Sensitive data must be protected from eavesdropping and malicious tampering during transit. An Obfuscated Transfer Object [[Steel 2005]] that is strongly encrypted can protect data in exchanges that involve multiple business tiers or end user systems. This approach is known as sealing the object. To guarantee object integrity, apply a digital signature to the sealed object.
Sealing and signing objects is the preferred mechanism to secure data when
- Serializing or transporting sensitive data is necessary
- A secure communication channel such as SSL is absent or is too costly for limited transactions
- Some sensitive data must persist over an extended period of time (e.g. on an external hard drive)
Avoid using home-brewed cryptographic algorithms; such algorithms almost certainly introduce unnecessary vulnerabilities. Applications that apply home-brewed "cryptography" in the readObject() and writeObject() methods are prime examples of anti-patterns.
Furthermore, Abadi and Needham have suggested [[Abadi 1996]] a useful principle of secure software design
When a principal signs material that has already been encrypted, it should not be inferred that the principal knows the content of the message. On the other hand, it is proper to infer that the principal that signs a message and then encrypts it for privacy knows the content of the message.
The rationale is that any malicious party can intercept the originally signed encrypted message from the originator, strip the signature and add its own signature to the encrypted message. Both the malicious party, and the receiver have no information about the contents of the original message as it is encrypted and then signed (it can only be decrypted after verifying the signature). The receiver has no way of confirming the sender's identity unless the legitimate sender's public key is obtained over a secure channel. One of the three CCITT X.509 standard protocols was susceptible to such an attack [[CCITT 1988]].
This rule involves the intential serialization of sensitive information. See SER03-J. Prevent serialization of unencrypted, sensitive data about preventing the unintentional serialization of sensitive information.
The subsequent code examples all involve the following code sample. This code sample posits a map that is serializable, as well as a function to populate the map with interesting values, and a function to check the map for those values.
class SerializableMap<K,V> implements Serializable {
final static long serialVersionUID = -2648720192864531932L;
private Map<K,V> map;
public SerializableMap() {
map = new HashMap<K,V>();
}
public Object getData(K key) {
return map.get(key);
}
public void setData(K key, V data) {
map.put(key, data);
}
}
public class MapSerializer {
static public SerializableMap< String, Integer> buildMap() {
SerializableMap< String, Integer> map = new SerializableMap< String, Integer>();
map.setData("John Doe", new Integer( 123456789));
map.setData("Richard Roe", new Integer( 246813579));
return map;
}
static public void InspectMap(SerializableMap< String, Integer> map) {
System.out.println("John Doe's number is " + map.getData("John Doe"));
System.out.println("Richard Roe's number is " + map.getData("Richard Roe"));
}
public static void main(String[] args) {
// ...
}
}
Noncompliant Code Example
This noncompliant code example simply serializes the map and then deserializes it. Thus, the map is capable of being serialized and transferred across different business tiers. Unfortunately, there are no safeguards against byte stream manipulation attacks while the binary data is in transit. Likewise, anyone can reverse engineer the serialized stream data from its hexadecimal notation to reveal the data in the HashMap.
public static void main(String[] args)
throws IOException, ClassNotFoundException {
// Build map
SerializableMap< String, Integer> map = buildMap();
// Serialize map
ObjectOutputStream out = new ObjectOutputStream(new FileOutputStream("data"));
out.writeObject( map);
out.close();
// Deserialize map
ObjectInputStream in = new ObjectInputStream(new FileInputStream("data"));
map = (SerializableMap< String, Integer>) in.readObject();
in.close();
// Inspect map
InspectMap( map);
}
If the data in the map is considered sensitive, this example will also violate SER03-J. Prevent serialization of unencrypted, sensitive data.
Noncompliant Code Example (Seal)
To provide message confidentiality, use the javax.crypto.SealedObject class. This class encapsulates a serialized object and encrypts (or seals) it. A strong cryptographic algorithm that uses a secure cryptographic key and padding scheme must be employed to initialize the Cipher object parameter. The seal and unseal utility methods provide the encryption and decryption facilities respectively.
This noncompliant code example encrypts the map into a SealedObject, rendering the data inaccessable to prying eyes. However, since the data is not signed, it provides no proof of authentication.
public static void main(String[] args)
throws IOException, GeneralSecurityException, ClassNotFoundException {
// Build map
SerializableMap< String, Integer> map = buildMap();
// Generate sealing key & seal map
KeyGenerator generator;
generator = KeyGenerator.getInstance("DES");
generator.init(new SecureRandom());
Key key = generator.generateKey();
Cipher cipher = Cipher.getInstance("DES");
cipher.init( Cipher.ENCRYPT_MODE, key);
SealedObject sealedMap = new SealedObject( map, cipher);
// Serialize map
ObjectOutputStream out = new ObjectOutputStream(new FileOutputStream("data"));
out.writeObject( sealedMap);
out.close();
// Deserialize map
ObjectInputStream in = new ObjectInputStream(new FileInputStream("data"));
sealedMap = (SealedObject) in.readObject();
in.close();
// Unseal map
cipher = Cipher.getInstance("DES");
cipher.init( Cipher.DECRYPT_MODE, key);
map = (SerializableMap< String, Integer>) sealedMap.getObject(cipher);
// Inspect map
InspectMap( map);
}
Noncompliant Code Example (Seal, Sign)
Use the java.security.SignedObject class to sign an object, when the integrity of the object is to be ensured. The two new arguments passed in to the SignedObject() method to sign the object are Signature and a private key derived from a KeyPair object. To verify the signature, a PublicKey as well as a Signature argument is passed to the SignedObject.verify() method.
This noncompliant code example signs the object as well as sealing it. Unfortunately, the signing occurs after the sealing. As discussed above, anyone can sign a sealed object, and so it cannot be assumed that the signer is the true originator of the object.
public static void main(String[] args)
throws IOException, GeneralSecurityException, ClassNotFoundException {
// Build map
SerializableMap< String, Integer> map = buildMap();
// Generate sealing key & seal map
KeyGenerator generator;
generator = KeyGenerator.getInstance("DES");
generator.init(new SecureRandom());
Key key = generator.generateKey();
Cipher cipher = Cipher.getInstance("DES");
cipher.init( Cipher.ENCRYPT_MODE, key);
SealedObject sealedMap = new SealedObject( map, cipher);
// Generate signing public/private key pair & sign map
KeyPairGenerator kpg = KeyPairGenerator.getInstance("DSA");
KeyPair kp = kpg.generateKeyPair();
Signature sig = Signature.getInstance("SHA1withDSA");
SignedObject signedMap = new SignedObject( sealedMap, kp.getPrivate(), sig);
// Serialize map
ObjectOutputStream out = new ObjectOutputStream(new FileOutputStream("data"));
out.writeObject( signedMap);
out.close();
// Deserialize map
ObjectInputStream in = new ObjectInputStream(new FileInputStream("data"));
signedMap = (SignedObject) in.readObject();
in.close();
// Unsign map
if (!signedMap.verify(kp.getPublic(), sig)) {
throw new GeneralSecurityException("Map failed verification");
}
sealedMap = (SealedObject) signedMap.getObject();
// Unseal map
cipher = Cipher.getInstance("DES");
cipher.init( Cipher.DECRYPT_MODE, key);
map = (SerializableMap< String, Integer>) sealedMap.getObject(cipher);
// Inspect map
InspectMap( map);
}
Compliant Solution (Sign, Seal)
This compliant solution correctly signs the object before sealing it. This provides a guarantee of authenticity to the object, in addition to protection from man-in-the-middle attacks.
public static void main(String[] args)
throws IOException, GeneralSecurityException, ClassNotFoundException {
// Build map
SerializableMap< String, Integer> map = buildMap();
// Generate signing public/private key pair & sign map
KeyPairGenerator kpg = KeyPairGenerator.getInstance("DSA");
KeyPair kp = kpg.generateKeyPair();
Signature sig = Signature.getInstance("SHA1withDSA");
SignedObject signedMap = new SignedObject( map, kp.getPrivate(), sig);
// Generate sealing key & seal map
KeyGenerator generator;
generator = KeyGenerator.getInstance("DES");
generator.init(new SecureRandom());
Key key = generator.generateKey();
Cipher cipher = Cipher.getInstance("DES");
cipher.init( Cipher.ENCRYPT_MODE, key);
SealedObject sealedMap = new SealedObject( signedMap, cipher);
// Serialize map
ObjectOutputStream out = new ObjectOutputStream(new FileOutputStream("data"));
out.writeObject( sealedMap);
out.close();
// Deserialize map
ObjectInputStream in = new ObjectInputStream(new FileInputStream("data"));
sealedMap = (SealedObject) in.readObject();
in.close();
// Unseal map
cipher = Cipher.getInstance("DES");
cipher.init( Cipher.DECRYPT_MODE, key);
signedMap = (SignedObject) sealedMap.getObject(cipher);
// Unsign map
if (!signedMap.verify(kp.getPublic(), sig)) {
throw new GeneralSecurityException("Map failed verification");
}
map = (SerializableMap<String, Integer>) signedMap.getObject();
// Inspect map
InspectMap( map);
}
Exceptions
SER02-EX0: A reasonable use for signing a sealed object is to certify the authenticity of a sealed object passed from elsewhere. In the spirit of the [[Abadi 1996]] quotation above, this represents a commitment about the sealed object itself rather than about its content.
SER02-EX1: Signing and sealing is only required for objects that must cross a trust boundary. Objects that never leave the trust boundary need not be signed or sealed. For instance, if an entire network is contained within a trust boundary, then objects that never leave this network need not be signed or sealed.
Risk Assessment
Failure to sign and/or seal objects during transit can lead to loss of object integrity or confidentiality.
Rule |
Severity |
Likelihood |
Remediation Cost |
Priority |
Level |
|---|---|---|---|---|---|
SER02-J |
medium |
probable |
high |
P4 |
L3 |
Automated Detection
Not amenable to static analysis in the general case.
Related Vulnerabilities
Search for vulnerabilities resulting from the violation of this rule on the CERT website.
Bibliography
[[API 2006]]
[[Gong 2003]] 9.10 Sealing Objects
[[Harold 1999]] Chapter 11: Object Serialization, Sealed Objects
[[Neward 2004]] Item 64: Use SignedObject to provide integrity of Serialized objects and Item 65: Use SealedObject to provide confidentiality of Serializable objects
[[MITRE 2009]] CWE ID 319
"Cleartext Transmission of Sensitive Information"
[[Steel 2005]] Chapter 10: Securing the Business Tier, Obfuscated Transfer Object
16. Serialization (SER) SER03-J. Prevent serialization of unencrypted, sensitive data