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When one of the methods any method from the highlighted table shown below is invoked on a Class, ClassLoader or Thread object, a comparison is run between the method's immediate caller's class loader and that of the object on which the method is invoked. As an example of what constitutes the immediate caller and the object, consider the method java.lang.Class.newInstance(). Here, the immediate caller is the class that contains this method call whereas the object is called the Class object , the one on which the newInstance() method is being invoked is referred to as the Class object (classObjectName.newInstance()). If a security manager is present, untrusted code that does not have the permissions to use the API directly is also denied from indirectly using trusted code containing the API call, to perform the operation.

However, the security manager checks are bypassed if the class loader of the immediate caller is the same as or the delgation ancestor of the class loader of the object on which the API is invoked. Consequently, untrusted callers who do not have the required permissions are able to perform sensitive operations if the trusted code invokes these APIs on their behalf Wiki MarkupAccording to the Java Language Specification \[[JLS 05|AA. Java References#JLS 05]\] section 4.3.2 "The Class {{Object}}": "The method {{getClass}} returns the {{Class}} object that represents the class of the object". The first ten methods shown below can be used on a {{Class}} object.

According to the Java Language Specification \[[JLS 05|AA. Java References#JLS 05]\] section 4.3.2 "The Class {{Object}}": "The method {{getClass}} returns the {{Class}} object that represents the class of the object". 

The first ten methods shown in the table can be invoked on a Class object. Care must be taken when using these APIs. In particular, trusted code should not accept Class objects from untrusted code for further use. For example, if trusted code is loaded by the bootstrap class loader, it can create an instance of a sensitive system class by using the the newInstance() method on the Class object. If the method that creates the instance is visible to untrusted code, no security manager checks are carried out to prohibit the utrusted code from indirectly creating the class instance. Similarly, instances of trusted Class objects should not be returned to untrusted code. Security vulnerabilities can arise if the untrusted code's class loader is the same as or the delegation ancestor of the trusted code's class loader.

The table also shows APIs that use the ClassLoader object. Classloaders facilitate isolation of trusted components from untrusted ones. They also ensure that the untrusted components do not interfere with each other. The proper choice of the class loader to load a class is of utmost importance. Using less trusted class loaders for performing operations of sensitive nature in trusted code can expose security vulnerabilities.

Security With respect to the ClassLoader object APIs, security manager checks may also get bypassed depending on the immediate caller's class loader. Consider for instance, the ClassLoader.getSystemClassLoader() and ClassLoader.getParent() methods that operate on a ClassLoader object. In the presence of a security manager, these methods succeed only if the immediate caller's class loader is the delegation ancestor of the current ClassLoader object's class loader or if the immediate caller's class loader is the same as the the current ClassLoader object's class loader or if the code in the current execution context has the RunTimePermission, namely "getClassLoader".

Untrusted code can bypass the security checks if its classloader is either the same or a delegation ancestor of the current class loader. Consequently, care must be taken while specifying the parent of a trusted classloader. Likewise, trusted code should not use a classloader instance supplied by untrusted code. For instance, a class loader instance obtained from untrusted code should not be used to load a trusted class that performs some sensitive operation. Also, a trusted classloader that performs security sensitive operations should never be made available to untrusted code by returning its instance.

Noncompliant Code Example

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This noncompliant code example shows the declaration of a {{Digester}} instance in the {{org.apache.catalina.startup.ContextConfig}} class. "A {{Digester}} processes an XML input stream by matching a series of element nesting patterns to execute Rules that have been added prior to the start of parsing" \[[Tomcat 09|AA. Java References#Tomcat 09]\]. The {{createWebDigester()}} method is responsible for creating the {{Digester}}. This method internally calls {{createWebXMLDigester()}} which requests the method {{DigesterFactory.newDisternewDigester()}} to create a new digester instance and sets a {{boolean}} flag {{useContextClassLoader}} to {{true}}. This means that the context class loader, in this case the _WebappClassLoader_, is used to create the digester. Later, when the {{Digester.getParser()}} method is internally called by Tomcat to process the web.xml and other files, according to the search rules, the parser installed by the untrusted web application is preferred, otherwise, the default parser is used. The underlying problem is that the {{newInstance()}} method is being invoked on behalf of an untrusted web application's classloader.

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