Security Threats from Deviant Java Byte Code

Copyright (c) 1997 Mark D. LaDue

1. The State of Java Security

The Java programming language has recently turned one year old. In its first year, Java has had a number of spectacular holes punched in its security model by Ed Felten and the Safe Internet Programming Team at Princeton University [McGF]. Since February of 1996 the Hostile Applets Home Page at the Georgia Institute of Technology's School of Mathematics has featured a collection of evil applets (with complete source code), and yet Sun Microsystems and its corporate partners have shown little progress in combatting hostile applets [LaD1, LaD2]. A year ago, when Java was first gaining notoriety, few people imagined that so many serious flaws would surface so quickly, and even fewer believed that threats from hostile applets would persist. In Java's first year it has become clear that many of the problems posed by executable content have not been solved. The power and complexity of the language make it extremely likely that security holes will continue to appear in years to come.

The scope of Java Security has been defined so narrowly as to exclude significant programmed threats, and wider issues of security within the Java Platform have scarcely been addressed. The almost exclusive focus of the Java community on executable content has left numerous avenues unexplored for threats. Java applications can pose a more profound threat to system security than applets. Java applications will also provide extremely potent tools for hackers, and any lack of security awareness on the part those who write and use them may translate into security breaches. When the notion of trusted code becomes a part of the Java programming environment, the distinction between applications and applets will begin to disappear. Thus it is wise to move beyond issues of executable content and to begin exploring other facets of Java Security [LaD3, McGF].

It has been observed that there is no one-to-one correspondence between Java source code (programs) and Java byte code (class files) [McGF, LaD4]. While every program written in Java can be compiled to byte code by a Java compiler, it is possible to create class files which no Java compiler can produce, and yet, which pass the Java Verifier with flying colors. Not only is it possible to create such deviant class files, it is a simple matter to do so, and the number of these non-compiler class files greatly exceeds the number of those producible by Java compilers. This fact has one very serious implication - No matter what claims are made, and even formally demonstrated, for the security of the Java language, all bets are off when it comes to byte code running in the Java Virtual Machine. Deviant class files that pass the Verifier and exploit unenforced, or improperly implemented, Verifier rules can reduce Java Security to rubble.

This article will explore some of the implications of this curious lack of coherence between Java source code and byte code. It will also illustrate how easy it is to alter Java class files for malicious purposes. Section 2 contains an overview of some salient facts about the Java class file format. It highlights the ease with which class files can be altered to do things other than those intended by their source code authors. Section 3 describes the problem of incoherence between Java source code and byte code. It points out some very surprising properties of byte code as well as several rules unenforced by the Java Verifier, all of which could lead to security breaches. Section 4 then introduces a number of examples in order to illustrate the threats. Finally, Section 5 recounts recent experience with some rudimentary Java Platform viruses. The conclusion is inescapable - Java Security has a fundamental weakness, and much work remains to be done before even its most basic assertions can be tested for validity.

2. An Overview of the Java Class File Format

When Java source code is compiled, the result is a class file, having a .class extension and containing platform-independent byte code in a very specific format. A class file should be regarded as a stream of 8-bit bytes, with 16-bit, 32-bit, and 64-bit quantities being constructed in big-endian order from two, four, and eight consecutive 8-bit bytes, respectively. The Java Virtual Machine (JVM) Specification represents a class file in a C-like structure notation as follows [Lind]:

ClassFile {
    u4 magic;
    u2 minor_version;
    u2 major_version;
    u2 constant_pool_count;
    cp_info constant_pool[constant_pool_count - 1];
    u2 access_flags;
    u2 this_class;
    u2 super_class;
    u2 interfaces_count;
    u2 interfaces[interfaces_count];
    u2 fields_count;
    field_info fields[fields_count];
    u2 methods_count;
    method_info methods[methods_count];
    u2 attributes_count;
    attribute_info attributes[attributes_count];
Here the notation un refers to an unsigned n-byte quantity. While this structure gives some idea of the nature of Java class files, it will be helpful to take a closer look at a few of the details.

The 4-byte quantity magic has the value 0xCAFEBABE and identifies the class file as such. The 2-byte quantities minor_version and major_version specify which version of the Java compiler produced the class file. The constant_pool is a table of structures that represent an assortment of class, field, and method names as well as string and other constants used within the class file. The constant_pool_count specifies how many entries are present in the constant_pool, while each cp_info structure is one of eleven different types that may appear in the constant_pool. The 2-byte quantity access_flags is a mask of modifiers used to specify class and interface accessibility. An extended form of access_flags also occurs in the field_info and method_info structures, where it serves the same purpose. The 2-byte quantities this_class and super_class refer to the constant_pool entries containing precisely what their names indicate.

The remainder of the class file consists of four tables, with one table each for interfaces, fields, methods, and attributes. Each table is preceded by a 2-byte quantity specifying the number of entries in that table, and each entry in a particular table is a structure of a type appropriate for that table. While each of these tables is an integral part of the class file, the methods table contains the byte code to be run in the JVM, and so a closer look at it is in order.

The methods table of a Java class file contains methods_count entries, and each entry is a structure of type method_info, which has the following format:

method_info {
    u2 access_flags;
    u2 name_index;
    u2 descriptor_index;
    u2 attributes_count;
    attribute_info attributes[attributes_count];
The JVM class file specification offers six predefined types of attribute_info structures:
  1. Code
  2. ConstantValue
  3. Exceptions
  4. LineNumberTable
  5. LocalVariableTable
  6. SourceFile
The most important of these attribute_info structures is the Code attribute, which contains the JVM instructions for a single Java method and has the following format:
Code_attribute {
    u2 attribute_name_index;
    u4 attribute_length;
    u2 max_stack;
    u2 max_locals;
    u4 code_length;
    u1 code[code_length];
    u2 exception_table_length;
    table_info exception_table[exception_table_length];
    u2 attributes_count;
    attribute_info attributes[attributes_count];
The code array contains the bytes of code actually run by the JVM. Each byte of the code array is either a legal Java opcode, of which there are 201 at the present time, or an operand of an opcode. The code array, like the class file as a whole, is subject to a multitude of static and structural constraints, all of which must be checked by the Java Verifier [Lind].

While the class file format greatly enhances Java's security by making the verification process much more tractable, it also raises some security concerns of its own [McGF]. The well-defined format and level of detail present in class files make it a straightforward, though tedious, task to recover source code from them. The justly celebrated Mocha decompiler does precisely that. Using the Mocha decompiler, for example, it is an easy matter for one to decompile class files to source code and scour them for security weaknesses, and it is just as easy for a Java developer to decompile a business competitor's work and search for trade secrets. What can be read can often be rewritten, but one need not go to all of the effort of decompiling class files to source code, editing that source code, and re-compiling it to obtain hacked class files.

The hacker who knows a bit of Java programming, the class file format, and Java opcodes can easily insert, delete, or otherwise alter code in class files, all without effect on the class files' verifiability. To insert some code, for instance, one need only append entries to the constant_pool, append the appropriate opcodes to a suitable method's code array, and use the goto instruction (167 or 0xa7) to jump to and from the inserted code. One has only to be careful and adjust the appropriate counts in the class file to maintain its verifiability. When the class file attacked happens to be SecurityManager.class or AppletClassLoader.class, more dire consequences would be sure to follow. The class has handy methods for reading and writing Java primitive data types, including unsigned 1-, 2-, and 4-byte quantities, at arbitrary locations in files. With Java's power and ease of use, it takes a scant few hours to develop the knowledge and skills required for the task. Thus it is extremely simple to read and manipulate Java class files for evil purposes. Moreover, what can be done by a hacker can just as easily be done by a virus. In the future we should expect class files to become tempting targets for hackers and virus writers, and all who have Java applications will need to defend them from byte code bandits.

3. Java Source Code Versus Byte Code

The overview of the class file format in the previous section revealed how simple it is to systematically inspect and tamper with Java class files. That alone is cause for concern. But the lack of a one-to-one correspondence between Java source code and class files which pass the Java Verifier is cause for much greater concern. There is fundamental incoherence between the Java programming language and the byte code passed by the Verifier.

A Java compiler will take any valid Java program and produce a class file which will pass the Verifier, yet there are class files which pass the Verifier but correspond to no valid Java program. Such deviant class files contain code not derived from legitimate programming constructs. Thus the JVM allows byte code to extend the Java language far beyond its official boundaries. Since some of Java's security policies depend upon the language itself, this is a potential source of serious security breaches. To make matters worse, the ease with which class files can be altered entails that it is utterly simple to produce deviant byte code. It also entails that the quantity of deviant byte code is vastly greater than that of the legitimate byte code produced by Java compilers.

One source of the problem is that the class file format is entirely independent of the Java language. While the Java Verifier can check with 100% certainty whether or not a given file is a bona fide class file, the Verifier does not, and most likely cannot, determine whether or not that file was produced by a Java compiler. The Verifier's function is to perform a multitude of tests to make sure that a potentially hostile file is consistent with some of the Java language's most important constraints. Thus it should be no surprise that byte code is more powerful than source code. The words of the Sun programmer who wrote check_code.c, the heart of the Java Verifier, are particularly apt: "All currently existing code passes the test, but so does lots of bad code." How true.

To compound the problems, a number of significant constraints are not enforced by the Verifier, and some opcodes possess more functionality than is apparently used by Sun's Java compiler. Examples of this abound. The Java Virtual Machine Specification lists three specific properties of exception handlers which byte code produced by Sun's Java compiler always possesses, but which are not checked by its Verifier:

  1. The ranges of instructions protected by distinct exception handlers must be disjoint, or one must contain the other;
  2. An exception handler cannot occur in code protected by itself;
  3. Control cannot be passed to an exception handler's code by any means other than an exception.
These constraints are not enforced because supposedly "they do not pose a threat to the integrity of the Java Virtual Machine" [Lind, p.133]. It remains to be seen whether or not such a cavalier view of Java's security is justified.

One need not rest content with such officially documented examples; others are readily found. An apparently harmless example is that arbitrary bytes can be appended to class files without effecting their verifiability, contradicting the JVM Specification [Lind, p.125]. A more interesting case study is the goto instruction (167 or 0xa7). While the Java language studiously avoided the goto statement, one is built into the JVM instruction set. Although Sun's Java compiler seems to always employ it in the forward direction, passing goto a sufficiently large offset allows it to pass control backwards. This allows deviant byte code to achieve arbitrary transfer of control within a method. In particular, arbitrary branches into and out of catch and finally blocks work perfectly well, and the better controlled jsr (168 or 0xa8) and ret (169 or 0xa9) instructions can be bypassed. It is quite possible that a deviant use of goto together with highly non-standard exception handling could lead to a meltdown of Java's security. In any event, deviant byte code running in the JVM is unpredictable and makes a mockery of Java's claims to security.

The surprising flexibility of the goto instruction has another interesting use. It is especially easy to append opcodes to any code array, execute that code, and return. This is indeed very good news for virus writers. Moreover, the existence of such appended code exposes another weakness in the Verifier - appended code is not consistently verified. If control is transferred to it at some point, then it almost certainly is verified, but dead opcodes appended are sometimes inspected and sometimes not. This is troubling and further raises the prospects of deviant byte code harboring nasty surprises. This combination of language-dependent security, rules not enforced by the Verifier, undocumented behavior of opcodes, and inconsisent code checking entails a great deal of risk. At the very least it implies that very little can be asserted with certainty about Java Security. Clearly much more work remains to be done before Java can be declared safe.

4. Battle of the Byte Code

The plea to ponder potential security threats always seems less convincing in the absence of concrete examples. In order to illustrate the threats posed by deviant and subversive byte code, the author created a number of examples [LaD5]. Some are lighthearted, while others are more serious. In pondering these examples, one should recall Frederick Cohen's amusing tale of an expert's reaction to one of his early demonstrations [Coh2, pp.35-36]:

... and he got really upset. He said "I don't know why we had you here, you're the worst programmer I have ever seen. In 15 minutes, I could write a program to do this, it would go through the system like a hot knife through butter, and nobody would even notice it." So I said, "That's the point!" He finally got the point.

As a first example, the author wrote and then attacked, which was contrived to try reading a nonexistent file, catch the resulting IOException and print an error message ("Oops!"), and finally print one last message ("Help!"). After inspecting Beginner.class, the author wrote, which inserted 3 bytes of code into Beginner.class at the end of its finally block: goto followed by the 2-byte offset necessary to return control to the beginning of the program. Attacker also adjusted the attribute_length and code_length of the proper Code_attribute structure to maintain verifiability. The altered Beginner.class readily passed the Verifier, and when run, it proceeded into an infinite loop of printing "Oops!" and "Help!" messages, as expected. This deviant class file could not have been produced by a Java compiler, and the Mocha decompiler failed to decompile it. A second version of was able to insert similar code into the catch block and achieved a similar effect, though it also had to alter the Exceptions table to preserve verifiability.

A more interesting example is Given a target directory, this Java application searches it and all of its subdirectories for Java class files. Once a class file is located, PublicEnemy alters the contents of its access_flags for the class, its fields, and its methods. The results are the following:

  1. The class becomes public.
  2. Any final fields and methods become non-final; any non-public fields and methods become public; and all public fields and methods remain so.
Attacked classes pass the Verifier and continue to run just as before, and their sizes do not change. But they are then open to the maximum amount of inspection and abuse by other Java classes. The system whose netscape.applet.* classes fell prey to PublicEnemy would allow Java applets to rampage through it. Thus PublicEnemy would be a fearsome payload for a Java Trojan horse or virus to carry. It also makes an important point about the dangers of trust for Java - Inadvertently trusting a single hostile class a single time can lead to a swift, silent, and thorough compromise of all Java-related security.

It has been observed that the Java class file format makes it a simple matter to recover valid source code using decompilers such as Mocha. As was the case with the altered Beginner.class, deviant byte code resists decompilation because it corresponds to no Java source code. This suggests that class files could be protected from decompilation by making them deviant, while preserving their functioning. The Java application does precisely that. Its operation is simple; it alters a class file by adding a dead opcode (for the pop instruction in this case) after each method's return. It so happens that the inconsistent Verifier ignores this frivolous pop, so that the altered class files are successfully verified and continue to function as designed. But the Mocha decompiler, trying desperately to do the impossible, gets a segmentation violation and dies when fed such deviant class files. If a future release of Mocha were to start defending itself, new strategies could be devised to protect class files from decompilation into source code. More sophisticated tools could rewrite most byte code in non-standard ways and make class files much more resistant to decompilation. It should now be clear that the power of byte code greatly exceeds the power of Java source code.

In the examples considered so far, Java class files have altered other class files to achieve dubious ends. But a class file is certainly capable of inducing mutations in itself, while remaining acceptable to the Verifier, and taking actions based upon its history. A very simple application,, was created to illustrate this capability. By altering a single byte of its own class file each time it runs, Mutator keeps track of the number of times it has been run and deletes itself, together with its source code, if present, on the sixth attempted run. This example shows that even simple class files are capable of adaptation and learning. The power of the Java language, and the greater power of byte code, when combined with the transparency of the class file format, make it feasible to create armies of intelligent class files that can attack, defend, and maneuver in file systems by exploiting the Java Virtual Machine.

5. More Virulent Threats

From the preceding examples it is but a short step to the realm of the virus. The threats posed by PC viruses are well understood, and ample tools for their prevention, detection, and elimination are readily available. There is a widely held myth, however, that UNIX systems are somehow immune to viruses [Rada]. Of course some of Frederick Cohen's pioneering work on viruses was carried out on UNIX systems [Coh1, Coh2], and in the late 1980's Tom Duff and M. Douglas McIlroy developed several strains of UNIX viruses at AT&T Bell Laboratories [Duf1, Duf2, McIl]. Java's "write once, run anywhere" capability offers the possibility of universal computer viruses, which would lay to rest the myth by putting UNIX systems on a par with DOS machines as havens for viruses. The examples of the preceding section illustrate numerous skills useful to Viruses, and class files can offer fine mobile homes for them.

As proof-of-concept the author created several applications, which, when inadvertently trusted and run, infect the unfortunate user's system with viruses:

  1. generates and executes, a Bourne shell script virus which would work on any UNIX platform by appending itself to all Bourne shell scripts in a user's home directory. It would be just as easy to write a Java Trojan horse to detect the user's platform and infect it with an appropriate virus.
  2. is a Java Trojan that subverts Sun's javac by adding a hostile main class to the user's CLASSPATH ahead of In this case the subverted compiler simply announces its presence and appends the string "Hijacked!" to class files that it produces, but it could just as easily infect them with a Java Platform virus.
  3. attacks Java source code in a user's home directory by inserting the necessary code to make it viral. When compiled and run, infected applications do likewise.
  4. also attacks Java source code. It compiles the infected source code and restores it to its original state, leaving only infected class files, which do likewise. It also makes applets able to run as applications, increasing the odds of further infection.
Of course these last two Java viruses are necessarily slow in their activities and obvious in their effects; they stand no chance of being successful in the wild and simply serve to illustrate the possibilities.

A more realistic example is provided by This application attacks Java class files directly, inserting the necessary byte code to make them viral and making all of the adjustments needed to insure that infected class files continue to pass the Verifier and run as before. ClassHacker uses the same techniques for manipulating class files as the examples of the preceding section, and the amount of code that it inserts is about 2K bytes. Though it acts swiftly, the viral code makes no effort to conceal or defend itself, and so it is unlikely to be very successful.

While the examples of Java Platform viruses considered here are not particularly threatening, more virulent threats are sure to arise in the future. The power of byte code beyond the Java language and the ease with which class files can be attacked virtually guarantee this. The evil that hostile byte code can perpetrate is limited only by the power of the Java Virtual Machine and human ingenuity. At the very least, the JVM will require integrity maintenance mechanisms to defend its crucial classes from tampering, and users will need to be enabled to better defend the files in their CLASSPATHs. Additional levels of trusted code in the Java programming environment will complicate matters and may create more problems than they solve. Key management is sure to come under attack by hostile byte code, and the stakes will be higher. One misapplication of trust can lead to a total breakdown of all Java-related security. As long as there remains such a vast gulf between the Java programming language and the byte code accepted by the Java Verifier, nothing can be asserted with any certainty about Java Security. Given the complete independence of the Java language and the class file format, bridging this gulf is going to be a very difficult task. Thus it seems extremely likely that the problems will continue as Java grows up and moves from its childhood sandbox to a much harsher adulthood.