02. Platform Security (SEC)

Introduction

According to the principle of least privilege, code should not be granted more privileges than those required for performing the particular task. This means that sections of code that require elevated privileges should be kept to a minimum. McGraw and Felten [[McGraw 2000]] enlist various goals of the principle of least privilege in the context of the Java programming language :

• [1] We want to grant each applet or application the minimum privileges it needs.

• [2] Rather than assigning a given applet's [or application's] entire collection of privileges to all of its classes, we want each class to get just what it needs.

• [3] We want a class's privileges to be "turned off" except for brief periods of time.

• [4] We even want to reduce the privileges of some of the built-in system classes.

[Our numbering.]

The ways in which these goals can be achieved are discussed below.

1. Applets rarely require elevated privileges. Sign only those applets that require elevated privileges; other applets should not be signed. (See guideline ENV00-J. Do not sign code that performs only unprivileged operations.) For applications, the security policy that defines the set of permissions should be as restrictive as possible. The default security policy file grants permissions sparingly, however, the flexible security model allows the user to grant additional permissions to applications by defining a custom security policy. Several guidelines deal with granting or limiting permissions:
   • ENV00-J. Do not sign code that performs only unprivileged operations
   • ENV03-J. Never grant AllPermission to untrusted code
   • ENV04-J. Do not grant ReflectPermission with target suppressAccessChecks
   • ENV05-J. Do not grant RuntimePermission with target createClassLoader
2. There are several ways of satisfying the second goal of privilege separation to enhance security. Applets or applications that need to be signed can coexist with unsigned classes in the same package (or JAR file). It is recommended that all privileged code be packaged together. (See guideline ENV01-J. Place all privileged code in a single package and seal the package for more information.) Furthermore, it is possible to grant privileges to code on the basis of the code base and/or its signer using a security policy.
3. The third goal can be realized using the AccessController mechanism. This mechanism allows only certain parts of code to acquire elevated privileges. When a class needs to assert its privileges, it executes the privileged code in a doPrivileged block. The AccessController mechanism works in conjunction with the security policy in effect. Because users may be unaware of the details of the security model and incapable of correctly configuring security policies tailored to their requirements, privileged code present within the doPrivileged blocks must be kept to a minimum to avoid security vulnerabilities.
4. The fourth goal can be achieved using a security manager to control the functions the trusted Java API can perform. (See guideline ENV02-J. Create a secure sandbox using a Security Manager.) When untrusted code should be disallowed from accessing system classes, it should be granted specific permissions to prevent it from accessing trusted classes in the specified packages. The accessClassInPackage permission provides the required functionality. (See guideline SEC12-J. Do not grant untrusted code access to classes in inaccessible packages.) Doing so does not limit what system classes can do; however, it restricts the range of system packages that can be used from less-privileged code.

Guidelines

SEC00-J. Avoid granting excess privileges

SEC01-J. Minimize the accessibility of classes and their members

SEC02-J. Guard doPrivileged blocks against untrusted invocations

SEC03-J. Do not allow tainted variables in doPrivileged blocks

SEC04-J. Do not expose standard APIs that may bypass Security Manager checks to untrusted code

SEC05-J. Do not expose standard APIs that use the immediate caller's class loader instance to untrusted code

SEC06-J. Do not use APIs that perform access checks against the immediate caller

SEC07-J. Classes that derive from a sensitive class or implement a sensitive interface should be declared to be final

SEC08-J. Enforce security checks in code that performs sensitive operations

SEC09-J. Do not base security checks on untrusted sources

SEC10-J. Define custom security permissions for fine grained security

SEC11-J. Call the superclass's getPermissions method when writing a custom class loader

SEC12-J. Do not grant untrusted code access to classes in inaccessible packages

SEC13-J. Do not allow unauthorized construction of classes in inaccessible packages

SEC14-J. Provide sensitive mutable classes with unmodifiable wrappers

SEC15-J. Prefer using SSLSockets over Sockets for secure data exchange

SEC16-J. Sign and seal sensitive objects before transit

SEC17-J. Create and sign a SignedObject before creating a SealedObject

SEC18-J. Define wrappers around native methods

SEC19-J. Do not rely on the default automatic signature verification provided by URLClassLoader and java.util.jar

SEC20-J. Do not expect java.lang.reflect.method.invoke() to behave as the immediate caller

Risk Assessment Summary

ENV10-J. Do not disable bytecode verification      The CERT Oracle Secure Coding Standard for Java      SEC00-J. Avoid granting excess privileges