CWE - CWE-329: Not Using an Unpredictable IV with CBC Mode (4.4)

Weakness ID: 329

Abstraction: Variant
Structure: Simple

Status: Draft

Presentation Filter:

Description

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.

Extended Description

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).

Relationships

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.

Relevant to the view "Research Concepts" (CWE-1000)

Nature	Type	ID	Name
ChildOf	Class - a weakness that is described in a very abstract fashion, typically independent of any specific language or technology. More specific than a Pillar Weakness, but more general than a Base Weakness. Class level weaknesses typically describe issues in terms of 1 or 2 of the following dimensions: behavior, property, and resource.	573	Improper Following of Specification by Caller
ChildOf	Base - a weakness that is still mostly independent of a resource or technology, but with sufficient details to provide specific methods for detection and prevention. Base level weaknesses typically describe issues in terms of 2 or 3 of the following dimensions: behavior, property, technology, language, and resource.	1204	Generation of Weak Initialization Vector (IV)

Background Details

CBC mode is a commonly used mode of operation for a block cipher. It works by XOR-ing an IV with the initial block of a plaintext prior to encryption and then XOR-ing each successive block of plaintext with the previous block of ciphertext before encryption.

C_0 = IV
C_i = E_k{M_i XOR C_{i-1}}

When used properly, CBC mode provides security against chosen plaintext attacks. Having an unpredictable IV is a crucial underpinning of this. See [REF-1171].

Modes Of Introduction

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.

Phase	Note
Architecture and Design	Designers might assume a non-cryptographic context for a cryptographic variable.
Implementation	Developers might dismiss the importance of an unpredictable IV and choose an easier implementation to save effort, weakening the scheme in the process.

Applicable Platforms

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)

Common Consequences

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.

Scope	Impact	Likelihood
Confidentiality	Technical Impact: Read Application Data If the IV is not properly initialized, data that is encrypted can be compromised and leak information.

Likelihood Of Exploit

Medium

Demonstrative Examples

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.

Observed Examples

Reference	Description
CVE-2020-5408	encryption functionality in an authentication framework uses a fixed null IV with CBC mode, allowing attackers to decrypt traffic in applications that use this functionality
CVE-2017-17704	messages for a door-unlocking product use a fixed IV in CBC mode, which is the same after each restart
CVE-2017-11133	application uses AES in CBC mode, but the pseudo-random secret and IV are generated using math.random, which is not cryptographically strong.
CVE-2007-3528	Blowfish-CBC implementation constructs an IV where each byte is calculated modulo 8 instead of modulo 256, resulting in less than 12 bits for the effective IV length, and less than 4096 possible IV values.
CVE-2011-3389	BEAST attack in SSL 3.0 / TLS 1.0. In CBC mode, chained initialization vectors are non-random, allowing decryption of HTTPS traffic using a chosen plaintext attack.

Potential Mitigations

Phase: Implementation

NIST recommends two methods of generating unpredictable IVs for CBC mode [REF-1172]. The first is to generate the IV randomly. The second method is to encrypt a nonce with the same key and cipher to be used to encrypt the plaintext. In this case the nonce must be unique but can be predictable, since the block cipher will act as a pseudo random permutation.

Functional Areas

Cryptography

Memberships

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.

Nature	Type	ID	Name
MemberOf	Category - a CWE entry that contains a set of other entries that share a common characteristic.	959	SFP Secondary Cluster: Weak Cryptography

Taxonomy Mappings

Mapped Taxonomy Name	Node ID	Fit	Mapped Node Name
CLASP			Not using a random IV with CBC mode

References

[REF-62] Mark Dowd, John McDonald and Justin Schuh. "The Art of Software Security Assessment". Chapter 2, "Initialization Vectors", Page 42. 1st Edition. Addison Wesley. 2006.

[REF-18] Secure Software, Inc.. "The CLASP Application Security Process". 2005. <https://cwe.mitre.org/documents/sources/TheCLASPApplicationSecurityProcess.pdf>.

[REF-1171] Matthew Green. "Why IND-CPA implies randomized encryption". Appendix C. 2018-08-24. <https://blog.cryptographyengineering.com/why-ind-cpa-implies-randomized-encryption/>.

Content History

Submissions
Submission Date	Submitter	Organization
2006-07-19	CLASP
2006-07-19
Modifications
Modification Date	Modifier	Organization
2008-07-01	Eric Dalci	Cigital
2008-07-01	updated Time_of_Introduction
2008-09-08	CWE Content Team	MITRE
2008-09-08	updated Background_Details, Common_Consequences, Functional_Areas, Relationships, Taxonomy_Mappings
2011-06-01	CWE Content Team	MITRE
2011-06-01	updated Common_Consequences
2012-05-11	CWE Content Team	MITRE
2012-05-11	updated References, Relationships
2012-10-30	CWE Content Team	MITRE
2012-10-30	updated Demonstrative_Examples, Potential_Mitigations
2014-07-30	CWE Content Team	MITRE
2014-07-30	updated Relationships
2017-11-08	CWE Content Team	MITRE
2017-11-08	updated Applicable_Platforms, Demonstrative_Examples
2019-06-20	CWE Content Team	MITRE
2019-06-20	updated Demonstrative_Examples
2020-02-24	CWE Content Team	MITRE
2020-02-24	updated Relationships
2021-03-15	CWE Content Team	MITRE
2021-03-15	updated Background_Details, Common_Consequences, Demonstrative_Examples, Description, Modes_of_Introduction, Name, Observed_Examples, Potential_Mitigations, References, Relationships
Previous Entry Names
Change Date	Previous Entry Name
2021-03-15	Not Using a Random IV with CBC Mode


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Common Weakness Enumeration

CWE-329: Not Using an Unpredictable IV with CBC Mode