SEC01-J. Do not allow tainted variables in privileged blocks

Do not operate on unvalidated or untrusted data (also known as tainted data) in a privileged block. An attacker can supply malicious input that could result in privilege escalation attacks. Appropriate mitigations include hard coding values rather than accepting arguments (when appropriate) and validating or sanitizing data before performing privileged operations (see rule IDS00-J. Sanitize untrusted data passed across a trust boundary).

Noncompliant Code Example

This noncompliant code example accepts a tainted path or file name as an argument. An attacker can access a protected file by supplying its path name as an argument to this method.

private void privilegedMethod(final String filename) 
                              throws FileNotFoundException {
  try {
    FileInputStream fis =
        (FileInputStream) AccessController.doPrivileged(
          new PrivilegedExceptionAction() {
        public FileInputStream run() throws FileNotFoundException {
          return new FileInputStream(filename);
        }
      }
    );
    // do something with the file and then close it
  } catch (PrivilegedActionException e) {
    // forward to handler
  }
}

Compliant Solution (Input Validation)

This compliant solution invokes the cleanAFilenameAndPath() method to sanitize malicious inputs. Successful completion of the sanitization method indicates that the input is acceptable and the doPrivileged() block can be executed.

private void privilegedMethod(final String filename) 
                              throws FileNotFoundException {
  final String cleanFilename;
  try {
    cleanFilename = cleanAFilenameAndPath(filename);
  } catch (/* exception as per spec of cleanAFileNameAndPath */) {
    // log or forward to handler as appropriate based on specification
    // of cleanAFilenameAndPath
  }
  try {
    FileInputStream fis =
        (FileInputStream) AccessController.doPrivileged(
          new PrivilegedExceptionAction() {
        public FileInputStream run() throws FileNotFoundException {
          return new FileInputStream(cleanFilename);
        }
      }
    );
    // do something with the file and then close it
  } catch (PrivilegedActionException e) {
    // forward to handler
  }
}

One potential drawback of this approach is that effective sanitization methods can be difficult to write. A benefit of this approach is that it works well in combination with taint analysis (see the Automated Detection section for this rule). For more information on how to perform secure file operations see rule FIO00-J. Do not operate on files in shared directories.

Compliant Solution (Built-in File Name and Path)

Sanitization of tainted inputs always carries the risk that the data is not fully sanitized. Both file and path name equivalence and directory traversal are common examples of vulnerabilities arising from the improper sanitization of path and file name inputs (see rule IDS02-J. Canonicalize path names before validating them). A design that requires an unprivileged user to access an arbitrary, protected file (or other resource) is always suspect. Consider alternatives such as using a hard coded resource name or permitting the user to select only from a list of options that are indirectly mapped to the resource names.

This compliant solution both explicitly hard codes the name of the file and declares the variable as static final to prevent it from being modified. This ensures that no malicious file can be loaded by exploiting the privileged method.

static final String FILEPATH = "/path/to/protected/file/fn.ext";

private void privilegedMethod() throws FileNotFoundException {
  try {
    FileInputStream fis =
        (FileInputStream) AccessController.doPrivileged(
            new PrivilegedExceptionAction() {
        public FileInputStream run() throws FileNotFoundException {
          return new FileInputStream(FILEPATH);
        }
      }
    );
    // do something with the file and then close it
  } catch (PrivilegedActionException e) {
    // forward to handler and log
  }
}

Risk Assessment

Allowing tainted inputs in privileged operations can result in privilege escalation attacks.

Automated Detection

Tools that support taint analysis enable assurance of code usage that is substantially similar to the first compliant solution. Typical taint analyses assume that one or more methods exist that can sanitize potentially tainted inputs, providing untainted outputs (or appropriate errors). The taint analysis then ensures that only untainted data is used inside the doPrivileged block. Note that the static analyses must necessarily assume that the sanitization methods are always successful, while in reality, this may not be the case.

Related Guidelines

Android Implementation Details

The code examples using the java.security package are not applicable to Android but the principle of the rule is applicable to Android apps.

Bibliography

      14. Platform Security (SEC)