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Java language-based security (in general and for mobile phones in particular)

Java language-based security (in general and for mobile phones in particular). Erik Poll Digital Security group Radboud University Nijmegen. mobile code. extensible application P comprising various (possibly less trusted ) extensions. P. Q. Internet. Code extension. OS. Resources.

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Java language-based security (in general and for mobile phones in particular)

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  1. Java language-based security(in general and for mobile phones in particular) Erik Poll Digital Security group Radboud University Nijmegen

  2. mobile code • extensible application P comprising various (possibly less trusted) extensions P Q Internet Code extension OS Resources

  3. Example: browser plugin Firefox Internet libraries OS Browser plugin Resources

  4. Example: Java enabled mobile phones midlet 1 midlet 2 midlet n Internet JVM APIs code download Mobile phone

  5. Need for safe programming language • Some 'modules' are less trusted than others • This requires a 'safe' programming language: • which can restrict the way one (less trusted) component can affect another (more trusted) component Eg • (untrusted) Java code executing on (trusted) Java platform • C# code executing on .NET platform • code of various sources executed on same platform, eg • applet in browser, • midlet on mobile phone • driver in operating system?

  6. Language-based security in Java Java provides • type system • enforced by static type checking & runtime checks • includes visibility modifiers: private, protected, public • sandboxing • using stack inspection aka stack walking that provide • type safety and memory safety • code-based access control even in the presence ofuntrusted - buggy or malicious - code

  7. Java language guarantees – part I • applet A can only access visible methods & fields • eg not to private fields • no pointer arithmetic to access memory "illegally" applet B applet A package C JRE (Java Runtime Environment) VM APIs Security Manager Class Loader hardware (CPU + peripherals)

  8. Java language guarantees – part II • if applet A accesses a public API method, • stack inspection restrictions may restrict this • even if untrusted applet A tricks trusted applet B • into doing this applet B applet A package C JRE (Java Runtime Environment) VM APIs Security Manager Class Loader hardware (CPU + peripherals)

  9. Buffer overflows are built-in vulnerability in some programming languages • pointer arithmetic, lack of array bounds checks, and lack of type safety cause problems • Java is immune to this to a large extent, provided • no use of Java native interface • no VM vulnerability • no flaw in type checker • bytecode verifier on some phones known to be broken.... • Does this mean Java does not have vulnerabilities?

  10. Attacking Java security (1): typing • The sandbox relies on typing: • if type system is not sound, you can escape the sandbox • there may bugs in the bytecode verifier (bcv), which checks type correctness. These may be exploitable...

  11. Attacking Java Security (2): native code Original release date: January 22, 2007 Source: US-CERT Overview The Sun Java Runtime Environment(JRE) contains multiple vulnerabilities that can allow a remote, unauthenticated attacker to execute arbitrary code on a vulnerable system. Exploit code is publicly available. Vulnerability Note VU#149457 Sun Java JRE vulnerable to arbitrary code execution via an undetermined error Two buffer overflow vulnerabilities in the Sun JRE may independently allow an untrusted applet to elevate its privileges. For example, an applet may grant itself permissions to read and write local files or execute local applications that are accessible to the user running the untrusted applet. Vulnerability Note VU#388289 Sun Microsystems Java GIF image processing buffer overflow ...

  12. Attacking Java security (3) • The sandbox relies on correctness of • java.lang.Classloader • java.lang.SecurityManager • java.lang.String • ... Implementation bugs in these may be exploited...

  13. Security flaw in code signing check (Magic Coat) JavaSoft’s implementation of JDK1.1.1 package java.lang; public class Class { private String[] signers; /** Obtain list of signers of given class */ public String[] getSigners() { return signers; } Can you spot the fatal security flaw?

  14. Security flaw in code signing check (Magic Coat) • Absence of pointer arithmetic in Java does not rule out all problems with pointers. • Ways to prevent this kind of bugs is an active area of research • goal: (type) system that can enforce some alias control, confinement, encapsulation, ownership of references, ...

  15. class should be final to prevent malicious subclasses fields should be not be public to prevent unauthorised changes, ie. preserve integrity Spot the defect in java.lang.String package java.lang public class String{ public char[] contents; public int offset, len; // idea: String is content[offset ... offset+len] String() {contents=null; offset=0; len=0;} String(char[] a) { contents = a; offset = 0; len = a.len;} String substring(int take) { if (take<=0) throw new NegativeSizeException(); String s = new String(); s.content=this.content; s.offset=0; s.len=Math.min(take,s.len); return s; } int getLength() { return len; } ... fields should be final (for thread-safety) array should be cloned to prevent representation exposure which allows unauthorised changes

  16. Programming guidelines Even if • type system is sound • type checker that implements it is correct • sandboxing is sound & implemented correcty • ie no exploitable bugs in platfrom API classes likejava.lang.Class, java.lang.SecurityManager • sandboxing policy file is correct a particular Java applet may still be vulnerable to attacks by untrusted code Programming guidelines have been proposed to rule out known vulnerabilities • eg no public fields, ...

  17. Java security programming guidelines Online sources • Twelve rules by McGraw & Felten • Java secure programming HOWTO by Wheeler • .... Note the context of these rules: they are for • applications that are – or may someday be – extended with less trusted or untrusted code • API components that are by definition extended with less trusted code

  18. Java-enabled mobile phones MIDP • aka J2ME (Java 2 Micro Edition), MIDP (Mobile Information Device Profile), with CLDC (Connected Limited Device Configuration) API • special API functionality • eg. support for sms:// as well as http:// • fine-grained sandboxing of applications, called midlets

  19. J2ME MIDP security model • sandbox offering fine-grained access control to "dangerous" functionality • dangerous = costs money, eg. using network to phone or sms • code is trusted or not depending on digital signatures • trusted code can use network, • untrusted code is denied network access, • semi-trusted code has to ask user permission • via pop-up message • permission may have to be asked only once, once per session, or once per sms, depending how trusted the code

  20. Midlet sandboxing • Midlet has to be given permission to • access network • send sms • read PIM data (eg phonebook) • ... • Permissions can be given • without any restrictions, or • with added restriction to ask the user • once for every single event, or • once for every program run, or • only once for the lifetime of the midlet

  21. mobile phone application security threats? • malicious midlets making expensive calls, sending expensive sms messages, subscribing to sms services • SMS spam by rogue midlets • stealing confidential data: phone book or diary content, location data • unwanted information flow • Denial-of-Service • X-rated contents, eg via backdoor in game • ... Telecom providers want to avoid malicious or buggy midlets that cause problems • costs them money and loses them customers!

  22. MIDP security bugs • Phenoelit attack midlet on Siemens SS55 phone • creates race condition to let user unwittingly authorise SMS text message OK to send SMS to 6492? Do you want to play game?

  23. limits of MIDP security model But even without such bugs in platform User cannot make security decisions • user gets confused • will press ok anyway • can be tricked ot tempted into making bad decisions • can't recognize expensive numbers • can't spot information leaking • ... as illustrated by the Mobius game

  24. limits of MIDP security model • Provider might want to certify compliance with richer security policies, eg • midlet will only dial to numbers beginning with 06 or +316 • midlet will only dial number supplied by user or taken from phone book • midlet will not calculate phone number • eg dial((5*x+y)/2); is very suspicious code • midlet will send at most 3 SMS • ... • Can we make such policies precise? • Can we enforce these policies statically?

  25. Policy for #SMS in JML specifications public class Connection { //@ static ghost int smsCount = 0; //@ ensures smsCount == \old(smsCount) + 1; public void sendSMS(/*@ non_null @*/ String number,/*@ non_null @*/ String message); } public class Example { //@ ensures APIClass.smsCount == \old(APIClass.smsCount)+2; public void oneSMS() { connection.sendSMS("+31612345678, "Hello"); connection.sendSMS("+31612345678, "Goodbye"); } }

  26. JavaVerified current practice for describing behaviour of midlets: graph showing screens & transitions between them conformance checked by testing

  27. midlet navigation graph added: sensitive API calls

  28. Conformance to navigation graphs • We want to translate such navigation graphs to JML • This formal model could be used to prove that midlet behaves as specified in the graph • using program verification tool, eg ESC/Java2 • It could also be used for model-based testing • Big challenge: midlet code is multi-threaded • program verification tools for multi-threaded code still in their infancy

  29. Lighterweight mechanism than program specification & verificationthan JML: Java tags

  30. Spot the defect { ... if (spec!=null) f.add(spec); if(isComplete(spec)) prefs.add(spec); ....} boolean isComplete(Preference spec){ return spec.getColorKey() != null && spec.getColorValue() != null && spec.getTextKey() != null; } isComplete should not get a null argument

  31. Spot the defect { ... if (spec!=null) f.add(spec); if(isComplete(spec)) prefs.add(spec); ....} boolean isComplete(@NonNull Preference spec){ return spec.getColorKey() != null && spec.getColorValue() != null && spec.getTextKey() != null; } annotation expresses intent and makes analysis – by human or tool - easier

  32. Java metadata tags • introduced in Java 1.5 (JSR 175) • JSR 305 "Annotations for Software Defect Detection" currently in progress • @NonNull, @Nullable • @Tainted, @Untainted to find input validation problems • @NonNegative • @WillClose, @WillNotClose • @CheckReturnValue • enables static analysis / special typecheckers

  33. Information flow policies • Another category of security requirements: • information flow policies • Eg • "untrusted input data should not be fed into sensitive API calls" • "only numbers obtained from phonebook should be used as 1st argument of sendSMS"

  34. Information flow policies using Java tags • Java tags can be used to enrich type information • eg @PhoneNr , @Tainted, @ Confidential, @CreditCardNr that can be used by compilers & typecheckers public class Connection { public void sendSMS(@PhoneNr String number) {...} } public class Phonebook { public @PhoneNr StringgetNumber(int offset) {...} }

  35. Information flow policies using Java tags • Checking input validation using tags for (un)tainted info • cf Pearl in tainting mode public class Input { public @Tainted Stringread() {...}; } public class System { public void sensitiveAction(@Untainted String st) {..} } public class InputValidation{ public @Untainted Stringvalidate(@Tainted String st){..} }

  36. Tainting approach works for explicit information flow, but not for implicit flows, eg for(i = 0; i< password.length; i++){ for(c ='a'..'z') if (password[0]=c) print(c); } leaks password but does not have explicit flow • JSR (Java Specification Request) 175 to define standard tags that can be used by variety of tools • also @NonNull : very basic JML specs can be expressed using Java tags

  37. Questions?

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