Serialization and Deserialization

Protocol decoders and file format parsers are often the most-exposed part of an application because they are exposed with little or no user interaction and before any authentication and security checks are made. They are also difficult to write robustly in languages which are not memory-safe.

Recommendations for Manually-written Decoders

For C and C++, the advice in Recommendations for Pointers and Array Handling applies. In addition, avoid non-character pointers directly into input buffers. Pointer misalignment causes crashes on some architectures.

When reading variable-sized objects, do not allocate large amounts of data solely based on the value of a size field. If possible, grow the data structure as more data is read from the source, and stop when no data is available. This helps to avoid denial-of-service attacks where little amounts of input data results in enormous memory allocations during decoding. Alternatively, you can impose reasonable bounds on memory allocations, but some protocols do not permit this.

Protocol Design

Binary formats with explicit length fields are more difficult to parse robustly than those where the length of dynamically-sized elements is derived from sentinel values. A protocol which does not use length fields and can be written in printable ASCII characters simplifies testing and debugging. However, binary protocols with length fields may be more efficient to parse.

In new datagram-oriented protocols, unique numbers such as sequence numbers or identifiers for fragment reassembly (see Fragmentation) should be at least 64 bits large, and really should not be smaller than 32 bits in size. Protocols should not permit fragments with overlapping contents.

Fragmentation

Some serialization formats use frames or protocol data units (PDUs) on lower levels which are smaller than the PDUs on higher levels. With such an architecture, higher-level PDUs may have to be fragmented into smaller frames during serialization, and frames may need reassembly into large PDUs during deserialization.

Serialization formats may use conceptually similar structures for completely different purposes, for example storing multiple layers and color channels in a single image file.

When fragmenting PDUs, establish a reasonable lower bound for the size of individual fragments (as large as possible—limits as low as one or even zero can add substantial overhead). Avoid fragmentation if at all possible, and try to obtain the maximum acceptable fragment length from a trusted data source.

When implementing reassembly, consider the following aspects.

  • Avoid allocating significant amount of resources without proper authentication. Allocate memory for the unfragmented PDU as more and more and fragments are encountered, and not based on the initially advertised unfragmented PDU size, unless there is a sufficiently low limit on the unfragmented PDU size, so that over-allocation cannot lead to performance problems.

  • Reassembly queues on top of datagram-oriented transports should be bounded, both in the combined size of the arrived partial PDUs waiting for reassembly, and the total number of partially reassembled fragments. The latter limit helps to reduce the risk of accidental reassembly of unrelated fragments, as it can happen with small fragment IDs (see Fragment IDs). It also guards to some extent against deliberate injection of fragments, by guessing fragment IDs.

  • Carefully keep track of which bytes in the unfragmented PDU have been covered by fragments so far. If message reordering is a concern, the most straightforward data structure for this is an array of bits, with one bit for every byte (or other atomic unit) in the unfragmented PDU. Complete reassembly can be determined by increasing a counter of set bits in the bit array as the bit array is updated, taking overlapping fragments into consideration.

  • Reject overlapping fragments (that is, multiple fragments which provide data at the same offset of the PDU being fragmented), unless the protocol explicitly requires accepting overlapping fragments. The bit array used for tracking already arrived bytes can be used for this purpose.

  • Check for conflicting values of unfragmented PDU lengths (if this length information is part of every fragment) and reject fragments which are inconsistent.

  • Validate fragment lengths and offsets of individual fragments against the unfragmented PDU length (if they are present). Check that the last byte in the fragment does not lie after the end of the unfragmented PDU. Avoid integer overflows in these computations (see Recommendations for Integer Arithmetic).

Fragment IDs

If the underlying transport is datagram-oriented (so that PDUs can be reordered, duplicated or be lost, like with UDP), fragment reassembly needs to take into account endpoint addresses of the communication channel, and there has to be some sort of fragment ID which identifies the individual fragments as part of a larger PDU. In addition, the fragmentation protocol will typically involve fragment offsets and fragment lengths, as mentioned above.

If the transport may be subject to blind PDU injection (again, like UDP), the fragment ID must be generated randomly. If the fragment ID is 64 bit or larger (strongly recommended), it can be generated in a completely random fashion for most traffic volumes. If it is less than 64 bits large (so that accidental collisions can happen if a lot of PDUs are transmitted), the fragment ID should be incremented sequentially from a starting value. The starting value should be derived using a HMAC-like construction from the endpoint addresses, using a long-lived random key. This construction ensures that despite the limited range of the ID, accidental collisions are as unlikely as possible. (This will not work reliable with really short fragment IDs, such as the 16 bit IDs used by the Internet Protocol.)

Library Support for Deserialization

There are too many subtleties when dealing with Deserialization to be discussed here. A more detailed and updated guide is available as OWASP Deserialization Cheat Sheet.

XML Serialization

External References

XML documents can contain external references. They can occur in various places.

  • In the DTD declaration in the header of an XML document:

    <!DOCTYPE html PUBLIC
  "-//W3C//DTD XHTML 1.0 Transitional//EN"
  "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

  • In a namespace declaration:

    <xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema">

  • In an entity definition:

    <!ENTITY sys SYSTEM "http://www.example.com/ent.adoc[]>
<!ENTITY pub PUBLIC "-//Example//Public Entity//EN"
  "http://www.example.com/pub-ent.adoc[]>

  • In a notation:

    <!NOTATION not SYSTEM "../not.adoc[]>

Originally, these external references were intended as unique identifiers, but by many XML implementations, they are used for locating the data for the referenced element. This causes unwanted network traffic, and may disclose file system contents or otherwise unreachable network resources, so this functionality should be disabled.

Depending on the XML library, external referenced might be processed not just when parsing XML, but also when generating it.

Entity Expansion

When external DTD processing is disabled, an internal DTD subset can still contain entity definitions. Entity declarations can reference other entities. Some XML libraries expand entities automatically, and this processing cannot be switched off in some places (such as attribute values or content models). Without limits on the entity nesting level, this expansion results in data which can grow exponentially in length with size of the input. (If there is a limit on the nesting level, the growth is still polynomial, unless further limits are imposed.)

Consequently, the processing internal DTD subsets should be disabled if possible, and only trusted DTDs should be processed. If a particular XML application does not permit such restrictions, then application-specific limits are called for.

XInclude Processing

XInclude processing can reference file and network resources and include them into the document, much like external entity references. When parsing untrusted XML documents, XInclude processing should be turned off.

XInclude processing is also fairly complex and may pull in support for the XPointer and XPath specifications, considerably increasing the amount of code required for XML processing.

Algorithmic Complexity of XML Validation

DTD-based XML validation uses regular expressions for content models. The XML specification requires that content models are deterministic, which means that efficient validation is possible. However, some implementations do not enforce determinism, and require exponential (or just polynomial) amount of space or time for validating some DTD/document combinations.

XML schemas and RELAX NG (via the xsd: prefix) directly support textual regular expressions which are not required to be deterministic.

Using Expat for XML parsing

By default, Expat does not try to resolve external IDs, so no steps are required to block them. However, internal entity declarations are processed. Installing a callback which stops parsing as soon as such entities are encountered disables them, see Disabling XML entity processing with Expat. Expat does not perform any validation, so there are no problems related to that.

Exemple 1. Disabling XML entity processing with Expat
// Stop the parser when an entity declaration is encountered.
static void
EntityDeclHandler(void *userData,
		  const XML_Char *entityName, int is_parameter_entity,
		  const XML_Char *value, int value_length,
		  const XML_Char *base, const XML_Char *systemId,
		  const XML_Char *publicId, const XML_Char *notationName)
{
  XML_StopParser((XML_Parser)userData, XML_FALSE);
}

This handler must be installed when the XML_Parser object is created (Creating an Expat XML parser).

Exemple 2. Creating an Expat XML parser
XML_Parser parser = XML_ParserCreate("UTF-8");
if (parser == NULL) {
  fprintf(stderr, "XML_ParserCreate failed\n");
  close(fd);
  exit(1);
}
// EntityDeclHandler needs a reference to the parser to stop
// parsing.
XML_SetUserData(parser, parser);
// Disable entity processing, to inhibit entity expansion.
XML_SetEntityDeclHandler(parser, EntityDeclHandler);

It is also possible to reject internal DTD subsets altogether, using a suitable XML_StartDoctypeDeclHandler handler installed with XML_SetDoctypeDeclHandler.

Using Qt for XML Parsing

The XML component of Qt, QtXml, does not resolve external IDs by default, so it is not required to prevent such resolution. Internal entities are processed, though. To change that, a custom QXmlDeclHandler and QXmlSimpleReader subclasses are needed. It is not possible to use the QDomDocument::setContent(const QByteArray &) convenience methods.

A QtXml entity handler which blocks entity processing shows an entity handler which always returns errors, causing parsing to stop when encountering entity declarations.

Exemple 3. A QtXml entity handler which blocks entity processing
class NoEntityHandler : public QXmlDeclHandler {
public:
  bool attributeDecl(const QString&, const QString&, const QString&,
		       const QString&, const QString&);
  bool internalEntityDecl(const QString&, const QString&);
  bool externalEntityDecl(const QString&, const QString&,
			    const QString&);
  QString errorString() const;
};

 bool
NoEntityHandler::attributeDecl
  (const QString&, const QString&, const QString&, const QString&,
   const QString&)
{
  return false;
}

bool
NoEntityHandler::internalEntityDecl(const QString&, const QString&)
{
  return false;
}

bool
NoEntityHandler::externalEntityDecl(const QString&, const QString&, const
				      QString&)
{
  return false;
}

QString
NoEntityHandler::errorString() const
{
  return "XML declaration not permitted";
}

This handler is used in the custom QXmlReader subclass in A QtXml XML reader which blocks entity processing. Some parts of QtXml will call the setDeclHandler(QXmlDeclHandler *) method. Consequently, we prevent overriding our custom handler by providing a definition of this method which does nothing. In the constructor, we activate namespace processing; this part may need adjusting.

Exemple 4. A QtXml XML reader which blocks entity processing
class NoEntityReader : public QXmlSimpleReader {
  NoEntityHandler handler;
public:
  NoEntityReader();
  void setDeclHandler(QXmlDeclHandler *);
};

 NoEntityReader::NoEntityReader()
{
  QXmlSimpleReader::setDeclHandler(&handler);
  setFeature("http://xml.org/sax/features/namespaces", true);
  setFeature("http://xml.org/sax/features/namespace-prefixes", false);
 }

void
NoEntityReader::setDeclHandler(QXmlDeclHandler *)
{
  // Ignore the handler which was passed in.
}

Our NoEntityReader class can be used with one of the overloaded QDomDocument::setContent methods. Parsing an XML document with QDomDocument, without entity expansion shows how the buffer object (of type QByteArray) is wrapped as a QXmlInputSource. After calling the setContent method, you should check the return value and report any error.

Exemple 5. Parsing an XML document with QDomDocument, without entity expansion
NoEntityReader reader;
QBuffer buffer(&data);
buffer.open(QIODevice::ReadOnly);
QXmlInputSource source(&buffer);
QDomDocument doc;
QString errorMsg;
int errorLine;
int errorColumn;
bool okay = doc.setContent
  (&source, &reader, &errorMsg, &errorLine, &errorColumn);

Using OpenJDK for XML Parsing and Validation

OpenJDK contains facilities for DOM-based, SAX-based, and StAX-based document parsing. Documents can be validated against DTDs or XML schemas.

The approach taken to deal with entity expansion differs from the general recommendation in Entity Expansion. We enable the the feature flag javax.xml.XMLConstants.FEATURE_SECURE_PROCESSING, which enforces heuristic restrictions on the number of entity expansions. Note that this flag alone does not prevent resolution of external references (system IDs or public IDs), so it is slightly misnamed.

In the following sections, we use helper classes to prevent external ID resolution.

Exemple 6. Helper class to prevent DTD external entity resolution in OpenJDK
class NoEntityResolver implements EntityResolver {
    @Override
    public InputSource resolveEntity(String publicId, String systemId)
            throws SAXException, IOException {
        // Throwing an exception stops validation.
        throw new IOException(String.format(
                "attempt to resolve \"%s\" \"%s\"", publicId, systemId));
    }
}
Exemple 7. Helper class to prevent schema resolution in OpenJDK
class NoResourceResolver implements LSResourceResolver {
    @Override
    public LSInput resolveResource(String type, String namespaceURI,
            String publicId, String systemId, String baseURI) {
        // Throwing an exception stops validation.
        throw new RuntimeException(String.format(
                "resolution attempt: type=%s namespace=%s " +
                "publicId=%s systemId=%s baseURI=%s",
                type, namespaceURI, publicId, systemId, baseURI));
    }
}

Java imports for OpenJDK XML parsing shows the imports used by the examples.

Exemple 8. Java imports for OpenJDK XML parsing
import javax.xml.XMLConstants;
import javax.xml.parsers.DocumentBuilder;
import javax.xml.parsers.DocumentBuilderFactory;
import javax.xml.parsers.ParserConfigurationException;
import javax.xml.parsers.SAXParser;
import javax.xml.parsers.SAXParserFactory;
import javax.xml.transform.dom.DOMSource;
import javax.xml.transform.sax.SAXSource;
import javax.xml.validation.Schema;
import javax.xml.validation.SchemaFactory;
import javax.xml.validation.Validator;

import org.w3c.dom.Document;
import org.w3c.dom.ls.LSInput;
import org.w3c.dom.ls.LSResourceResolver;
import org.xml.sax.EntityResolver;
import org.xml.sax.ErrorHandler;
import org.xml.sax.InputSource;
import org.xml.sax.SAXException;
import org.xml.sax.SAXParseException;
import org.xml.sax.XMLReader;

DOM-based XML parsing and DTD validation in OpenJDK

This approach produces a org.w3c.dom.Document object from an input stream. DOM-based XML parsing in OpenJDK use the data from the java.io.InputStream instance in the inputStream variable.

Exemple 9. DOM-based XML parsing in OpenJDK
DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance();
// Impose restrictions on the complexity of the DTD.
factory.setFeature(XMLConstants.FEATURE_SECURE_PROCESSING, true);

// Turn on validation.
// This step can be omitted if validation is not desired.
factory.setValidating(true);

// Parse the document.
DocumentBuilder builder = factory.newDocumentBuilder();
builder.setEntityResolver(new NoEntityResolver());
builder.setErrorHandler(new Errors());
Document document = builder.parse(inputStream);

External entity references are prohibited using the NoEntityResolver class in Helper class to prevent DTD external entity resolution in OpenJDK. Because external DTD references are prohibited, DTD validation (if enabled) will only happen against the internal DTD subset embedded in the XML document.

To validate the document against an external DTD, use a javax.xml.transform.Transformer class to add the DTD reference to the document, and an entity resolver which whitelists this external reference.

XML Schema Validation in OpenJDK

SAX-based validation against an XML schema in OpenJDK shows how to validate a document against an XML Schema, using a SAX-based approach. The XML data is read from an java.io.InputStream in the inputStream variable.

Exemple 10. SAX-based validation against an XML schema in OpenJDK
SchemaFactory factory = SchemaFactory.newInstance(
        XMLConstants.W3C_XML_SCHEMA_NS_URI);

// This enables restrictions on the schema and document
// complexity.
factory.setFeature(XMLConstants.FEATURE_SECURE_PROCESSING, true);

// This prevents resource resolution by the schema itself.
// If the schema is trusted and references additional files,
// this line must be omitted, otherwise loading these files
// will fail.
factory.setResourceResolver(new NoResourceResolver());

Schema schema = factory.newSchema(schemaFile);
Validator validator = schema.newValidator();

// This prevents external resource resolution.
validator.setResourceResolver(new NoResourceResolver());

validator.validate(new SAXSource(new InputSource(inputStream)));

The NoResourceResolver class is defined in Helper class to prevent schema resolution in OpenJDK.

If you need to validate a document against an XML schema, use the code in DOM-based XML parsing in OpenJDK to create the document, but do not enable validation at this point. Then use Validation of a DOM document against an XML schema in OpenJDK to perform the schema-based validation on the org.w3c.dom.Document instance document.

Exemple 11. Validation of a DOM document against an XML schema in OpenJDK
SchemaFactory factory = SchemaFactory.newInstance(
        XMLConstants.W3C_XML_SCHEMA_NS_URI);

// This enables restrictions on schema complexity.
factory.setFeature(XMLConstants.FEATURE_SECURE_PROCESSING, true);

// The following line prevents resource resolution
// by the schema itself.
factory.setResourceResolver(new NoResourceResolver());

Schema schema = factory.newSchema(schemaFile);

Validator validator = schema.newValidator();

// This prevents external resource resolution.
validator.setResourceResolver(new NoResourceResolver());
validator.validate(new DOMSource(document));

Other XML Parsers in OpenJDK

OpenJDK contains additional XML parsing and processing facilities. Some of them are insecure.

The class java.beans.XMLDecoder acts as a bridge between the Java object serialization format and XML. It is close to impossible to securely deserialize Java objects in this format from untrusted inputs, so its use is not recommended, as with the Java object serialization format itself. See Library Support for Deserialization.

Protocol Encoders

For protocol encoders, you should write bytes to a buffer which grows as needed, using an exponential sizing policy. Explicit lengths can be patched in later, once they are known. Allocating the required number of bytes upfront typically requires separate code to compute the final size, which must be kept in sync with the actual encoding step, or vulnerabilities may result. In multi-threaded code, parts of the object being deserialized might change, so that the computed size is out of date.

You should avoid copying data directly from a received packet during encoding, disregarding the format. Propagating malformed data could enable attacks on other recipients of that data.

When using C or C++ and copying whole data structures directly into the output, make sure that you do not leak information in padding bytes between fields or at the end of the struct.

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