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The Java classes used by a program are not necessarily loaded upon program startup. Many JVMs load classes only when they need them.
If untrusted code is permitted to load classes, it may possess the ability to load sensitive classes required by trusted code. If the trusted code has not already loaded these classes, attempts to subsequently do so may result in untrusted classes being substituted for the sensitive classes. Therefore, if a program permits untrusted code to load classes, it must first 'preload' any sensitive classes it needs. Once properly loaded, these sensitive classes may not be 'overridden' by untrusted code.
This text comes from SCG 2007, it is instructional and maybe useful in an introduction. ~DS
When any method from the following table 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. ([SCG 2007])
APIs capable of bypassing SecurityManager's checks |
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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 on which the newInstance()
method is being invoked is referred to as the Class
object (classObjectName.newInstance()
). According to the Java Language Specification [[JLS 2005]], the method getClass()
returns the Class
object that represents the class of the object.
If a security manager is present, untrusted code that does not have the permissions to use the API directly is prevented 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 delegation ancestor of the class loader of the object on which the API is invoked. Consequently, untrusted callers who do not have the required permissions but are capable of passing the class loader check are able to perform sensitive operations if the trusted code invokes these APIs on their behalf.
The Java SE 6 class loader delegation hierarchy shown below summarizes several cases and highlights those where security checks are bypassed:
Immediate Caller |
ICC* |
Class Object |
COC** |
Class Loader Check |
Security Check |
---|---|---|---|---|---|
C1 |
A |
C2, C3, C4, C5 |
Application, B, C |
A is not a delegation ancestor of Application, B or C |
Yes |
C2 |
Application |
C1 |
A |
Application is not a delegation ancestor of A |
Yes |
C2 |
Application |
C3, C4, C5 |
B and C |
Application is a delegation ancestor of B and C |
No |
C3 |
B |
C4 |
B |
The class loader is same for C3 and C4 (B) |
No |
C4 |
B |
C3 |
B |
The class loader is same for C4 and C3 (B) |
No |
C5 |
C |
C1, C2, C3, C4 |
Application, A, B, C |
C is not a delegation ancestor of Application, A, B or C |
Yes |
* ICC: Intermediate caller's class loader
** COC: Class object's class loader
Care must be taken when using these APIs that trusted code does 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 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 untrusted code from indirectly creating the class instance (untrusted code must pass the class loader comparison check).
Similarly, instances of trusted Class
objects should not be returned to untrusted code. An untrusted caller can invoke the affected APIs and bypass security checks if its 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
class object. Class loaders 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 untrusted class loaders for performing operations of sensitive nature in trusted code can result in vulnerabilities.
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 current ClassLoader
object's class loader or if the code in the current execution context has the getClassLoader
RunTimePermission
.
Untrusted code can bypass the security checks if its class loader is either the same or a delegation ancestor of the trusted code's class loader. Consequently, care should be taken while specifying the parent of a trusted class loader. Likewise, trusted code is forbidden to use any class loader instance supplied by untrusted code. For instance, a class loader instance obtained from untrusted code may never be used to load a trusted class that performs some sensitive operation. Also, a trusted class loader that performs security sensitive operations must never be made available to untrusted code by returning its instance.
Noncompliant Code Example (Tomcat)
This noncompliant code example illustrates a vulnerability present in several versions of the Tomcat HTTP web server (fixed in v 6.0.20), that allows untrusted web applications to override the default XML parser used by the system to process web.xml, context.xml and tld files of other web applications deployed on the Tomcat instance. Consequently, untrusted web applications that install a parser could view and/or alter these files under limited circumstances.
The noncompliant code example shows the code associated with initialiation of a new 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 2009]]. The code to initialize the Digester
follows:
protected static Digester webDigester = null; if (webDigester == null) { webDigester = createWebDigester(); }
The createWebDigester()
method is responsible for creating the Digester
. This method internally calls createWebXMLDigester()
, which invokes the method DigesterFactory.newDigester()
. Thie method creates the new digester instance and sets a boolean
flag useContextClassLoader
to true
.
// This method exists in the class DigesterFactory and is called by ContextConfig.createWebXmlDigester() // which is in turn called by ContextConfig.createWebDigester() // webDigester finally contains the value of digester defined in this method public static Digester newDigester(boolean xmlValidation, boolean xmlNamespaceAware, RuleSet rule) { Digester digester = new Digester(); // ... digester.setUseContextClassLoader(true); // ... return digester; }
The useContextClassLoader
flag is used by Digester
to decide which ClassLoader
to use when loading new classes. When true, it uses the WebappClassLoader
, which is untrusted, because it loads whatever classes are requested by various web applications.
public ClassLoader getClassLoader() { // ... if (this.useContextClassLoader) { // Uses the context class loader which was previously set to the WebappClassLoader ClassLoader classLoader = Thread.currentThread().getContextClassLoader(); } return classloader; }
Later, the Digester.getParser()
method is internally called by Tomcat to process web.xml and other files:
// Digester.getParser() calls this method. It is defined in class Digester public SAXParserFactory getFactory() { if (factory == null) { factory = SAXParserFactory.newInstance(); // Uses WebappClassLoader // ... } return (factory); }
The underlying problem is that the newInstance()
method is being invoked on behalf of a web application's class loader, the WebappClassLoader
, and it loads classes before Tomcat has loaded all the classes it needs. If a web application has loaded its own javax.xml.parsers.SAXParserFactory
, then when Tomcat tries to access a SAXParserFactory
, it will access the incorrect SaxParserFactory
used by the web app, rather than the standard Java SAXParserFactory
that it depends on.
Compliant Solution (Tomcat)
In this compliant solution, Tomcat initializes the SAXParserFactory
when it creates the Digester
. This guarantees that the SAXParserFactory
is constructed using the container's class loader, rather than the WebappClassLoader
.
The webDigester
is also marked final. This prevents any subclasses from assigning a new object reference to webDigester
. See [OBJ04-J. Do not use public static non-final variables] for more information. It also prevents a race condition where another thread could access webDigester
before it is fully initialized; see [OBJ05-J. Prevent access to partially initialized objects] for more information.
protected static final Digester webDigester = init(); protected Digester init() { Digester digester = createWebDigester(); digester.getParser(); // Does not use the context Classloader at initialization, so safe return digester; }
Later, even if the Tomcat server continues to use the WebappClassLoader
to create the parser instance when attempting to process the web.xml and other files, the explicit call to getParser()
in init()
ensures that the default parser has been set during prior initialization and is impossible to replace. Because this is a one-time setting, future attempts to change the parser are futile.
Note that the Class.newInstance()
method requires the class to contain a no-argument constructor. If this requirement is not satisfied, a runtime exception results, which indirectly prevents a security breach.
Risk Assessment
Allowing untrusted code to load classes enables untrusted code to replace benign classes with malicious classes.
Rule |
Severity |
Likelihood |
Remediation Cost |
Priority |
Level |
---|---|---|---|---|---|
SEC04-J |
high |
probable |
medium |
P12 |
L1 |
Related Vulnerabilities
Search for vulnerabilities resulting from the violation of this rule on the CERT website.
Bibliography
[[CVE 2008]] |
|
[[Gong 2003]] |
Section 4.3.2, Class Loader Delegation Hierarchy |
[[JLS 2005]] |
Section 4.3.2, "The Class |
[[SCG 2007]] |
Guideline 6-2 Safely invoke standard APIs that bypass SecurityManager checks depending on the immediate caller's class loader |
[[Tomcat 2009]] |
Bug ID 29936 |
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