*`	Exactly one DNS segment	`*.example.com` matches `api.example.com`
`**`	One or more DNS segments	`**.example.com` matches `a.b.c.example.com`

Host matching is case-insensitive with IDN normalization.

Port Specs:

Format	Example	Description
Integer	`443`	Single port
String range	`"80-443"`	Inclusive port range

Instrumented APIs:

java.net.Socket: constructors, connect()
java.nio.channels.SocketChannel.connect()

network.listen(portSpec?)

Bind server sockets to listen for connections.

entitle module to network.listen;              // Any port
entitle module to network.listen(8080);        // Specific port
entitle module to network.listen("8080-8090"); // Port range

Instrumented APIs:

java.net.ServerSocket: constructors, bind()
java.nio.channels.ServerSocketChannel.bind()

Concurrency

threads.create

Create new threads.

entitle com.example.app.worker.. to threads.create;

Instrumented APIs:

java.lang.Thread.start()

Native Code

native.load(pattern?)

Load native libraries via JNI.

entitle module to native.load;           // Any library
entitle module to native.load("mylib*"); // Library name pattern

Instrumented APIs:

java.lang.System: loadLibrary(), load()
java.lang.Runtime: loadLibrary(), load()

Environment

env.read(pattern?)

Read environment variables.

entitle module to env.read;          // Any env var (including bulk access)
entitle module to env.read("HOME");  // Specific variable
entitle module to env.read("APP_*"); // Pattern match

Bulk API Gating

System.getenv() with no arguments requires the no-arg form or explicit * pattern.

Instrumented APIs:

java.lang.System: getenv(), getenv(String)

System Properties

system.property.read(pattern?)

Read system properties.

entitle module to system.property.read;              // Any property
entitle module to system.property.read("java.home"); // Specific property
entitle module to system.property.read("app.**");    // All descendants
entitle module to system.property.read("app.*");     // Direct children only

Bulk API Gating

System.getProperties() requires the no-arg form or explicit * pattern.

Instrumented APIs:

java.lang.System: getProperty(String), getProperty(String, String), getProperties()

system.property.write(pattern?)

Write system properties.

entitle module to system.property.write;              // Any property
entitle module to system.property.write("app.config"); // Specific property
entitle module to system.property.write("app.**");    // All descendants

Bulk API Gating

System.setProperties() requires the no-arg form or explicit * pattern.

Instrumented APIs:

java.lang.System: setProperty(), setProperties(), clearProperty()

Process Execution

process.exec(pattern?)

Execute external processes.

entitle module to process.exec;                    // Any process
entitle module to process.exec("/usr/bin/java");   // Specific command
entitle module to process.exec("/opt/app/bin/*");  // Pattern match

Security Warning

Process execution is one of the most dangerous capabilities. Only grant to trusted code that absolutely requires it.

Instrumented APIs:

java.lang.Runtime: exec(String), exec(String[])
java.lang.ProcessBuilder.start()

Cryptography

crypto.provider

Modify JCE cryptographic providers.

entitle com.example.security.. to crypto.provider;

This capability guards installation and removal of JCE security providers, preventing rogue crypto implementations.

Instrumented APIs:

java.security.Security: addProvider(), insertProviderAt(), removeProvider(), setProperty()

Runtime Lifecycle

runtime.exit

Terminate the JVM. Prevents rogue libraries from calling System.exit() and killing your application.

entitle com.example.main to runtime.exit;

Server Uptime

Without this restriction, any library (like embedded databases or ML frameworks) could terminate your server, destroying uptime SLAs.

Instrumented APIs:

java.lang.System.exit(int)
java.lang.Runtime: exit(int), halt(int)

runtime.shutdown_hook

entitle com.example.lifecycle.. to runtime.shutdown_hook;

Instrumented APIs:

java.lang.Runtime: addShutdownHook(Thread), removeShutdownHook(Thread)

Capability Summary Table

Capability	Arguments	Example
`fs.read`	root, glob	`fs.read("config", "*/.json")`
`fs.write`	root, glob	`fs.write("logs", "*.log")`
`fs.hardlink`	root, glob	`fs.hardlink("data", "**")`
`network.outbound`	host?, port?	`network.outbound("*.example.com", 443)`
`network.listen`	port?	`network.listen("8080-8090")`
`threads.create`	none	`threads.create`
`native.load`	pattern?	`native.load("mylib*")`
`env.read`	pattern?	`env.read("HOME")`
`system.property.read`	pattern?	`system.property.read("java.*")`
`system.property.write`	pattern?	`system.property.write("app.**")`
`process.exec`	pattern?	`process.exec("/usr/bin/*")`
`crypto.provider`	none	`crypto.provider`
`runtime.exit`	none	`runtime.exit`
`runtime.shutdown_hook`	none	`runtime.shutdown_hook`

Enforcement Modes

Mode	Behavior	Use Case
STRICT (default)	Block denied access, block on errors	Production - fail secure
PERMISSIVE	Block denied access, allow on errors	Migration - allow recovery
AUDIT	Log only, never block	Policy development

Override at runtime:

./gradlew runWithAgent -Pjguard.mode=audit

Or via system property:

java -Djguard.mode=audit -javaagent:jguard-agent.jar -jar app.jar

Policy Lifecycle

┌─────────────────┐
│ module-info     │
│   .jguard       │ ← Source (human-readable)
└────────┬────────┘
         │ jguardc / Gradle plugin
         ▼
┌─────────────────┐
│  policy.bin     │ ← Binary (runtime)
└────────┬────────┘
         │ Embedded in JAR
         ▼
┌─────────────────┐
│ META-INF/jguard │
│   /policy.bin   │ ← Shipped with application
└─────────────────┘

Next Steps

External Policies - Grant/deny semantics for third-party code
Getting Started - Installation and quickstart
Use Cases - Real-world examples

Core Concepts​

Capabilities, Not Permissions​

Deny by Default​

Modules as Principals​

Policy File Format​

Subject Patterns​

Example Policy​

Supported Capabilities​

Filesystem​

fs.read(root, glob)​

fs.write(root, glob)​

fs.hardlink(root, glob)​

Network​

network.outbound(hostPattern?, portSpec?)​

network.listen(portSpec?)​

Concurrency​

threads.create​

Native Code​

native.load(pattern?)​

Environment​

env.read(pattern?)​

System Properties​

system.property.read(pattern?)​

system.property.write(pattern?)​

Process Execution​

process.exec(pattern?)​

Cryptography​

crypto.provider​

Runtime Lifecycle​

runtime.exit​

runtime.shutdown_hook​

Capability Summary Table​

Enforcement Modes​

Policy Lifecycle​

Next Steps​

Core Concepts

Capabilities, Not Permissions

Deny by Default

Modules as Principals

Policy File Format

Subject Patterns

Example Policy

Supported Capabilities

Filesystem

fs.read(root, glob)

fs.write(root, glob)

fs.hardlink(root, glob)

Network

network.outbound(hostPattern?, portSpec?)

network.listen(portSpec?)

Concurrency

threads.create

Native Code

native.load(pattern?)

Environment

env.read(pattern?)

System Properties

system.property.read(pattern?)

system.property.write(pattern?)

Process Execution

process.exec(pattern?)

Cryptography

crypto.provider

Runtime Lifecycle

runtime.exit

runtime.shutdown_hook

Capability Summary Table

Enforcement Modes

Policy Lifecycle

Next Steps