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When Java source code is compiled, it is converted into bytecode, saved in one or more class files, and executed by the Java Virtual Machine (JVM). Java class files may be compiled on one machine and executed on another machine. A properly generated class file is said to be conforming. When the JVM loads a class file, it has no way of knowing whether the class file is conforming. The class file could have been created by some other process, or an attacker may have tampered with a conforming class file.

The Java bytecode verifier is an internal component of the JVM and that is responsible for detecting non-confirming nonconforming Java codebytecode. It performs tasks such as ensuring ensures that the class file is in the proper Java class format, that illegal type casts are not performed and preventing operand stack overflows or underflows.

Non-Compliant Code Example

are avoided, that operand stack underflows are impossible, and that each method eventually removes from the operand stack everything pushed by that method.

Users often assume that Java class files obtained from a trustworthy source will be conforming and, consequently, safe for execution. This belief can erroneously lead them to see bytecode verification as a superfluous activity for such classes. Consequently, they might disable bytecode verification, undermining Java's safety and security guarantees. The bytecode verifier must not be suppressed.

Noncompliant Code Example

The bytecode verification process runs by default. The -Xverify:none flag on the JVM command line suppresses the verification process. This noncompliant code example uses the flag to disable bytecode verification:In the non-compliant code snippet shown below, two classes, Ssn and SsnVerify are defined. If at some later time, the programmer changes the access modifier of the ssn field from public to private, a possibility exists that only the modified Ssn class is recompiled but the SsnVerify class is overlooked. As a result, SsnVerify can now illegally access the private ssn field of the Ssn class.

package ssnvault.values;
public class Ssn {
  public String ssn = "001 01 0001";
}

package ssnvault.values;
public class SsnVerify {
  public static void main(String[] args) {
    Ssn number = new Ssn();
    System.out.println("Please enter last four digits of your SSN:");
    //perform verification
  }
}java -Xverify:none ApplicationName

Compliant Solution

It is vital to re-compile both Ssn and SsnVerify classes so that the bytecode verifier can be applied to detect the non-conforming code.

Most JVM implementations perform bytecode verification by default; it is also performed during dynamic class loading.

Specifying the -Xverify:all flag on the command line requires the JVM to enable bytecode verification (even when it would otherwise have been suppressed), as shown in this compliant solution:

java -Xverify:all ApplicationName

Exceptions

ENV04-J-EX0: The verification process is automatically initiated unless the -noverify flag is specified at command line. On Java 2 systems, classes loaded by the primordial class loader (that loads classes is permitted to omit bytecode verification of classes loaded from the boot class path) are not required to perform bytecode verification.

References

. These system classes are protected through platform and file system protections rather than by the bytecode verification process.

Risk Assessment

Bytecode verification ensures that the bytecode contains many of the security checks mandated by the Java Language Specification. Omitting the verification step could permit execution of insecure Java code.

Automated Detection

Static checking of this rule is not feasible in the general case.

Android Implementation Details

Under the default settings, bytecode verification is enabled on the Dalvik VM. To change the settings, use the adb shell to set the appropriate system property: for example, adb shell setprop dalvik.vm.dexopt-flags v=a or pass -Xverify:all as an argument to the Dalvik VM.

Bibliography

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Image Added Image Added Image AddedJava Security, Scott Oaks pg. 50, The Bytecode Verifier
Enterprise Security: Building Secure J2EE Applications, 7.3 The Class File Verifier