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The bytecode verifier is an internal component of the JVM and is responsible for detecting non-confirming Java code. It ensures that the class file is in proper format, that illegal type casts are not performed and prevents operand stack overflows or underflows. Users sometimes, assume safe runtime environments and forgo bytecode verification by disabling it. This practice is extremely dangerous.

Noncompliant Code Example

In this noncompliant code example, 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 classThe verification process is automatically initiated unless the -Xverify:none flag is specified on the command line. This noncompliant example uses this flag.

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
  }
}

Version Specific Details

The behavior described in the Noncompliant Code Example has been demonstrated in Java 1.5.0 versions running on Mac OS X Tiger, Solaris 10/Sparc64, and Ubuntu Linux 8.04, but not in Java 1.6.0 versions running on Windows XP or Linux (unless the target version of both class files is specified as 1.4 or earlier).

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.

java -Xverify:none application.java

Compliant Solution

Bytecode verification happens by default in most implementations. If it doesn't, the Alternatively, to force bytecode verification when the unmodified class is loaded, the -Xverify:all flag can be specified on the java command line.

The verification process is automatically initiated unless the -Xverify:none flag is specified on the 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. The verification is automatically performed when a classloader loads a class dynamically.

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 The code that is not subject to bytecode verification can bypass security 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

\[[Oaks 01|AA. Java References#Oaks 01]\] The Bytecode Verifier
\[[Pistoia 04|AA. Java References#Pistoia 04]\] Section 7.3, The Class File Verifier

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