FIO54-JG. Prevent Arbitrary File Upload

Java based web applications that must accept file uploads must ensure that an adversary cannot upload or transfer malicious files. If a restricted file containing code is executed by the target system, it can result in misuse of privileges. For example, an application that permits HTML files to be uploaded could allow malicious code to be executed - an attacker can submit a valid HTML file with a cross-site-scripting payload that will execute in the absence of an output escaping routine. For this reason, many applications restrict the type of files that can be uploaded.

It may also be possible to upload files with dangerous extensions such as .exe and .sh which may cause arbitrary code execution on server-side applications.

For providing file upload functionality, a typical Java Server Pages (JSP) page consists of code such as

<s:form action="uploadAction" method="POST" enctype="multipart/form-data">
  <s:file name="uploadFile" label="Choose File" size="40" />
  <s:submit value="Upload" name="submit" />
</s:form>

Many Java enterprise frameworks provide configuration settings intended to be used as a defense against arbitrary file upload. Unfortunately, most of them fail to provide adequate protection. Mitigation of this vulnerability involves checking file size, content type and file contents among other metadata attributes.

Noncompliant Code Example

This noncompliant code example shows some XML code from the upload action of a Struts 2 application.  The interceptor code is responsible for allowing file uploads.

<action name="doUpload" class="com.example.UploadAction">
  <interceptor-ref name="fileUpload">
    <param name="maximumSize"> 10240 </param>
    <param name="allowedTypes"> text/plain,image/JPEG,text/html </param>
  </interceptor-ref>        
</action>

The code for file upload is present in the Upload class:

public class Upload extends ActionSupport {
  private File uploadedFile;
  // setter and getter for uploadedFile
  
  public String execute() {
    try {
      // File path and file name are hardcoded for illustration
      File fileToCreate = new File("filepath", "filename");
      // Copy temporary file content to this file
      FileUtils.copyFile(uploadedFile, fileToCreate);
      return "SUCCESS";
    } catch (Exception e) {
      e.printStackTrace();
      addActionError(e.getMessage());
      return "ERROR";
    }
  }
} 

The value of the parameter type maximumSize ensures that a particular Action does not receive a very large file. The allowedType parameter defines the type of files that are accepted.

However, this approach does not ensure that the uploaded file conforms to the security requirements as interceptor checks can be trivially bypassed. If an attacker uses a proxy tool to change the content type in the raw HTTP request in transit, the framework would not prevent the file's upload. Consequently, an attacker can upload a malicious file having a .exe extension.

This code appears to violate ERR08-J. Do not catch NullPointerException or any of its ancestors. But it does not violate that rule, because it falls under the exception ERR08-EX2.

Compliant Solution 

The file upload must only succeed if the content type matches the content present within the file. For example, a file with an image header must only contain an image and not executable code. This compliant solution uses the Apache Tika library to detect and extract metadata and structured text content from documents using existing parser libraries [Apache Tika|http://tika.apache.org/index.html]. The checkMetaData() method must be called before invoking execute().

public static boolean checkMetaData(File f, String getContentType) {
  InputStream is = null;
  try {
    is = new FileInputStream(f);
    ContentHandler contenthandler = new BodyContentHandler();
    Metadata metadata = new Metadata();
    metadata.set(Metadata.RESOURCE_NAME_KEY, f.getName());
    Parser parser = new AutoDetectParser();
    parser.parse(is, contenthandler, metadata, new ParseContext());
    
    if (metadata.get(Metadata.CONTENT_TYPE).equalsIgnoreCase(getContentType)) {
      return true;
    }
    else {
      return false;
    }
  } catch (Exception e) {
    e.printStackTrace();
    return false;
  } finally {
    if (is != null) {
      try {
        is.close();
      } catch (IOException e) {
        e.printStackTrace();
      }
    }
  }
}

The AutoDetectParser selects the best available parser based on the content type of file to be parsed.

Applicability

An arbitrary file upload vulnerability could result in privilege escalation and execution of arbitrary code.

Bibliography