Code is usually signed when it requires more than the default set of permissions to perform some tasks. Although it is often a bad idea to sign code (c.f. see ENV00-J. Do not sign code that performs only unprivileged operations), some actions necessitate require this step. For example, if an application requires an http HTTP connection with an external host to download plugins or extensions, its vendor may provide signed code rather than requiring that the user deal with complex security policies. Because executing signed code can be extremely dangerous, verifying the authenticity of its origin is of utmost importance.
Java based technologies typically use the Java Archive (JAR) feature for packaging files to facilitate platform independent deployment. JAR files are the preferred means of distribution for Enterprise Java Beans (EJB), MIDlets (J2ME) and Weblogic Server J2EE applications, for example. The point and click installation provided by Java Web Start also relies on the JAR file format for packaging. Vendors sign their JAR files when required. This certifies the authenticity of the code; it cannot guarantee the safety security of the code.
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According to the Java Tutorials \[[Tutorials 2008|AA. Bibliography#Tutorials 08]\] |
If you are creating applet code that you will sign, it needs to be placed in a JAR file. The same is true if you are creating application code that may be similarly restricted by running it with a security manager. The reason you need the JAR file is that when a policy file specifies that code signed by a particular entity is permitted one or more operations, such as specific file reads or writes, the code is expected to come from a signed JAR file. (The term "signed code" is an abbreviated way of saying "code in a class file that appears in a JAR file that was signed.")
Client code may lack programatic programmatic checks of code signatures. For example, any instances of URLClassLoader and its subclasses and java.util.jar automatically verify signatures of signed JAR files. Developer-implemented custom classloaders class loaders that subclass java.lang.ClassLoader may lack this check. Moreover, even in the URLClassLoader case, the automatic verification performs only an integrity check; it fails to authenticate the loaded class because the check uses a public key contained within the JAR. The legitimate JAR file may be replaced with a malicious JAR file containing a different public key along with appropriately modified digest values.
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This noncompliant code example demonstrates the JarRunner application which can be used to dynamically execute a particular class residing within a JAR file (abridged version of the class in The Java Tutorials [Tutorials 2008]). It creates a JarClassLoader that loads an application update, plugin plug-in or patch over an untrusted network such as the Internet. The URL to fetch the code is specified as the first argument (for example, http://somewebsite.comwww.securecoding.cert.org/software-updates.jar
); any other arguments specify the arguments that are to be passed to the class that is loaded. JarRunner uses reflection to invoke the main method of the loaded class. Unfortunately, by default, JarClassLoader verifies the signature using the public key contained within the JAR file.
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public class JarRunner {
public static void main(String[] args) throws IOException,
ClassNotFoundException, NoSuchMethodException, InvocationTargetException {
URL url = new URL(args[0]);
// Create the class loader for the application jar file
JarClassLoader cl = new JarClassLoader(url);
// Get the application's main class name
String name = cl.getMainClassName();
// Get arguments for the application
String[] newArgs = new String[args.length - 1];
System.arraycopy(args, 1, newArgs, 0, newArgs.length);
// Invoke application's main class
cl.invokeClass(name, newArgs);
}
}
final class JarClassLoader extends URLClassLoader {
private URL url;
public JarClassLoader(URL url) {
super(new URL[] { url });
this.url = url;
}
public String getMainClassName() throws IOException {
URL u = new URL("jar", "", url + "!/");
JarURLConnection uc = (JarURLConnection) u.openConnection();
Attributes attr = uc.getMainAttributes();
return attr != null ? attr.getValue(Attributes.Name.MAIN_CLASS) : null;
}
public void invokeClass(String name, String[] args)
throws ClassNotFoundException, NoSuchMethodException,
InvocationTargetException {
Class c = loadClass(name);
Method m = c.getMethod("main", new Class[] { args.getClass() });
m.setAccessible(true);
int mods = m.getModifiers();
if (m.getReturnType() != void.class || !Modifier.isStatic(mods)
|| !Modifier.isPublic(mods)) { throw new NoSuchMethodException("main"); }
try {
m.invoke(null, new Object[] { args });
} catch (IllegalAccessException e) { System.out.println("Access denied"); }
}
}
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<ac:structured-macro ac:name="unmigrated-wiki-markup" ac:schema-version="1" ac:macro-id="a9bc9bfc3064093d-35d8c43f-48e14625-84059fbe-228e9ce4e447b5c2bad87657"><ac:plain-text-body><![CDATA[ | [ISO/IEC TR 24772:2010 | http://www.aitcnet.org/isai/] | "Improperly Verified Signature [XZR]" | ]]></ac:plain-text-body></ac:structured-macro> |
CWE ID 300, "Channel Accessible by Non-Endpoint (aka 'Man-in-the-Middle')" | ||||
| CWE ID 319, "Cleartext Transmission of Sensitive Information" | |||
| CWE ID 494, "Download of Code Without Integrity Check" | |||
| CWE ID 347, "Improper Verification of Cryptographic Signature" |
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<ac:structured-macro ac:name="unmigrated-wiki-markup" ac:schema-version="1" ac:macro-id="2e75c951c967d2b9-3a00591b-413544f4-ba1a965d-e7dfb0cb08144d7eca0850ce"><ac:plain-text-body><![CDATA[ | [[API 2006 | AA. Bibliography#API 06]] |
| ]]></ac:plain-text-body></ac:structured-macro> | |||
<ac:structured-macro ac:name="unmigrated-wiki-markup" ac:schema-version="1" ac:macro-id="851d2d49c992db65-0d99f35b-487e4464-a1e6879e-d703e283aa8a88b0a07b3c66"><ac:plain-text-body><![CDATA[ | [[Bea 2008 | AA. Bibliography#Bea 08]] |
| ]]></ac:plain-text-body></ac:structured-macro> | |||
<ac:structured-macro ac:name="unmigrated-wiki-markup" ac:schema-version="1" ac:macro-id="1f892fbe247e4aa1-6b95c2dc-4c5942c9-a9e79c92-c6f7b3f22b9f42a6a7da231d"><ac:plain-text-body><![CDATA[ | [[Eclipse 2008 | AA. Bibliography#Eclipse 08]] | [JAR Signing | http://wiki.eclipse.org/JAR_Signing] and [Signed bundles and protecting against malicious code | http://help.eclipse.org/stable/index.jsp?topic=/org.eclipse.platform.doc.isv/guide] | ]]></ac:plain-text-body></ac:structured-macro> | |
<ac:structured-macro ac:name="unmigrated-wiki-markup" ac:schema-version="1" ac:macro-id="b426907bbaebf968-facc97dc-4eae4ade-a10ea1e3-6b3935245b8f67bde04baa23"><ac:plain-text-body><![CDATA[ | [[Fairbanks 07 | AA. Bibliography#Fairbanks 07]] |
| ]]></ac:plain-text-body></ac:structured-macro> | |||
<ac:structured-macro ac:name="unmigrated-wiki-markup" ac:schema-version="1" ac:macro-id="3da0a5daecaef3c3-7890550a-4b6449d9-8f50b963-1fbb9a38448af889fd551884"><ac:plain-text-body><![CDATA[ | [[Flanagan 2005 | AA. Bibliography#Flanagan 05]] | Chapter 24. The java.util.jar Package | ]]></ac:plain-text-body></ac:structured-macro> | |||
<ac:structured-macro ac:name="unmigrated-wiki-markup" ac:schema-version="1" ac:macro-id="7c36ceebd2d59850-77b9edf2-4b164cd9-a2f1ad9d-cb794251554a5f8e725ce324"><ac:plain-text-body><![CDATA[ | [[Gong 2003 | AA. Bibliography#Gong 03]] | 12.8.3 jarsigner | ]]></ac:plain-text-body></ac:structured-macro> | |||
<ac:structured-macro ac:name="unmigrated-wiki-markup" ac:schema-version="1" ac:macro-id="f7171dcc0cfa0a3f-874e06c6-4cf6489e-9ce58c6f-71fef9ab06ee6e58aff9172c"><ac:plain-text-body><![CDATA[ | [[Halloway 2001 | AA. Bibliography#Halloway 01]] |
| ]]></ac:plain-text-body></ac:structured-macro> | |||
<ac:structured-macro ac:name="unmigrated-wiki-markup" ac:schema-version="1" ac:macro-id="0b3cb92d76c151d2-8260c8b3-4ef647d5-ab83bce4-6b8e4f86061763c33b8d5172"><ac:plain-text-body><![CDATA[ | [[JarSpec 2008 | AA. Bibliography#JarSpec 08]] | Signature Validation |
| ]]></ac:plain-text-body></ac:structured-macro> | ||
<ac:structured-macro ac:name="unmigrated-wiki-markup" ac:schema-version="1" ac:macro-id="3c600e55226d1c2f-92a7fee9-4d0c48c8-82db91fe-5c339d5cb5988468dfbb0959"><ac:plain-text-body><![CDATA[ | [[Oaks 2001 | AA. Bibliography#Oaks 01]] | Chapter 12: Digital Signatures, Signed Classes | ]]></ac:plain-text-body></ac:structured-macro> | |||
<ac:structured-macro ac:name="unmigrated-wiki-markup" ac:schema-version="1" ac:macro-id="dad10e70c057fab0-24b6ccbb-49294d2f-b33088b6-cd48cff5c67b345d8b39d40c"><ac:plain-text-body><![CDATA[ | [[Muchow 2001 | AA. Bibliography#Muchow 01]] |
| ]]></ac:plain-text-body></ac:structured-macro> | |||
<ac:structured-macro ac:name="unmigrated-wiki-markup" ac:schema-version="1" ac:macro-id="b36bf057dc808c77-860331a8-4ad54b02-9fcc9b28-78a122fa626d530780f79feb"><ac:plain-text-body><![CDATA[ | [[Tutorials 2008 | AA. Bibliography#Tutorials 08]] | [The JarRunner Class | http://java.sun.com/docs/books/tutorial/deployment/jar/jarrunner.html], [Lesson: API and Tools Use for Secure Code and File Exchanges | http://java.sun.com/docs/books/tutorial/security/sigcert/index.html] and [Verifying Signed JAR Files | http://java.sun.com/docs/books/tutorial/deployment/jar/verify.html] | ]]></ac:plain-text-body></ac:structured-macro> |
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