Applications of Programming Language

Theory:Java Security

David WalkerCOS 441

With slides stolen from:Steve ZdancewicUniversity of Pennsylvania

COS 441 2

Mobile Code Modern languages like Java and C#

have been designed for Internet applications and extensible systems

PDAs, Cell Phones, Smart Cards, …

operating system

web browser

applet applet applet

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Applet Security Problems Protect OS & other valuable resources. Applets should not:

crash browser or OS execute “rm –rf /” be able to exhaust resources

Applets should: be able to access some system resources

(e.g. to display a picture) be isolated from each other

Principles of least privilege and complete mediation apply

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Java and C# Security Static Type Systems (Bytecode Verification)

Memory safety and jump safety Enforces encapsulation boundaries (e.g. private

fields)

Run-time checks for Array index bounds Downcasts Access controls

Garbage Collected Eliminates memory management errors

Library support Cryptography, authentication, …

This lecture

Most of the

course

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Access Control for Applets What level of granularity?

Applets can touch some parts of the file system but not others

Applets can make network connections to some locations but not others

Different code has different levels of trustworthiness www.l33t-hax0rs.com vs. www.java.sun.com

Trusted code can call untrusted code e.g. to ask an applet to repaint its window

Untrusted code can call trusted code e.g. the paint routine may load a font

How is the access control policy specified?

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Outline Java Security Model (C# similar)

Stack inspection Concrete examples

Semantics from a PL perspective Formalizing stack inspection

how exactly does it work?

Reasoning about programs that use stack inspection

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Java Security Model

a.classb.classc.classd.classe.class

Domain A

Domain B

Permissions

Permissions

Security PolicyVM Runtime

http://java.sun.com/j2se/1.4.2/docs/guide/security/spec/security-specTOC.fm.html

ClassloaderSecurityManager

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Kinds of PermissionsPermissions are implemented by the

java.security.Permission class for which there are many subclasses

perm = new java.io.FilePermission("/tmp/abc","read");

java.security.AllPermission java.security.SecurityPermission java.security.UnresolvedPermission java.awt.AWTPermission java.io.FilePermission java.io.SerializablePermission java.lang.reflect.ReflectPermission java.lang.RuntimePermissionjava.net.NetPermission java.net.SocketPermission …

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Code Trustworthiness How does one decide what protection

domain the code is in? Source (e.g. local or applet) Digital signatures

How does one decide what permissions a protection domain has? Configurable – administrator file or

command line

Enforced by the classloader

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Classloaders In order to pull new code into the virtual machine,

we use an object from the ClassLoader class A class loader will look in the file system, or

across the network for a class file, or possibly dynamically generate the class When loading the first class of an application, a new

instance of the URLClassLoader is used. When loading the first class of an applet, a new instance

of the AppletClassLoader is used. Class loaders are responsible for placing classes

into their security domains AppletClassLoader places classes in domains depending

on where they are from Other ClassLoaders places classes in domains based on

digital signatures, or origin (such as local file system)

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Classloader Hierarchy

ClassLoader

SecureClassLoader URLClassLoader

AppletClassLoader

Primordial ClassLoade

r

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Associating Privileges with Domains

grant codeBase “http://www.l33t-hax0rz.com/*” { permission java.io.FilePermission(“/tmp/*”, “read,write”);}

grant codeBase “file://$JAVA_HOME/lib/ext/*” { permission java.security.AllPermission;}

grant signedBy “trusted-company.com” { permission java.net.SocketPermission(…); permission java.io.FilePermission(“/tmp/*”, “read,write”); …}

Policy information stored in: $JAVA_HOME/lib/security/java.policy $USER_HOME/.java.policy (or passed on command line)

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Summary so Far We’ve seen what privileges are and how

to assign them to fragments of code

Next: how does the system use privileges to enforce an access control policy?

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Example Trusted Code

void fileWrite(String filename, String s) { SecurityManager sm = System.getSecurityManager(); if (sm != null) { FilePermission fp = new FilePermission(filename,“write”); sm.checkPermission(fp); /* … write s to file filename (native code) … */ } else { throw new SecurityException(); }}

public static void main(…) { SecurityManager sm = System.getSecurityManager(); FilePermission fp = new FilePermission(“/tmp/*”,“write,…”); sm.enablePrivilege(fp); UntrustedApplet.run();}

Code in the System protection domain

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Example Client

class UntrustedApplet { void run() { ... s.FileWrite(“/tmp/foo.txt”, “Hello!”); ... s.FileWrite(“~dpw/grades.txt”, “Nick: A+”); ... }}

Applet code obtained from http://www.l33t-hax0rz.com/

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Stack Inspection Stack frames are annotated with their

protection domains and any enabled privileges.

During inspection, stack frames are searched from most to least recent: fail if a frame belonging to someone not

authorized for privilege is encountered succeed if activated privilege is found in

frame

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Stack Inspection Example

main(…){ fp = new FilePermission(“/tmp/*”,“write,…”); sm.enablePrivilege(fp); UntrustedApplet.run();}

Policy

Data

base

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Stack Inspection Example

main(…){ fp = new FilePermission(“/tmp/*”,“write,…”); sm.enablePrivilege(fp); UntrustedApplet.run();}

fp

Policy

Data

base

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Stack Inspection Example

main(…){ fp = new FilePermission(“/tmp/*”,“write,…”); sm.enablePrivilege(fp); UntrustedApplet.run();}

void run() { … s.FileWrite(“/tmp/foo.txt”, “Hello!”); …}

fp

Policy

Data

base

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Stack Inspection Example

main(…){ fp = new FilePermission(“/tmp/*”,“write,…”); sm.enablePrivilege(fp); UntrustedApplet.run();}

void run() { … s.FileWrite(“/tmp/foo.txt”, “Hello!”); …}

void fileWrite(“/tmp/foo.txt”, “Hello!”) { fp = new FilePermission(“/tmp/foo.txt”,“write”) sm.checkPermission(fp); /* … write s to file filename … */

fp

Policy

Data

base

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Stack Inspection Example

main(…){ fp = new FilePermission(“/tmp/*”,“write,…”); sm.enablePrivilege(fp); UntrustedApplet.run();}

void run() { … s.FileWrite(“/tmp/foo.txt”, “Hello!”); …}

void fileWrite(“/tmp/foo.txt”, “Hello!”) { fp = new FilePermission(“/tmp/foo.txt”,“write”) sm.checkPermission(fp); /* … write s to file filename … */

fp

Policy

Data

base

Succeed!

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Stack Inspection Example

main(…){ fp = new FilePermission(“/tmp/*”,“write,…”); sm.enablePrivilege(fp); UntrustedApplet.run();}

void run() { … s.FileWrite(“~dpw/grades.txt”, “Nick: A+”);}

fp

Policy

Data

base

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Stack Inspection Example

main(…){ fp = new FilePermission(“/tmp/*”,“write,…”); sm.enablePrivilege(fp); UntrustedApplet.run();}

void fileWrite (“~dpw/grades.txt”, “Nick: A+”) { fp = new FilePermission(“important.txt”, “write”); sm.checkPermission(fp);

fp

Policy

Data

base

void run() { … s.FileWrite(“~dpw/grades.txt”, “Nick: A+”);}

Fail

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Other Possibilities The fileWrite method could enable

the write permission itself Potentially dangerous, should not base the

file to write on data from the applet

A trusted piece of code could disable a previously granted permission Terminate the stack inspection early

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Stack Inspection AlgorithmcheckPermission(T) { // loop newest to oldest stack frame foreach stackFrame { if (local policy forbids access to T by class executing in stack frame) throw ForbiddenException;

if (stackFrame has enabled privilege for T) return; // allow access

if (stackFrame has disabled privilege for T) throw ForbiddenException; }

// end of stack if (Netscape || …) throw ForbiddenException; if (MS IE4.0 || JDK 1.2 || …) return;}

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Two Implementations On demand –

On a checkPermission invocation, actually crawl down the stack, checking on the way

Used in practice

Eagerly – Keep track of the current set of available

permissions during execution (security-passing style Wallach & Felten)

+ more apparent (could print current perms.)- more expensive (checkPermission occurs

infrequently)

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Stack Inspection Stack inspection seems appealing:

Fine grained, flexible, configurable policies Distinguishes between code of varying degrees of

trust But…

How do we understand what the policy is? Semantics tied to the operational behavior of the

program (defined in terms of stacks!) How do we compare implementations Changing the program (e.g. optimizing it) may

change the security policy Policy is distributed throughout the software, and is

not apparent from the program interfaces. Is it any good?

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Stack Inspection Literature

Stack Inspection: Theory and VariantsCédric Fournet and Andrew D. Gordon Use operational semantics like in class

Understanding Java Stack InspectionDan S. Wallach and Edward W. Felten Formalize Java Stack Inspection using a special logic

of authentication

Formalizing Stack Inspection

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Abstract Stack Inspection Abstract permissions

p,q Permissions (left abstract in the

theory) R,S Sets of permissions (models an

entity)

Examples:System = {fileWrite(“f1”), fileWrite(“f2”),…}Applet = {fileWrite(“f1”)}

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sec Syntax Language syntax:

e ::= expressions x variable x.e function e1 e2 application R{e} framed expr enable p in e enable test p then e1 else e2 check perm. fail failure

v ::= x | x.e valueso ::= v | fail outcomes

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Modelling the Classloader Models the Classloader that marks the

(unframed) code with its protection domain:

Load(R,x) = x

Load(R,x.e) = x. R{ Load(R,e) }

Load(R,e1 e2) = Load(R,e1) Load(R,e2)

Load(R,enable p in e) = enable p in Load(R,e)

Load(R,test p then e2 else e2)

= test p then Load(R,e1) else Load(R,e2)

Load(R,fail) = fail

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Example

writeFile = fileName.System{ test fileWrite(fileName) then

“f2 contents” // primitive file IO else fail }

Applet{writeFile “f2”} -->* fail System{writeFile “f2”} -->* “f2 contents”

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sec Operational Semantics Evaluation contexts:

E ::= [] Hole E e Eval function

v E Eval argenable p in E Tag on stack

frame R{E} Stack frame

E models the control stack

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sec Operational SemanticsE[(x.e) v] --> E[e{v/x}]

E[enable p in v] --> E[v]

E[R{v}] --> E[v]

E[fail] --> fail

E[test p then e else f] --> E[e] if Stack(E) |-- p

E[test p then e else f] --> E[f] if (Stack(E) |-- p)

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Formal Stack Inspection

E = Applet{System{[]}}e = test fileWrite(“f2”) then

“f2 contents” else fail

When does stack(E) allow permissionfileWrite(“f2”)?

Stack(E) |-- fileWrite(“f2”)

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Formal Stack Inspection

Structure of Stacks:

s ::= . (Empty Stack) | s.R (Stack for code of principal R) | s.enable(p) (Privelege p enabled)

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Stack of an Eval. Context

Stack([]) = .Stack(E e) = Stack(E)Stack(v E) = Stack(E)Stack(enable p in E) = enable(p).Stack(E)Stack(R{E}) = R.Stack(E)

Stack(E’) = Stack(Applet{System{[]}}) = Applet.Stack(System{[]}) = Applet.System.Stack([]) = Applet.System.

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Abstract Stack Inspection

. |-- p empty stack axiom

s |-- p p Rs.R |-- p

s |-- ps.enable(q) |-- p

protection domain check

p q irrelevant enable

s |= ps.enable(p) |-- p

check enable

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Abstract Stack Inspection

. |= p empty stack enables all

p Rx.R |= p enable succeeds

x |= px.enable(q) |=

p

irrelevant enable

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Reason about optimization: Which programs are equal? (Is the optimized program the same as the unoptimized program?)

Eg: Let C[] be an arbitrary program context. Define e = e’ iff

for all C[], C[e] terminates whenever C[e’] terminates

What Can You Do with an Operational Semantics?

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Conclusions What security properties does the Java

security model guarantee?

What optimizations are legal?

Formal semantics helps us find the answers & suggests improvements