Unsafe use of getResource¶

ID: java/unsafe-get-resource
Kind: problem
Security severity: 
Severity: warning
Precision: medium
Tags:
   - quality
   - reliability
   - correctness
Query suites:
   - java-security-and-quality.qls

Click to see the query in the CodeQL repository

Using the Class.getResource method is a common way of including some non-code resources with an application.

There are problems when this is called using x.getClass().getResource(), for some variable x. This is not a safe way to retrieve a resource. The method getClass returns the run-time class of x (that is, its actual, “most derived” class, rather than its declared type), which causes two potential problems:

If the run-time type of the receiving object is a subclass of the declared type and is in a different package, the resource path may be interpreted differently. According to its contract, Class.getResource qualifies non-absolute paths with the current package name, thus potentially returning a different resource or failing to find the requested resource.
Class.getResource delegates finding the resource to the class loader that loaded the class. At run time, there is no guarantee that all subclasses of a particular type are loaded by the same class loader, resulting in resource lookup failures that are difficult to diagnose.

Recommendation¶

Rather than using the getClass method, which relies on dynamic dispatch and run-time types, use class literals instead. For example, instead of calling getClass().getResource() on an object of type Foo, call Foo.class.getResource(). Class literals always refer to the declared type they are used on, removing the dependency on run-time types.

Example¶

In the following example, the calls to getPostalCodes return different results, depending on which class the call is made on: the class Address is in the package framework and the class UKAddress is in the package client.

package framework;
class Address {
    public URL getPostalCodes() {
        // AVOID: The call is made on the run-time type of 'this'.
        return this.getClass().getResource("postal-codes.csv");
    }
}

package client;
class UKAddress extends Address {
    public void convert() {
        // Looks up "framework/postal-codes.csv"
        new Address().getPostalCodes();
        // Looks up "client/postal-codes.csv"
        new UKAddress().getPostalCodes();
    }
}

In the following corrected example, the implementation of getPostalCodes is changed so that it always calls getResource on the same class.

package framework;
class Address {
    public URL getPostalCodes() {
        // GOOD: The call is always made on an object of the same type.
        return Address.class.getResource("postal-codes.csv");
    }
}

package client;
class UKAddress extends Address {
    public void convert() {
        // Looks up "framework/postal-codes.csv"
        new Address().getPostalCodes();
        // Looks up "framework/postal-codes.csv"
        new UKAddress().getPostalCodes();
    }
}

References¶

Java API Specification: Class.getResource().

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