Most methods lack security manager checks because they do not provide access to sensitive parts of the system, such as the file system. Most methods that do provide security manager checks verify that every class and method in the call stack is authorized before they proceed. This security model allows restricted programs, such as Java applets, to have full access to the core Java library. It also prevents a sensitive method from acting on behalf of a malicious method that hides behind trusted methods in the call stack.
However, certain methods use a reduced-security check that checks only that the calling method is authorized rather than checking every method in the call stack. Any code that invokes these methods must guarantee that they cannot be invoked on behalf of untrusted code. These methods are listed in the following table.
Methods That Check the Calling Method Only
getAccessible() methods are used to instruct the Java Virtual Machine (JVM) to override the language access checks, they perform standard (and more restrictive) security manager checks and consequently lack the vulnerability described by this guideline. Nevertheless, these methods should also be used with extreme caution. The remaining
get* field reflection methods perform only the language access checks and are consequently vulnerable.
Class loaders allow a Java application to be dynamically extended at runtime by loading additional classes. For each class that is loaded, the JVM tracks the class loader that was used to load the class. When a loaded class first refers to another class, the virtual machine requests that the referenced class be loaded by the same class loader that was used to load the referencing class. Java's class loader architecture controls interaction between code loaded from different sources by allowing the use of different class loaders. This separation of class loaders is fundamental to the separation of code: it prevents malicious code from gaining access to and subverting trusted code.
Several methods that are charged with loading classes delegate their work to the class loader of the class of the method that called them. The security checks associated with loading classes are performed by class loaders. Consequently, any method that invokes one of these class loading methods must guarantee that these methods cannot act on behalf of untrusted code. These methods are listed in the following table.
Methods That Use the Calling Method's Class Loader
With the exception of the
load() methods, the tabulated methods do not perform any security manager checks; they delegate security checks to the appropriate class loader.
In practice, the trusted code's class loader frequently allows these methods to be invoked, whereas the untrusted code's class loader may lack these privileges. However, when the untrusted code's class loader delegates to the trusted code's class loader, the untrusted code gains visibility to the trusted code. In the absence of such a delegation relationship, the class loaders would ensure namespace separation; consequently, the untrusted code would be unable to observe members or to invoke methods belonging to the trusted code.
The class loader delegation model is fundamental to many Java implementations and frameworks. Avoid exposing the methods listed in the preceding tables to untrusted code. Consider, for example, an attack scenario where untrusted code is attempting to load a privileged class. If its class loader lacks permission to load the requested privileged class on its own, but the class loader is permitted to delegate the class loading to a trusted class's class loader, privilege escalation can occur. Furthermore, if the trusted code accepts tainted inputs, the trusted code's class loader could be persuaded to load privileged, malicious classes on behalf of the untrusted code.
Classes that have the same defining class loader will exist in the same namespace, but they can have different privileges depending on the security policy. Security vulnerabilities can arise when privileged code coexists with unprivileged code (or less privileged code) that was loaded by the same class loader. In this case, the less privileged code can freely access members of the privileged code according to the privileged code's declared accessibility. When the privileged code uses any of the tabulated APIs, it bypasses security manager checks (with the exception of
This guideline is similar to SEC03-J. Do not load trusted classes after allowing untrusted code to load arbitrary classes. Many examples also violate SEC00-J. Do not allow privileged blocks to leak sensitive information across a trust boundary.
Noncompliant Code Example
In this noncompliant code example, a call to
System.loadLibrary() is embedded in a
This code is insecure because it could load a library on behalf of untrusted code. In essence, the untrusted code's class loader may be able to use this code to load a library even though it lacks sufficient permissions to do so directly. After loading the library, the untrusted code can call native methods from the library, if those methods are accessible, because the
doPrivileged block stops any security manager checks from being applied to callers further up the execution stack.
Nonnative library code can also be susceptible to related security flaws. Suppose there exists a library that contains a vulnerability that is not directly exposed, perhaps because it lies in an unused method. Loading this library may not directly expose a vulnerability. However, an attacker could then load an additional library that exploits the first library's vulnerability. Moreover, nonnative libraries often use
doPrivileged blocks, making them attractive targets.
This compliant solution hard codes the name of the library to prevent the possibility of tainted values. It also reduces the accessibility of the
load() method from
private. Consequently, untrusted callers are prohibited from loading the
Noncompliant Code Example
This noncompliant code example returns an instance of
java.sql.Connection from trusted to untrusted code.
Untrusted code that lacks the permissions required to create a SQL connection can bypass these restrictions by using the acquired instance directly. The
getConnection() method is unsafe because it uses the
url argument to indicate a class to be loaded; this class serves as the database driver.
This compliant solution prevents malicious users from supplying their own URL to the database connection, thereby limiting their ability to load untrusted drivers.
Noncompliant Code Example (CERT Vulnerability 636312)
CERT Vulnerability Note VU#636312 describes a vulnerability in Java 1.7.0 update 6 that was widely exploited in August 2012. The exploit actually used two vulnerabilities; the other one is described in SEC05-J. Do not use reflection to increase accessibility of classes, methods, or fields.)
The exploit runs as a Java applet. The applet class loader ensures that an applet cannot directly invoke methods of classes present in the
com.sun.* package. A normal security manager check ensures that specific actions are allowed or denied depending on the privileges of all of the caller methods on the call stack (the privileges are associated with the code source that encompasses the class).
The first goal of the exploit code was to access the private
sun.awt.SunToolkit class. However, invoking
class.forName() directly on the name of this class would cause a
SecurityException to be thrown. Consequently, the exploit code used the following method to access any class, bypassing the security manager:
java.beans.Expression.execute() method delegates its work to the following method:
com.sun.beans.finder.ClassFinder.resolveClass() method delegates its work to the
Although this method is called in the context of an applet, it uses
Class.forName() to obtain the requested class.
Class.forName() delegates the search to the calling method's class loader. In this case, the calling class (
com.sun.beans.finder.ClassFinder) is part of core Java, so the trusted class loader is used in place of the more restrictive applet class loader, and the trusted class loader loads the class, unaware that it is acting on behalf of malicious code.
Compliant Solution (CVE-2012-4681)
Oracle mitigated this vulnerability in Java 1.7.0 update 7 by patching the
com.sun.beans.finder.ClassFinder.findClass() method. The
checkPackageAccess() method checks the entire call stack to ensure that
Class.forName(), in this instance only, fetches classes only on behalf of trusted methods.
Noncompliant Code Example (CVE-2013-0422)
Java 1.7.0 update 10 was widely exploited in January 2013 because of several vulnerabilities. One vulnerability in the
MBeanInstantiator class granted unprivileged code the ability to access any class regardless of the current security policy or accessibility rules. The
MBeanInstantiator.findClass() method could be invoked with any string and would attempt to return the
Class object named after the string. This method delegated its work to the
loadClass() method, whose source code is shown here:
This method delegates the task of dynamically loading the specified class to the
Class.forName() method, which delegates the work to its calling method's class loader. Because the calling method is
MBeanInstantiator.loadClass(), the core class loader is used, which provides no security checks.
Compliant Solution (CVE-2013-0422)
Oracle mitigated this vulnerability in Java 1.7.0 update 11 by adding an access check to the
MBeanInstantiator.loadClass() method. This access check ensures that the caller is permitted to access the class being sought:
Allowing untrusted code to invoke methods with reduced-security checks can result in privilege escalation. Likewise, allowing untrusted code to perform actions using the immediate caller's class loader may allow the untrusted code to execute with the same privileges as the immediate caller.
Methods that avoid using the immediate caller's class loader instance fall outside the scope of this guideline. For example, the three-argument
java.lang.Class.forName() method requires an explicit argument that specifies the class loader instance to use.
Do not use the immediate caller's class loader as the third argument when instances must be returned to untrusted code.