Code Security

Gordon College

Stephen Brinton

Virtual Machine Security• Building a fence around your code

– JVM – Java Virtual Machine• Originally developed by Sun Microsystems• Executes Java Bytecode

– Execution by: Interpretation (JVM) or Compilation (JIT)

– Common Language Runtime • Developed by Microsoft• Executes Common Intermediate Language (CIL)

– Verified and then run as native code on machine

Java’s Security• Strongly Typed Compiler

– Eliminate programming bugs– Help enforce language semantics

• Bytecode verifier– Makes sure the rules of Java are followed in the compiled

code

• Classloader– Finds, loads, and defines classes (runs verifier on them)

• Security Manager– Main interface between the system and the Java Code

Trusted vs Untrusted Code• Trusted

– Java API code – Code loaded from the classpath– resides outside the “sandbox”

• Untrusted– Code loaded from outside the classpath (usually from a

network)– Confined to the “sandbox”

• Java Apps (by default) live outside the sandbox and Java Applets are confined within the sandbox

What’s a “Sandbox”?

• Applets run inside a “sandbox”– If you download code, it has to play within

the JVM (Sandbox)– SecurityManager is

called for certain methods, and can

forbid access

• JDK1.1 introduced the notions of code-signing and “trusted applets”

SandboxesHow do you protect your computer bad code?

• The solution: Make untrusted code play within a sandbox.

• Need for varying security policies increases– You assign “permissions” to pieces of code– JDK 1.1 (digital signatures) – if user trusts the

digitally signed code – users could allow normally untrusted code to access resources

• Enforcement Mechanism for Policy (sandbox)?– Security Managers

Java SandboxThe sandbox for untrusted Java applets, for

example, prohibits many activities, including:

• Reading or writing to the local disk• Making a network connection to any host, except

the host from which the applet came• Creating a new process• Loading a new dynamic library and directly calling

a native method

Java Sandbox• The fundamental components responsible for

Java's sandbox are:

* Safety features built into the Java virtual machine (and the language)

* The class loader architecture

* The class file verifier

* The security manager and the Java API

Creating a security manager in JDK 1.1 (that allows reading files, but disallows writing files)

public class MySecurityManager extends java.lang.SecurityManager {

public void checkRead(String file) throws SecurityException {// reading is allowed, so just return

return;}

public void checkWrite(String file) throws SecurityException {// writing is not allowed, so throw the exception

throw new SecurityException("Writing is not allowed");}

} // end MySecurityManager

Creating a security manager in JDK 1.1 (that allows reading files, but only with extension “txt”)

public class MySecurityManager2 extends java.lang.SecurityManager {

public void checkRead(String file) throws SecurityException {

//check the file extension to see if it ends in ".txt"

int index=file.lastIndexOf('.');String result=file.substring(index, file.length());if(result.equalsIgnoreCase(".txt")){return;}else{throw new SecurityException("Cannot read file: "+file);}}

}

Security Manager prior to JDK 1.2

Only way security was controlled prior to JDK 1.2

Advantages:– Easy to provide a binary security model (yes, you can or no, you can't) – Methods in the SecurityManager class are called by the Java

API; there's no need for you to call the code at all – Interface of this system is constant across all JVM platforms;

one security manager can run everywhere – Additional size of a simple security manager is negligible – Security manager & class loader work hand-in-hand to

ensure neither is compromised by accident or an act of evil

Security Manager prior to JDK 1.2

Disadvantages:– Control of security is in the developer's hands, not

in a security specialist's hands – Not easy to provide a customizable security model

that varies from user to user – Only way to change an existing policy is to change

or subclass the existing security manager; not all users have the capability of programming in Java

– New security policies (non-system resource policies, for example) are difficult to implement

Install a SecurityManager

• Applications don’t start, by default, with a SecurityManager

• You must install one, either from within the code, or

using a command line argument

java -Djava.security.manager

Building a bigger and better sandbox

• To provide greater control to user and developer -– Traditional sandbox (java.security

package) was expanded to include:• AccessController class

– Muscle of the security manager – enforces policy

• Permission class• Policy class

AccessController

java -new -usepolicy FileApp test.txt

The Policytool

Policy Tool Main Window

The Policytool

Policy Tool Edit Entry Window

The Policytool

Policy Tool Add Grant Entry Window

.NET Framework

• Different administrative software model

• Fixed location (traditional)

• Dynamic nature of software (present)– Dynamic downloads and execution– Remote execution– Security is essential

Security Models

• Role-based security– Users have access to resources based on roles– Model used by most operating systems

• Code access security (new with .Net)– Also called “evidence-based security”– Even if user is trusted - the code may not be.– Tackles the problem with mobile code

Both models are found in .Net framework

Code Access Securitymechanism of the CLR

– Manages code and depends on level of trust– CLR – will lessen and tighten its grip based on

permission and trust level• Very similar to the sandbox view

– Two aspects:• Control the access level given to an application

(assembly)• Control access to a particular resource (like a database)

.Net execution• Runtime framework

– Runs both managed and unmanaged code• Managed - under control of runtime

– Has access to certain features: memory management, JIT, and security services

– MSIL (MS intermediate language)– Can be compiled to native code prior to execution

• Unmanaged - compiled for a certain system– Can not directly use the runtime

MSIL : object-oriented assembly language for an abstract, stack-based machine

CILcommon intermediate language

C#

public double GetVolume()

{

double volume = height*width*thickness;

if(volume<0) return 0;

return volume;

}

.method public hidebysig instance float64 GetVolume() cil managed { // Code size 51 (0x33) .maxstack 2 .locals init ([0] float64 volume, [1] float64 CS$00000003$00000000) IL_0000: ldarg.0 IL_0001: ldfld float64 OOP.Aperture::height IL_0006: ldarg.0 IL_0007: ldfld float64 OOP.Aperture::width IL_000c: mul IL_000d: ldarg.0 IL_000e: ldfld float64 OOP.Aperture::thickness IL_0013: mul IL_0014: stloc.0 IL_0015: ldloc.0 IL_0016: ldc.r8 0.0 IL_001f: bge.un.s IL_002d IL_0021: ldc.r8 0.0 IL_002a: stloc.1 IL_002b: br.s IL_0031 IL_002d: ldloc.0 IL_002e: stloc.1 IL_002f: br.s IL_0031 IL_0031: ldloc.1 IL_0032: ret } // end of method Aperture::GetVolumeobject-oriented

assembly language for an abstract, stack-based machine

CLRcommon language runtime

CLRcommon language runtime

3 different runtimes1. A single CLR that runs all ASP.NET apps

2. Browsers (IE) uses a single CLR that executes all dowloaded controls

3. CLR used to execute commands run from the OS shell.

Verification

• 2 Forms of verification done at runtime– MSIL - verified

• Invalid - JIT compiler cannot make native exe• Valid - can be made into native code• Type safe - interacts with types through

exposed contracts• Verifiable - can be proved to be type-safe

– Assembly metadata - validated• Metadata - describes aspects of the code file

Verification

• Integrated with the compiler

• If code is trustedskip verification and compile

otherwise

MSIL verification

assembly metadata verification

if successful verification - compile

Code Access Security

• Assigns permissions to assemblies based on assembly evidence

• Evidence identifies code: location, etc.

• Evidence is attached to assembly when loaded for execution

• Evidence limits what program can doOpt-in approach

Permissions• Authority to perform protected

operations– Accessing files, registry, network, GUI,

execution environment, skip verification

• Assembly load time– Evidence -> grant permissions

Evidence

• Zone

• URL

• Hash (encrypted value)

• Strong name - unique ID for program

• Site

• Application Directory

• Publisher certificate

Evidence<System.Security.Policy.Zone version="1"> <Zone>MyComputer</Zone></System.Security.Policy.Zone><System.Security.Policy.Url version="1"> <Url> file:///C:/winnt/microsoft.net/framework/v1.0.2728/mscorlib.dll </Url></System.Security.Policy.Url><StrongName version="1" Key="00000000000000000400000000000000" Name="mscorlib" Version="1.0.2411.0"/><System.Security.Policy.Hash version="1"> <RawData>4D5A90000300000004000000FFFF0000B8000000000000... 0000000000000000000000000000000000000000000000000000 </RawData></System.Security.Policy.Hash>

Evidence Based Security

• The loader discovers evidence of the origin of the code– Evidence is info about the assembly– Used to determine the permissions granted to an assembly– Evidence is the input to the security policy

• Publisher Authenticode signer• Strong Name public key+name+version• Site Web site of code origin• URL URL of code origin• Zone zone of code origin

– Extensible for new kinds of evidence (custom)

Security Policy

Mapping {AssemblyEvidence}

{PermissionsGranted}

Control by administratorPolicy levels: * Enterprise Policy Level * Machine Policy Level * User Policy Level * Application Domain Policy Level

Security Policy

Determining the set of granted permissions:

1. Policy levels evaluate the evidence and generate a set of permissions.

2. Permission sets calculated for each policy level are intersected with each other.

3. Resulting permission set is compared with the set of permissions the assembly declared necessary to run.

Security Policy

.NET Framework• Code groups

– Bring together code with similar characteristics• Evidence

– Information used to place code into code groups– where code is from (internet or intranet), publisher, strong name,

URI from download, etc.

• Arranged in a hierarchy

• Permissions– Actions you allow each code group to perform

• For example: “able to access the user interface”

– Managed by system admin. at the enterprise, machine and user levels

.NET Framework

.NET Framework

Condition: All code, Permission Set: Nothing Condition: Zone: Internet, Permission Set: Internet Condition: URL: www.monash.edu.au, Permission Set: MonashPSet Condition: Strong Name: m-Commerce, Permission Set: m-CommercePSet

Microsoft Management Console snap-in

Condition: All code, Permission Set: Nothing Condition: Zone: Internet, Permission Set: Internet Condition: URL: www.monash.edu.au, Permission Set: MonashPSet Condition: Strong Name: m-Commerce, Permission Set: m-CommercePSet

Permission Sets• FullTrust: Allows unrestricted access to system resources.• SkipVerification: Allows an assembly to skip verification.• Execution: Allows code to execute.• Nothing: No permissions. Not granting the permission to

execute effectively stops code from running• Internet: Appropriate for code coming from the Internet.

(Limited) Code will not receive access to the file system or registry, but can do limited user interface actions as well as use the safe file system called Isolated Storage.

Predefined Psets - can be accessed within code.

Stack Walk

• When are the permissions generated by the code-access security module checked?– To determine whether code is authorized to access a

resource or perform an operation, the runtime's security system walks the call stack, comparing the granted permissions of each caller to the permission being demanded.

a protected resource may demand a stack walk

Stack Walk

• When are the permissions generated by the code-access security module checked?– To determine whether code is authorized to access a

resource or perform an operation, the runtime's security system walks the call stack, comparing the granted permissions of each caller to the permission being demanded.

Security Policy Topology

• Multiple policy levels of administration– Enterprise: policy distributed across organization– Machine: policy for all users of a machine– User: policy specific to logged on user

• Effective policy is the intersection of levels

Enterprise policy

Machine1 policy Machine2 policy

UserA

UserB

UserC

UserD

Evaluating Policy Per Level

• Each Policy Level contains a set of code groups– Code groups are arranged in a tree– Every code group has a membership condition and a set of

granted permissions– An assembly is mapped to one of more code groups based

on the evidence that the assembly provides

ALL CODE? None

Internet? Intranet? Local?P1 P2 P3

YES

NO NOYES

Completing Policy Resolve

• Matching code groups evaluated– Union of matching permission sets per level, this is

the permissions allowed by this level– Intersection of Policy Levels produces the final

ALLOWED permission set for the assembly• ALLOWED = Enterprise Machine User

Policy Tools

• caspol.exe

• Console Management snap-in– .NET Framework 2.0 Configuration

Example

caspol.exe –ldlook at the code groups on a machine

caspol.exe –listgroupslook at the code groups on a machine (more compact)

caspol.exe –resolvegroup simpleSecure.exeview an assembly’s code groupings (effective permission – intersection)

caspol.exe –resolveperm simpleSecure.exeview an assembly’s permissions (each code groups brings additional permissions)

.NET Framework 2.0 Configuration

Detail Information

“Building a bigger sandbox”http://www.javaworld.com/javaworld/jw-08-1998/jw-08-sandbox-p2.html

“Security in the .NET Framework” http://msdn2.microsoft.com/en-us/library/fkytk30f(vs.80).aspx

“Java vs. .NET Security”http://www.onjava.com/pub/a/onjava/2003/11/26/javavsdotnet.html