SEC36-J. Ensure that the bytecode verifier is applied to all involved code upon any modification

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

Noncompliant Code Example

In the noncompliant 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
  }
}

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.

Alternatively, to force bytecode verification when the unmodified class is loaded, the -verify flag can be specified on the java command line.

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 from the boot class path) are not required to perform bytecode verification.

Risk Assessment

If the bytecode verifier is not applied to all code then code could be loaded into a java system that does not conform to the Java Language Specification. This code could bypass checks that are normally expected to be performed by Java code, thereby compromising security.

Automated Detection

TODO

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

References

[[Oaks 01]] The Bytecode Verifier
[[Pistoia 04]] Section 7.3, The Class File Verifier