CWE-329: Not Using an Unpredictable IV with CBC Mode
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Not using an unpredictable initialization Vector (IV) with Cipher Block Chaining (CBC) Mode causes algorithms to be susceptible to dictionary attacks when they are encrypted under the same key. CBC mode eliminates a weakness of Electronic Code Book (ECB) mode by allowing identical plaintext blocks to be encrypted to different ciphertext blocks. This is possible by the XOR-ing of an IV with the initial plaintext block so that every plaintext block in the chain is XOR'd with a different value before encryption. If IVs are reused, then identical plaintexts would be encrypted to identical ciphertexts. However, even if IVs are not identical but are predictable, then they still break the security of CBC mode against Chosen Plaintext Attacks (CPA). The table(s) below shows the weaknesses and high level categories that are related to this weakness. These relationships are defined as ChildOf, ParentOf, MemberOf and give insight to similar items that may exist at higher and lower levels of abstraction. In addition, relationships such as PeerOf and CanAlsoBe are defined to show similar weaknesses that the user may want to explore. ![]()
The different Modes of Introduction provide information about how and when this weakness may be introduced. The Phase identifies a point in the life cycle at which introduction may occur, while the Note provides a typical scenario related to introduction during the given phase.
The listings below show possible areas for which the given weakness could appear. These may be for specific named Languages, Operating Systems, Architectures, Paradigms, Technologies, or a class of such platforms. The platform is listed along with how frequently the given weakness appears for that instance. Languages Class: Language-Independent (Undetermined Prevalence) The table below specifies different individual consequences associated with the weakness. The Scope identifies the application security area that is violated, while the Impact describes the negative technical impact that arises if an adversary succeeds in exploiting this weakness. The Likelihood provides information about how likely the specific consequence is expected to be seen relative to the other consequences in the list. For example, there may be high likelihood that a weakness will be exploited to achieve a certain impact, but a low likelihood that it will be exploited to achieve a different impact.
Example 1 In the following examples, CBC mode is used when encrypting data: (bad code) Example Language: C EVP_CIPHER_CTX ctx;
char key[EVP_MAX_KEY_LENGTH]; char iv[EVP_MAX_IV_LENGTH]; RAND_bytes(key, b); memset(iv,0,EVP_MAX_IV_LENGTH); EVP_EncryptInit(&ctx,EVP_bf_cbc(), key,iv); (bad code) Example Language: Java public class SymmetricCipherTest {
public static void main() {
byte[] text ="Secret".getBytes(); byte[] iv ={ 0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00 };KeyGenerator kg = KeyGenerator.getInstance("DES"); kg.init(56); SecretKey key = kg.generateKey(); Cipher cipher = Cipher.getInstance("DES/CBC/PKCS5Padding"); IvParameterSpec ips = new IvParameterSpec(iv); cipher.init(Cipher.ENCRYPT_MODE, key, ips); return cipher.doFinal(inpBytes); In both of these examples, the initialization vector (IV) is always a block of zeros. This makes the resulting cipher text much more predictable and susceptible to a dictionary attack.
This MemberOf Relationships table shows additional CWE Categories and Views that reference this weakness as a member. This information is often useful in understanding where a weakness fits within the context of external information sources.
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