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File scan API

Rationale

When a user uploads files via one of the Form Runner attachment controls, a virus scanner can verify the file before it gets attached to the current form.
Orbeon Forms provides a simple Java API which allows integrating with virus scanning solutions.
NOTE: At this time Orbeon doesn't provide integrations with specific virus scanning solutions.

API Version 2

Availability

[SINCE Orbeon Forms 2022.1]
This is an Orbeon Forms PE feature.
Version 2 of the API adds the following enhancements to the original version (called now Version 1):
  • The provider can return a modified file name, content type, or even replace the entire content.
  • The provider can return custom error messages in the current language of the form.
  • An extension mechanism via hash maps is provided.
While we recommend using Version 2 of the API, Version 1 is still supported. If a Version 2 provider is present, it takes precedence over a Version 1 provider.

API

Orbeon Forms provide the following FileScanProvider2 interface as a base for concrete providers:
package org.orbeon.oxf.xforms.upload.api.java;
interface FileScanProvider2 {
void init();
void destroy();
FileScan2 startStream(
String filename,
Map<String, String[]> headers,
String language,
Map<String, Object> extension
);
}
When Orbeon Forms initializes the provider, it:
  • creates an instance of the class
  • calls the init() method
When Orbeon Forms is starting to receive an upload file, it calls the startStream() method. The method receives:
  • filename: a file name as provided by the web browser (which means that it must not be trusted)
  • headers: HTTP header name/values as they are received by Orbeon Forms
  • language: the current form language at the time the user started the upload
  • extension: an extension immutable hashmap (empty as of Orbeon Forms 2022.1)
The provider implementation must return an implementation of a FileScan2.
The FileScan2 interface is as follows :
package org.orbeon.oxf.xforms.upload.api.java;
interface FileScan2 {
FileScanResult bytesReceived(byte[] bytes, int offset, int length);
FileScanResult complete(File file);
void abort();
}
The FileScanResult type replaces the earlier API's FileScanStatus:
package org.orbeon.oxf.xforms.upload.api.java;
public abstract class FileScanResult {
public final String message;
private FileScanResult(String message) {
this.message = message;
}
public static class FileScanAcceptResult extends FileScanResult {
public final String mediatype;
public final String filename;
public final java.io.InputStream content;
public final java.util.Map<String, Object> extension;
public FileScanAcceptResult() {
this(null, null, null, null, null);
}
public FileScanAcceptResult(
String message,
String mediatype,
String filename,
java.io.InputStream content,
java.util.Map<String, Object> extension
) {
super(message);
this.mediatype = mediatype;
this.filename = filename;
this.content = content;
this.extension = extension;
}
}
public static class FileScanRejectResult extends FileScanResult {
public FileScanRejectResult(String message) {
super(message);
}
}
public static class FileScanErrorResult extends FileScanResult {
public final Throwable throwable;
public FileScanErrorResult(String message, Throwable throwable) {
super(message);
this.throwable = throwable;
}
}
}
This essentially defines three possible results, all implementing FileScanResult:
  • FileScanAcceptResult: accept the uploaded content, possibly with modifications
  • FileScanRejectResult: reject the uploaded content (for example because it is suspected to contain a virus), possibly with a message
  • FileScanErrorResult: any other error happened, possibly with a message and Throwable
As bytes are received, the bytesReceived() method is called. This method allows incremental scanning of a file. A provider may or may not make use of this method. If it is unused, the provider must return a FileScanAcceptResult with all null content, for example with:
public FileScanResult bytesReceived(byte[] bytes, int offset, int length) {
return new FileScanResult.FileScanAcceptResult();
}
When the file is completely uploaded and stored into a temporary file, but not yet attached to the user's form, the complete() method is called with a pointer to the temporary file. The scanner can use this method to perform the entirety of the scan in case bytesReceived() is not used.
Upon processing the complete() method, resources associated with the file scan must be cleared by the provider, whether FileScanRejectResult or FileScanErrorResult is returned, or if any exception is thrown as part of the processing of complete().
Both bytesReceived() and complete() must return a value from the FileScanStatus enumeration:
If FileScanRejectResult or FileScanErrorResult is returned, or if any exception is thrown as part of startStream(), bytesReceived(), or complete(), the uploaded file is rejected and an error is shown to the user.
File scan error
If the user cancels the upload, or if any other error occurs on the Orbeon Forms side, the abort() method is called. In this case, the file scan for the given file must be stopped and associated resources must be cleared, if any.
The abort() method may be called after the complete() method.
The constructor parameters for FileScanAcceptResult, FileScanRejectResult, and FileScanErrorResult can all be set to null.
If the provider wants to accept the file but replace any of the following:
  • filename
  • mediatype
  • actual content
It can do so with a FileScanAcceptResult response, simply by passing non-null values to the respective fields in the constructor.
Replacing the entire content of the uploaded file is useful for example to:
  • strip metadata in images
  • recompress images (although there is a built-in Orbeon Forms feature for that)
  • standardize file formats

Registering a provider

The file scanning API uses the standard Java ServiceLoader API with a provider name of org.orbeon.oxf.xforms.upload.api.java.FileScanProvider2.
The provider must:
  • be a public class
  • have a public no-arguments constructor
  • implement the FileScanProvider2 Java interface
To enable a provider with Orbeon Forms:
  • create your provider as per the standard Java ServiceLoader
  • create a JAR file containing the code or your provider
  • place your JAR file under the Orbeon Forms WEB-INF/lib directory
The Orbeon Forms log files will log errors if any when starting if the provider was found but could not be instantiated.

Example

AcmeFileScanProvider2 is a Java example of a provider which does nothing but:
  • log method calls
  • reject files which contain in their name the string "virus"
  • replace files which contain in their name the string "replace"
  • prefix the file name with "My "
  • return messages in English and French
NOTE: This obviously is not the right way to determine whether a file contains a virus or not!
The service provider is described with this META-INF/services/org.orbeon.oxf.xforms.upload.api.java.FileScanProvider2 file.
The example project is provided here.

API Version 1

Availability

[SINCE Orbeon Forms 2017.2]
This is an Orbeon Forms PE feature.

API

Orbeon Forms provide the following FileScanProvider abstract class as a base for concrete providers:
package org.orbeon.oxf.xforms.upload.api.java;
public abstract class FileScanProvider {
public void init();
public void destroy();
public FileScan startStream(String filename, Map<String, String[]> headers);
}
When Orbeon Forms initializes the provider, it:
  • creates an instance of the class
  • calls the init() method
When Orbeon Forms is starting to receive an upload file, it calls the startStream() method. The method receives a file name as provided by the web browser (which means that it must not be trusted), and HTTP header name/values as they are received by Orbeon Forms.
The provider implementation must return an implementation of a FileScan.
The FileScan interface is as follows :
package org.orbeon.oxf.xforms.upload.api.java;
interface FileScan {
FileScanStatus bytesReceived(byte[] bytes, int offset, int length);
FileScanStatus complete(File file);
void abort();
}
As bytes are received, the bytesReceived() method is called. A provider may or may not make use of this method. If it is unused, the provider must return an ACCEPT FileScanStatus (see below). This method allows incremental scanning of a file.
When the file is completely uploaded and stored into a temporary file, but not yet attached to the user's form, the complete() method is called with a pointer to the temporary file. The scanner can use this method to perform the entirety of the scan in case bytesReceived() is not used.
Upon processing the complete() method, resources associated with the file scan must be cleared by the provider, whether REJECT or ERROR is returned, or if any exception is thrown as part of the processing of complete().
Both bytesReceived() and complete() must return a value from the FileScanStatus enumeration:
package org.orbeon.oxf.xforms.upload.api.java;
public enum FileScanStatus { ACCEPT, REJECT, ERROR }
  • ACCEPT: accept the uploaded content
  • REJECT: reject the uploaded content (for example because it is suspected to contain a virus)
  • ERROR: any other error happened
If REJECT or ERROR is returned, or if any exception is thrown as part of startStream(), bytesReceived(), or complete(), the uploaded file is rejected and an error is shown to the user.
File scan error
If the user cancels the upload, or if any other error occurs on the Orbeon Forms side, the abort() method is called. In this case, the file scan for the given file must be stopped and associated resources must be cleared, if any.
The abort() method may be called after the complete() method.

Registering a provider

The file scanning API uses the standard Java ServiceLoader API with a provider name of org.orbeon.oxf.xforms.upload.api.FileScanProvider.
The provider must:
  • be a public class
  • have a public no-arguments constructor
  • extend the FileScanProvider Java abstract class
To enable a provider with Orbeon Forms:
  • create your provider as per the standard Java ServiceLoader
  • create a JAR file containing the code or your provider
  • place your JAR file under the Orbeon Forms WEB-INF/lib directory
The Orbeon Forms log files will log errors if any when starting if the provider was found but could not be instantiated.

Example

AcmeFileScanProvider is a Java example of a provider which does nothing but:
  • log method calls
  • reject files which contain in their name the string "virus"
NOTE: This obviously is not the right way to determine whether a file contains a virus or not.
The service provider is described with this META-INF/services/org.orbeon.oxf.xforms.upload.api.FileScanProvider file.
The example project is provided here.