EXP00-J. Do not ignore values returned by methods

Methods return values to communicate failure or success and at other times, to update the caller's objects or fields. Security risks may arise when method return values are ignored or when the invoking method fails to take suitable action on their receipt. When getter methods are named after an action, such as ProcessBuilder.redirectErrorStream() for example, a programmer may fail to realize that a return value is expected. Note that the only purpose of the redirectErrorStream() method is report via its return value whether the process builder merges standard error and standard output; the method that actually performs redirection of the error stream is the overloaded single-argument version. It is important to read the API documentation so that return values are not ignored.

Noncompliant Code Example (File deletion)

This noncompliant code example attempts to delete a file, but fails to check whether the operation has succeeded.

  File someFile = new File("someFileName.txt");
  // do something with someFile
  someFile.delete();

Compliant Solution

This compliant solution checks the (boolean) value returned by the delete() method and handles errors as appropriate.

  File someFile = new File("someFileName.txt");
  // do something with someFile
  if (!someFile.delete()) {
    // handle failure to delete the file
  }

Noncompliant Code Example (String replacement)

This noncompliant code example ignores the return value of the String.replace method, and so fails to update the original string. The String.replace() method cannot modify the state of the String (because String objects are immutable); rather, it returns a reference to a new String object containing the desired result.

public class Ignore {
  public static void main(String[] args) {
    String original = "insecure";
    original.replace( 'i', '9' );
    System.out.println(original);
  }
}

Compliant Solution

This compliant solution correctly updates the String reference original with the return value from the String.replace method.

public class DoNotIgnore {
  public static void main(String[] args) {
    String original = "insecure";
    original = original.replace( 'i', '9' );
    System.out.println(original);
  }
}

See also guideline [FIO02-J. Keep track of bytes read and account for character encoding while reading data].

Risk Assessment

Ignoring method return values can lead to unanticipated program behavior.

Automated Detection

The Coverity Prevent Version 5.0 CHECKED_RETURN checker can detect the instance where Value returned from a function is not checked for errors before being used.

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this guideline on the CERT website.

Other Languages

This guideline appears in the C Secure Coding Standard as [seccode:EXP12-C. Do not ignore values returned by functions].

This guideline appears in the C++ Secure Coding Standard as [cplusplus:EXP12-CPP. Do not ignore values returned by functions or methods].

Bibliography

[[API 2006]] method delete() and method replace()
[[Green 2008]] "String.replace"
[[MITRE 2009]] CWE ID 252 "Unchecked Return Value"
[[Pugh 2009]] misusing putIfAbsent

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