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The
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Java
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classes
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used
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by
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a
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program
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are
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not
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necessarily
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loaded
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upon
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program
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startup.
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Many Java Virtual Machines (JVMs) load classes only when they need them.
If untrusted code is permitted to load classes, it may possess the ability to load a malicious class. This is a class that shares a fully-qualified name with a benign class that is required by trusted code. When the trusted code tries to load its benign class, the JVM provides it with the malicious class instead. As a result, if a program permits untrusted code to load classes, it must first preload any benign classes it needs. Once loaded, these benign classes cannot be replaced by untrusted code.
Noncompliant Code Example (Tomcat)
This noncompliant code example shows a vulnerability present in several versions of the Tomcat HTTP web server (fixed in version 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 tag library descriptor (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 certain circumstances.
The noncompliant code example shows the code associated with initialization of a new Digester instance in the org.apache.catalina.startup.ContextConfig
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class.
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"A
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Digester
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processes
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an
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XML
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input
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stream
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by
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matching
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a
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series
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of
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element
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nesting
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patterns
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to
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execute
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Rules
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that
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have
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been
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added
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prior
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to
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the
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start
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of
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parsing"
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[
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...
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].
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The
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code
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to
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initialize
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the
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Digester
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follows:
| Code Block | ||||
|---|---|---|---|---|
| =
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} protected static Digester webDigester = null; if (webDigester == null) { webDigester = createWebDigester(); } {code} The {{ |
The createWebDigester()
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method
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is
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responsible
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for
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creating
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the
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Digester
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.
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This
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method
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calls
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createWebXMLDigester()
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,
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which
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invokes
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the
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method
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DigesterFactory.newDigester(). This method creates the new digester instance and sets a boolean flag useContextClassLoader to true.
| Code Block | ||
|---|---|---|
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}}. Thie method creates the new digester instance and sets a {{boolean}} flag {{useContextClassLoader}} to {{true}}. {code:bgColor=#FFCCCC} // 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; } {code} 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. {code:bgColor=#FFCCCC} |
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.
| Code Block | ||
|---|---|---|
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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;
}
{code}
Later, the {{ |
The Digester.getParser()
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method
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is
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subsequently called
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by
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Tomcat
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to
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process
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web.xml
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and
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other
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files:
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} // Digester.getParser() calls this method. It is defined in class Digester public SAXParserFactory getFactory() { if (factory == null) { factory = SAXParserFactory.newInstance(); // Uses WebappClassLoader // ... } return (factory); } {code} |
The
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underlying
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problem
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is
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that
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the
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newInstance()
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method
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is
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being
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invoked
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on
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behalf
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of
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a
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web
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application's
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class
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loader,
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the
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WebappClassLoader
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,
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and
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it
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loads
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classes
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before
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Tomcat
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has
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loaded
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all
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the
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classes
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it
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needs.
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If
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a
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web
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application
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has
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loaded
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its
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own
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Trojan javax.xml.parsers.SAXParserFactory
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,
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when
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Tomcat
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tries
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to
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access
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a
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SAXParserFactory
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,
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it accesses the Trojan SaxParserFactory installed by the web application rather than the standard Java SAXParserFactory that Tomcat depends on.
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.
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 declared final. This prevents any subclasses from assigning a new object reference to webDigester. (See rule OBJ10-J. Do not use public static nonfinal fields for more information.) It also prevents a race condition where another thread could access webDigester before it is fully initialized. (See rule OBJ11-J. Be wary of letting constructors throw exceptions for more information.)
| Code Block | ||
|---|---|---|
| ||
will access the incorrect {{SaxParserFactory}} used by the web app, rather than the standard Java {{SAXParserFactory}} that it depends on. h2. 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 [OBJ10-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 [OBJ11-J. Prevent access to partially initialized objects] for more information. {code:bgColor=#ccccff} protected static final Digester webDigester = init(); protected Digester init() { Digester digester = createWebDigester(); digester.getParser(); // Does not use the context Classloader at initialization, so safe digester.getParser(); return digester; } {code} 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. h2. 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 | {color:red}{*}P12{*}{color} | {color:red}{*}L1{*}{color} | h3. Related Vulnerabilities Search for vulnerabilities resulting from the violation of this rule on the [CERT website|https://www.kb.cert.org/vulnotes/bymetric?searchview&query=FIELD+KEYWORDS+contains+SEC02-J]. h2. Bibliography | \[[CVE 2008|AA. Bibliography#CVE 08]\] | [CVE-2009-0783|http://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2009-0783] | | \[[Gong 2003|AA. Bibliography#Gong 03]\] | Section 4.3.2, Class Loader Delegation Hierarchy | | \[[JLS 2005|AA. Bibliography#JLS 05]\] | Section 4.3.2, "The Class {{Object}}" | | \[[SCG 2007|AA. Bibliography#SCG 07]\] | Guideline 6-2 Safely invoke standard APIs that bypass SecurityManager checks depending on the immediate caller's class loader | | \[[Tomcat 2009|AA. Bibliography#Tomcat 09]\] | [Bug ID 29936|https://issues.apache.org/bugzilla/show_bug.cgi?id=29936], API Class {{ |
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 cannot be replaced. Because this is a one-time setting, future attempts to change the parser are futile.
Risk Assessment
Allowing untrusted code to load classes enables untrusted code to replace benign classes with Trojan classes.
Rule | Severity | Likelihood | Remediation Cost | Priority | Level |
|---|---|---|---|---|---|
SEC03-J | high | probable | medium | P12 | L1 |
Automated Detection
| Tool | Version | Checker | Description |
|---|---|---|---|
| Parasoft Jtest | 9.5 | CERT.SEC03.ACL | Do not access the class loader in a web component |
Related Guidelines
Secure Coding Guidelines for the Java Programming Language, Version 3.0 | Guideline 6-3. Safely invoke standard APIs that bypass |
Android Implementation Details
On Android, the use of DexClassLoader or PathClassLoader requires caution.
Bibliography
[CVE 2011] | |
Section 4.3.2, Class Loader Delegation Hierarchy | |
[JLS 2005] | §4.3.2, The Class |
Bug ID 29936, API Class |
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