CWE-329: Generation of Predictable IV with CBC Mode

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).

Code Examples & CVEs

Demonstrative Examples

In the following examples, CBC mode is used when encrypting data:

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.

BadC

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);

In the following examples, CBC mode is used when encrypting data:

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.

BadJava

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);                            }                      }

Observed CVE Examples (5)

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

View Details

CVE-2017-17704

messages for a door-unlocking product use a fixed IV in CBC mode, which is the same after each restart

View Details

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.

View Details

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.

View Details

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.

View Details

Source-backed guidance

Additional facts reviewed against primary or authoritative security sources.

Verify controls for CWE-329 with SSDF evidence

Use NIST SSDF verification and vulnerability-response practices to detect CWE-329, Generation of Predictable IV with CBC Mode, throughout the product lifecycle. Derive review questions, static or dynamic checks, and negative tests from the CWE's causal behavior; define the components and lifecycle stages each check covers; and retain findings with enough evidence to distinguish the root cause from symptoms and impacts. Track escapes and false negatives, then improve the verification plan after every confirmed occurrence.

NIST SP 800-218 Secure Software Development Framework — National Institute of Standards and Technology

Address Generation of Predictable IV with CBC Mode during Implementation

MITRE associates mitigation with Implementation; documented detection approaches include Automated Static Analysis; recorded impacts include Read Application Data. Use these source-defined anchors to turn CWE-329 into implementation, review, and verification checks for the affected component.

CWE-329: Generation of Predictable IV with CBC Mode — MITRE CWE

Triage CWE-329 against known exploitation evidence

Use CISA's Known Exploited Vulnerabilities catalog to test whether a vulnerability mapped to CWE-329, Generation of Predictable IV with CBC Mode, has evidence of exploitation in the wild. Confirm the CVE-to-CWE root-cause mapping independently before attaching the example, then capture the affected product, required action, and remediation deadline. A missing KEV match is not evidence that the weakness is unexploited, and a KEV entry must not be generalized to every occurrence of this CWE.

Known Exploited Vulnerabilities Catalog — Cybersecurity and Infrastructure Security Agency

Apply precise root-cause mapping to CWE-329

Apply MITRE's full root-cause mapping guidance when using CWE-329, Generation of Predictable IV with CBC Mode. Separate weakness language from attacker prerequisites and technical impact, check the entry's abstraction and vulnerability-mapping notes, and prefer the most specific Base or Variant supported by the evidence. Record the rejected alternatives and require an independent review before the mapping is used for remediation trends or program metrics.

CVE to CWE Root Cause Mapping Guidance — MITRE CWE

Validate CWE-329 with root-cause mapping checks

Apply MITRE's root-cause mapping quick tips to CWE-329, Generation of Predictable IV with CBC Mode. Confirm the finding describes the causal weakness rather than an impact or attack pattern, compare the abstraction and mapping notes with plausible alternatives, and have a second reviewer challenge the selection. Preserve the evidence and reasoning so recurring defects can be measured against one consistent identifier.

CVE to CWE Root Cause Mapping Quick Tips — MITRE CWE

Frequently Asked Questions

What is CWE-329: Generation of Predictable IV with CBC Mode?+

CWE-329: Generation of Predictable IV with CBC Mode is a Common Weakness Enumeration (CWE) entry maintained by MITRE. The product generates and uses a predictable initialization Vector (IV) with Cipher Block Chaining (CBC) Mode, which 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).

What are the security consequences of Generation of Predictable IV with CBC Mode?+

If exploited, CWE-329 (Generation of Predictable IV with CBC Mode) it can compromise Confidentiality, leading to outcomes such as Read Application Data.

How do you prevent or mitigate Generation of Predictable IV with CBC Mode?+

Recommended mitigations for CWE-329 include: 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.

Which programming languages are affected by Generation of Predictable IV with CBC Mode?+

CWE-329 commonly affects Not Language-Specific. Note that weaknesses are often language-agnostic patterns, so secure coding practices apply broadly.

What are real-world examples of Generation of Predictable IV with CBC Mode?+

MITRE documents real CVEs mapped to CWE-329, including CVE-2020-5408, CVE-2017-17704, CVE-2017-11133, CVE-2007-3528 and CVE-2011-3389. You can look up the full details of each CVE, including CVSS scores and remediation guidance, on our CVE Lookup tool.

What is the difference between a CWE and a CVE?+

A CWE (Common Weakness Enumeration) like CWE-329 describes a category of software weakness — the underlying flaw type. A CVE (Common Vulnerabilities and Exposures) identifies a specific, real-world vulnerability in a particular product. In short, a CWE is the kind of mistake, and a CVE is an instance of that mistake being found in software.

CWE-329: Generation of Predictable IV with CBC Mode

Extended Description

Technical Details

Applicable To

Security Consequences

Scope

Impact

Mitigation Strategies

Phase

Description

Detection Methods

Code Examples & CVEs

Demonstrative Examples

Observed CVE Examples (5)

CWE Relationships

Parent Weaknesses

Source-backed guidance

Verify controls for CWE-329 with SSDF evidence

Address Generation of Predictable IV with CBC Mode during Implementation

Triage CWE-329 against known exploitation evidence

Apply precise root-cause mapping to CWE-329

Validate CWE-329 with root-cause mapping checks

Frequently Asked Questions

Learn More