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(Duke suicide picture by Gary McGraw.) CS201j: Engineering Software University of Virginia Computer Science Lecture 20: Security in Java / Java (In?) Security David Evans http://www.cs.virginia.edu/evans Menu • Low-level Code Safety – Java Byte Code Verifier • Policy Code Safety – Reference Monitors – Java’s Security Manager 14 November 2002 CS 201J Fall 2002 2 Java Security Real or Decaf? 14 November 2002 CS 201J Fall 2002 3 Java : Programming Language “A simple, object-oriented, distributed, interpreted, robust, secure, architecture neutral, portable, high-performance, multithreaded, and dynamic language.” [Sun95] 14 November 2002 CS 201J Fall 2002 4 What is a secure programming language? 1. Language is designed so it cannot express certain computations considered insecure. A few attempt to do this: PLAN, packet filters 2. Language is designed so that (accidental) program bugs are likely to be caught by the compiler or runtime environment instead of leading to security vulnerabilities. 14 November 2002 CS 201J Fall 2002 5 Safe Programming Languages • Type Safety – Compiler and run-time environment ensure that bits are treated as the type they represent • Memory Safety – Compiler and run-time environment ensure that program cannot access memory outside defined storage • Control Flow Safety – Can’t jump to arbitrary addresses Which of these does C/C++ have? Is Java the first language to have them? No way! LISP had them all in 1960. 14 November 2002 CS 201J Fall 2002 6 Java Safety • Type Safety – Most types checked statically – Coercions, array assignments type checked at run time • Memory Safety – No direct memory access (e.g., pointers) – Primitive array type with mandatory run-time bounds checking • Control Flow Safety – Structured control flow, no arbitrary jumps 14 November 2002 CS 201J Fall 2002 7 Malicious Code Can a safe programming language protect you from malcode? 1. Code your servers in it to protect from buffer overflow bugs 2. Only allow programs from untrustworthy origins to run if the are programmed in the safe language 14 November 2002 CS 201J Fall 2002 8 Safe Languages? • But how can you tell program was written in the safe language? – Get the source code and compile it (most vendors, and all malicious attackers refuse to provide source code) – Special compilation service cryptographically signs object files generated from the safe language (SPIN, [Bershad96]) – Verify object files preserve safety properties of source language (Java) 14 November 2002 CS 201J Fall 2002 9 JVML malcode.java Java Source Code javac Compiler malcode.class JVML Object Code JavaVM Joe User 14 November 2002 Joe wants to know JVML code satisfies Java’s safety properties. CS 201J Fall 2002 10 Does JVML satisfy Java’s safety properties? iconst_2 istore_0 aload_0 arraylength push integer constant 2 on stack store top of stack in variable 0 as int load object reference from variable 0 replace array on top of stack with its length No! This code violates Java’s type rules. 14 November 2002 CS 201J Fall 2002 11 Do Java Byte Codes provide these? • Type Safety iconst_3 iconst_3 fadd astore_1 iload_1 • Memory Safety iconst_3 iadd • Control Flow Safety ifeq 2359 14 November 2002 CS 201J Fall 2002 12 malcode.class JVML Object Code Bytecode Verifier Trusted Computing Base Java Bytecode Verifier Invalid “Okay” STOP JavaVM Joe User 14 November 2002 CS 201J Fall 2002 13 Bytecode Verifier • Checks class file is formatted correctly – Magic number: class file starts with 0xCAFEBABE – String table, code, methods, etc. • Checks JVML code satisfies safety properties 14 November 2002 CS 201J Fall 2002 14 Verifying Safety Properties • Type safe – Stack and variable slots must store and load as same type • Memory safe – Must not attempt to pop more values from stack than are on it check this in bytecode verifier, VM must – Array bounds? Can’t check array bounds at run time. • Control flow safe – Jumps must be to valid addresses within function, or call/return 14 November 2002 CS 201J Fall 2002 15 Are Java Bytecode Verifiers Complicated? • ~700 rules to enforce, JVML specification is (not all clearly specified) • Emin Gün Sirer found > 100 bugs in commercial bytecode verifiers (using automatic test generation) – At least 15 of them were security vulnerabilities • JVML includes jsr instruction (jump to subroutine), can be called with different types in variables and on stack Added to allow implementations to reuse code in finally blocks to save code size. Would have saved more space to keep original language name (“Oak”). 14 November 2002 CS 201J Fall 2002 16 Java malcode.java javac Compiler malcode.class JVML Trusted Computing Base Java Bytecode Verifier Invalid “Okay” STOP JavaVM Joe User 14 November 2002 CS 201J Fall 2002 17 JavaVM • Virtual machine – interpreter for JVML programs • Has complete access to host machine • Bytecode verifier ensures some safety properties, JavaVM must ensure rest: – Type safety of run-time casts, array assignments – Memory safety: array bounds checking – Resource use policy 14 November 2002 CS 201J Fall 2002 18 Reference Monitors 14 November 2002 CS 201J Fall 2002 19 Program Execution Monitor Program Speakers Network Disk 14 November 2002 Memory CS 201J Fall 2002 SuperSoaker 2000 20 Program Execution Reference Monitor Monitor Program Speakers Network Disk 14 November 2002 Memory CS 201J Fall 2002 SuperSoaker 2000 21 Ideal Reference Monitor 1. Sees everything a program is about to do before it does it 2. Can instantly and completely stop program execution (or prevent action) 3. Has no other effect on the program or system Can we build this? Probably not unless we can build a time machine... 14 November 2002 CS 201J Fall 2002 22 RealReference Monitor Ideal most things 1. Sees everything a program is about to do before it does it 2. Can instantly and completely stop program execution (or prevent action) limited 3. Has no other effect on the program or system 14 November 2002 CS 201J Fall 2002 23 Operating Systems • Provide reference monitors for most security-critical resources – When a program opens a file in Unix or Windows, the OS checks that the principal running the program can open that file • Doesn’t allow different policies for different programs • No flexibility over what is monitored – OS decides for everyone – Hence, can’t monitor inexpensive operations 14 November 2002 CS 201J Fall 2002 24 Reference Monitor as Finite State Automaton [Schneider99] All other instructions 0 All other instructions Aim Aim 1 Aim Fire 2 All other instructions Fire STOP Policy Violation 14 November 2002 CS 201J Fall 2002 25 What policies can be enforced? • Assume: – Security Automaton can see entire state of world, everything about instruction about to execute – Security Automaton has unlimited memory, can do unlimited computation • Are there interesting policies that still can’t be enforced? 14 November 2002 CS 201J Fall 2002 26 What’s a Security Policy? • What’s a program? – A set of possible executions • What’s an execution? – A sequence of states • What’s a security policy? – A predicate on a set of executions 14 November 2002 CS 201J Fall 2002 27 Reference Monitors cannot enforce all Security Policies • Some policies depend on: – Knowing about the future • If the program charges the credit card, it must eventually ship the goods – Knowing about all possible executions • Information flow – can’t tell if a program reveals secret information without knowing about other possible executions • Reference Monitors can only know about past of this particular execution 14 November 2002 CS 201J Fall 2002 28 Safety Policies • Reference monitors can only enforce safety policies • Safety policy is a predicate on a prefix of states (see Schneider98 for more formal definition) – Cannot depend on future: prefix means once it is false, it is always false – Cannot depend on other possible executions 14 November 2002 CS 201J Fall 2002 29 JavaVM Policy Enforcment [JDK 1.0 – JDK 1.1] From java.io.File: public boolean delete() { SecurityManager security = System.getSecurityManager(); if (security != null) { security.checkDelete(path); } if (isDirectory()) return rmdir0(); else return delete0(); } 14 November 2002 CS 201J Fall 2002 30 java.lang.SecurityManager /** Throws a SecurityException if the calling thread is not allowed to delete the specified file. This method is invoked for the current security manager by the delete method of class File. */ (Some other comments deleted.) public void checkDelete(String file) { throw new SecurityException(); } 14 November 2002 CS 201J Fall 2002 31 Security Manager • Reference monitor – How well does it satisfy the requirements? • Complete mediation • Can stop execution/prevent action • Limited effect on execution until policy violation • User/host application creates a subclass of SecurityManager to define a policy 14 November 2002 CS 201J Fall 2002 32 JavaVM Policy Enforcment [JDK 1.0 – JDK 1.1] From java.io.File: public boolean delete() { SecurityManager security = System.getSecurityManager(); if (security != null) { security.checkDelete(path); } if (isDirectory()) return rmdir0(); else return delete0(); } What could go seriously wrong with this?! 14 November 2002 CS 201J Fall 2002 33 HotJava’s Policy (JDK 1.1.7) public class AppletSecurity extends SecurityManager { ... public synchronized void checkDelete(String file) { checkWrite(file); } ... } 14 November 2002 CS 201J Fall 2002 34 AppletSecurity.checkWrite (some exception handling code removed) public synchronized void checkWrite(String file) { if (inApplet()) { if (!initACL) initializeACLs(); String realPath = (new File(file)).getCanonicalPath(); for (int i = writeACL.length ; i-- > 0 ;) { if (realPath.startsWith(writeACL[i])) return; } throw new AppletSecurityException ("checkwrite", file, realPath); } } Note: no checking if not inApplet! Very important this does the right thing. 14 November 2002 CS 201J Fall 2002 35 inApplet boolean inApplet() { return inClassLoader(); } Inherited from java.lang.SecurityManager: protected boolean inClassLoader() { return currentClassLoader() != null; } 14 November 2002 CS 201J Fall 2002 36 currentClassLoader /** Returns an object describing the most recent class loader executing on the stack. Returns the class loader of the most recent occurrence on the stack of a method from a class defined using a class loader; returns null if there is no occurrence on the stack of a method from a class defined using a class loader. */ protected native ClassLoader currentClassLoader(); 14 November 2002 CS 201J Fall 2002 37 Recap • java.io.File.delete calls before deleting • HotJava overrides SecurityManager with AppletSecurity to set policy • AppletSecurity.checkDelete calls SecurityManager.checkDelete AppletSecurity.checkWrite • AppletSecurity.checkWrite checks if any method on stack has a ClassLoader • If not no checks; if it does, checks ACL list 14 November 2002 CS 201J Fall 2002 38 JDK 1.0 Trust Model • When JavaVM loads a class from the CLASSPATH, it has no associated ClassLoader (can do anything) • When JavaVM loads a class from elsewhere (e.g., the web), it has an associated ClassLoader 14 November 2002 CS 201J Fall 2002 39 JDK Evolution • JDK 1.1: Signed classes from elsewhere and have no associated ClassLoader • JDK 1.2: – Different classes can have different policies based on ClassLoader – Explict enable/disable/check privileges – SecurityManager is now AccessController 14 November 2002 CS 201J Fall 2002 40 What can go wrong? • Java API doesn’t call right SecurityManager checks (63 calls in java.*) – Font loading bug, synchronization • ClassLoader is tricked into loading external class as internal • Bug in Bytecode Verifier can be exploited to circumvent SecurityManager • Policy is too weak and allows damaging behavior 14 November 2002 CS 201J Fall 2002 41 Hostile Applets • See http://java.sun.com/sfaq/chronology.html (about 1 new vulnerability/month) • Easy to write “annoying” applets (policy is too imprecise; no way to constrain many resource operations) • After your PS6 phylogeny programs finish on all the ITC machines, try http://www.cigital.com/hostile-applets/index.html 14 November 2002 CS 201J Fall 2002 42 Charge • No classes next week • Exams due before 4:30pm Tuesday, drop off to Brenda Perkins 14 November 2002 CS 201J Fall 2002 43