com.io7m.jxe

The jxe package implements a set of classes intended to both provide more secure defaults and to eliminate much of the boilerplate required to set up the standard JDK SAX parsers.

It provides a sane API for setting up secure-by-default validating SAX parsers that dynamically locate schemas for incoming documents from a whitelisted set of locations without those documents knowing or caring where those schemas are actually located.

The package is capable of setting up extremely strict validating parsers. For example, many applications that receive XML have the following requirements on incoming data:

• XML documents must be validated against one of a small set of schemas. Data that has not been validated must be rejected.
• Documents must declare the namespace to which their data belongs, but must not be required to actually state the physical location of the schema. This is security sensitive: A document should not be able to tell a parser where to find a schema, because hostile documents could cause the parser to read a schema that trivially accepts all data. This would allow the document to essentially pass through without having to conform to the structure that an application expects. Documents that do not declare a namespace must be rejected.
• The XML parser must not access the network except to explicitly permitted locations. This is security sensitive: A hostile document could declare a dependency on a schema that could cause the parser to contact attacker-controlled servers.
• The XML parser must be robust in the face of attacks such as entity expansion attacks.
• The XML parser must prevent path traversal attacks: Documents must not be able to cause files outside of a particular directory to be accessed.

The jxe package allows applications to enforce all of the above requirements via a very simple API:

// Incoming documents *must* be in the "urn:com.io7m.example:simple:1:0" namespace

URI schema_namespace =
  URI.create("urn:com.io7m.example:simple:1:0");

// When a document states that it is in the "urn:com.io7m.example:simple:1:0" namespace,
// the parser will open the schema at the URL returned by getResource("simple_1_0.xsd").
// All other namespaces will be rejected.

JXESchemaDefinition schema =
  JXESchemaDefinition.of(
    schema_namespace, "simple_1_0.xsd", Example.class.getResource("simple_1_0.xsd")));

// Declare an immutable map of schemas. In this example, there is only the
// one schema declared above.

final JXESchemaResolutionMappings schemas =
  JXESchemaResolutionMappings.builder()
    .putMappings(schema_namespace, schema)
    .build();

// Create a provider of hardened SAX parsers.

JXEHardenedSAXParsers parsers =
  new JXEHardenedSAXParsers();

// Specify a directory containing documents. The parser will not be allowed
// to access paths that are ancestors of the given directory. This prevents
// path traversal attacks such as trying to xinclude "../../../../etc/passwd".

Path document_directory = ...;

// Create an XInclude aware SAX parser.

XMLReader reader =
  parsers.createXMLReader(
    document_directory, JXEXInclude.XINCLUDE_ENABLED, schemas);

// Use the SAX parser.

reader.setContentHandler(...);
reader.parse(...);