Sensitive data must be protected from eavesdropping and malicious tampering. An obfuscated transfer object [[Steel 2005]] that is strongly encrypted can protect data. 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
- a secure communication channel such as SSL is absent or is too costly for limited transactions
- sensitive data must persist over an extended period of time (for example, on a 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 antipatterns.
This rule applies to the intentional serialization of sensitive information. Rule SER03-J. Do not serialize unencrypted, sensitive data is meant to prevent the unintentional serialization of sensitive information.
Noncompliant Code Example
Code examples are all based upon the following code example.
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 {
public static 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;
}
public static 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) {
// ...
}
}
This code sample defines a serializable map, a method to populate the map with values, and a method to check the map for those values.
This noncompliant code example simply serializes the map and then deserializes it. Consequently, 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. Do not serialize unencrypted, sensitive data.
Noncompliant Code Example (Seal)
This noncompliant code example uses the javax.crypto.SealedObject class to provide message confidentiality. 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 inaccessible to prying eyes. However, because 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("AES");
generator.init(new SecureRandom());
Key key = generator.generateKey();
Cipher cipher = Cipher.getInstance("AES");
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("AES");
cipher.init(Cipher.DECRYPT_MODE, key);
map = (SerializableMap<String, Integer>) sealedMap.getObject(cipher);
// Inspect map
InspectMap(map);
}
Noncompliant Code Example (Seal Then Sign)
This noncompliant code example uses 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 seals it. According to Abadi and Needham [[Abadi 1996]],
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]].
Because the signing occurs after the sealing, 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("AES");
generator.init(new SecureRandom());
Key key = generator.generateKey();
Cipher cipher = Cipher.getInstance("AES");
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("AES");
cipher.init(Cipher.DECRYPT_MODE, key);
map = (SerializableMap<String, Integer>) sealedMap.getObject(cipher);
// Inspect map
InspectMap(map);
}
Compliant Solution (Sign Then 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("AES");
generator.init(new SecureRandom());
Key key = generator.generateKey();
Cipher cipher = Cipher.getInstance("AES");
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("AES");
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. This represents a commitment about the sealed object itself rather than about its content [[Abadi 1996]].
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, objects that never leave that network need not be signed or sealed.
Risk Assessment
Failure to sign and then 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 Guidelines
Bibliography
<ac:structured-macro ac:name="unmigrated-wiki-markup" ac:schema-version="1" ac:macro-id="bcb7965c-d162-48c8-9654-0826b2d6e0cc"><ac:plain-text-body><![CDATA[ |
[[API 2006 |
AA. Bibliography#API 06]] |
|
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[[Gong 2003 |
AA. Bibliography#Gong 03]] |
9.10 Sealing Objects |
]]></ac:plain-text-body></ac:structured-macro> |
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[[Harold 1999 |
AA. Bibliography#Harold 99]] |
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[[Neward 2004 |
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Item 64: Use SignedObject to provide integrity of Serialized objects |
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Item 65: Use SealedObject to provide confidentiality of Serializable objects |
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[[Steel 2005 |
AA. Bibliography#Steel 05]] |
Chapter 10: Securing the Business Tier, Obfuscated Transfer Object |
]]></ac:plain-text-body></ac:structured-macro> |
13. Serialization (SER) SER03-J. Do not serialize unencrypted, sensitive data