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Java Security
Topics
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Intro to the Java Sandbox
Language Level Security
Run Time Security
Evolution of Security Sandbox Models
The Security Manager
Internet Security Needed
• Nowadays, code is downloaded from the
Internet and executed transparently by
millions of users.
• Downloaded software can hide all sorts of
hazardous code. Games and music are
often Trojan horses, spyware and virus
installers.
• There is a real need for a more security
mechanism for mobile code.
Writing Secure Code
• Software developers also have security
problems to handle. Programmers
unknowingly leave holes in their code that
hackers can exploit.
– Forgetting to deallocate resources.
– An open socket connection is like an open
invitation to a hacker.
– Memory leaks can be exploited
– Buffer overflow
The Java Solution
• Java Virtual Machine creates a sandbox
• Syntax - insecure operations cannot even
be represented
• Automatic garbage collection prevents
memory leaks
• Security Manager watches for anomalies
during execution and can take action
JVM
• Java is an interpreted language. Your
source code is “compiled” to “bytecode”
which is executed on the JVM.
• The Java Virtual Machine (JVM) observes
each instruction (bytecode) before it is
used.
• This allows Java to provide the idea of a
sandbox, which limits how an untrusted
program can affect the system it runs on.
Language Level Security
• No pointers or pointer arithmetic
• No way to represent unstructured memory
• Variables, methods and classes can be
final
• Compiler checks variable instantiation and
typecasts
No Pointers
• Pointers and pointer arithmetic allow
attackers to access any byte in memory.
• In C, strings and arrays are essentially
unstructured memory. They start with a
pointer and operations on the array are
done by manipulating the pointer. There is
no bounds checking.
• The Java programmer cannot represent or
manipulate pointers.
No Pointers
• You just can’t write a Java program to do
damage like this.
void main() {
int *randAddress;
randAddress = (int *) rand();
*randAddress = rand();
}
No Unstructured Memory Access
• Unstructured memory access (or
unenforced structure) can be exploited.
• In C and C++, character data can be
written to memory allocated as integer.
Character or integer data can be read and
interpreted as Boolean.
• Java prevents data of one type from being
used as a different type – cannot be
expressed in bytecode.
Unspecified Memory Layout
• The JVM stores several types of data to
execute a program
– Runtime stacks – one for each thread
– Bytecode for methods
– Dynamic memory heap and garbage
collection area
• The storage layout is not defined for the
JVM. Each implementation does it
differently.
The Keyword final
• This keyword can be used to prevent
variables, methods and classes from being
changed (and potentially exploited).
• The value of a variable is fixed for the
duration of the execution.
• A method cannot be modified in
subclasses (hacker tactic to use
permission levels of original method)
• Class cannot have subclasses (subclass
of API would have full system access).
The Compiler
• The compiler checks code and produces
bytecode (intermediate representation
interpreted by all JVMs).
• Checks that:
– variables are instantiated before they are
used.
– Type casts are legal (prevents unstructured
memory exploits)
– Methods called by appropriate type objects
Run-time security
• Java Virtual Machine – the runtime
environment
– Bytecode verifier, class loader, runtime
checks
– Sandbox evolution
– Security manager
Bytecode Verifier, Class Loader
• Bytecode verifier runs first and guards
against circumvention of compiler checks
with handwritten bytecode.
• Class loader checks permissions and
helps to prevent the loading of “Trojan
Horse” methods.
Run Time Checks
• Bounds checking on arrays (no buffer
overflow)
• Type cast checking
• Automatic garbage collection (memory
leaks can lead to DOS attacks
The Sandbox Idea
• The original Java release, jdk1.0, provided
a system that used the basic sandbox
model.
• Differentiated only between native code
(code stored locally, trusted, given full
access) and non-native code (applets
downloaded, not trusted).
JDK 1.0 Sandbox
• Trusted code can read, write and delete
any file, start and kill threads and open,
use and close socket connections without
any restrictions.
• Untrusted code cannot access any files,
threads are hidden and only socket
connections to the applet’s origin server
are allowed.
JDK 1.1: More Flexible
• Native code is trusted and treated as in
JDK1.0
• Non-native code can be trusted or nontrusted.
– If the .jar file has a valid digital signature and
comes from a “trusted developer” (list is part
of the JVM) code is considered trusted and
given same rights as native code.
– Otherwise, untrusted and restrictions apply.
JDK 1.2
• ALL code (even native) is subject to a
security policy that can be defined by the
user.
• Permissions are checked against the
policy when the class is loaded AND
whenever restricted actions are attempted.
• Promotes Principle of Least Priviledge
• Performs Complete Mediation
JDK 1.2 Restricted Actions
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Accept a socket connection
Open a socket connection
Wait for a connection on a port
Create a new process
Modify a thread
Cause the application to exit
Load a dynamic library that contains native methods
Access or modify system properties
Read from a file
Write to a file
Delete a file
Create a new class loader
Load a class from a specified package
Add a new class to a specified package
The Security Manager
• The part of the JVM that performs these
run time checks is called the security
manager
• JDK 1.2 or later (a.k.a. Access Manager)
• In addition the security manager watches
other potential security holes and can
react if needed.
Possible Problems
• Security features not automatic if Java program
is invoked on the command line
• And others as yet undiscovered …
Readings
• http://java.sun.com/sfaq
• http://www.javaworld.com/javaworld/jw-08-1997/jw-08-hood_p.html