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CWE-1204: Generation of Weak Initialization Vector (IV)

Weakness ID: 1204
Abstraction: Base
Structure: Simple
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+ Description
The product uses a cryptographic primitive that uses an Initialization Vector (IV), but the product does not generate IVs that are sufficiently unpredictable or unique according to the expected cryptographic requirements for that primitive.
+ Extended Description
By design, some cryptographic primitives (such as block ciphers) require that IVs must have certain properties for the uniqueness and/or unpredictability of an IV. Primitives may vary in how important these properties are. If these properties are not maintained, e.g. by a bug in the code, then the cryptography may be weakened or broken by attacking the IVs themselves.
+ Relationships
Section HelpThis table 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)
ChildOfClassClass - 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.330Use of Insufficiently Random Values
ParentOfVariantVariant - a weakness that is linked to a certain type of product, typically involving a specific language or technology. More specific than a Base weakness. Variant level weaknesses typically describe issues in terms of 3 to 5 of the following dimensions: behavior, property, technology, language, and resource.329Generation of Predictable IV with CBC Mode
+ Modes Of Introduction
Section HelpThe 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.
Architecture and Design
+ Applicable Platforms
Section HelpThis listing shows 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.


Class: Not Language-Specific (Undetermined Prevalence)

+ Common Consequences
Section HelpThis table 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.

Technical Impact: Read Application Data

If the IV is not properly initialized, data that is encrypted can be compromised and information about the data can be leaked. See [REF-1179].
+ Demonstrative Examples

Example 1

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

(bad code)
Example Language:
RAND_bytes(key, b);
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 ={
KeyGenerator kg = KeyGenerator.getInstance("DES");
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.

Example 2

The Wired Equivalent Privacy (WEP) protocol used in the 802.11 wireless standard only supported 40-bit keys, and the IVs were only 24 bits, increasing the chances that the same IV would be reused for multiple messages. The IV was included in plaintext as part of the packet, making it directly observable to attackers. Only 5000 messages are needed before a collision occurs due to the "birthday paradox" [REF-1176]. Some implementations would reuse the same IV for each packet. This IV reuse made it much easier for attackers to recover plaintext from two packets with the same IV, using well-understood attacks, especially if the plaintext was known for one of the packets [REF-1175].

+ Observed Examples
ZeroLogon vulnerability - use of a static IV of all zeroes in AES-CFB8 mode
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.
wireless router does not use 6 of the 24 bits for WEP encryption, making it easier for attackers to decrypt traffic
WEP card generates predictable IV values, making it easier for attackers to decrypt traffic
device bootloader uses a zero initialization vector during AES-CBC
crypto framework uses PHP rand function - which is not cryptographically secure - for an initialization vector
encryption routine does not seed the random number generator, causing the same initialization vector to be generated repeatedly
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
messages for a door-unlocking product use a fixed IV in CBC mode, which is the same after each restart
application uses AES in CBC mode, but the pseudo-random secret and IV are generated using math.random, which is not cryptographically strong.
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.
+ Potential Mitigations

Phase: Implementation

Different cipher modes have different requirements for their IVs. When choosing and implementing a mode, it is important to understand those requirements in order to keep security guarantees intact. Generally, it is safest to generate a random IV, since it will be both unpredictable and have a very low chance of being non-unique. IVs do not have to be kept secret, so if generating duplicate IVs is a concern, a list of already-used IVs can be kept and checked against.

NIST offers recommendations on generation of IVs for modes of which they have approved. These include options for when random IVs are not practical. For CBC, CFB, and OFB, see [REF-1175]; for GCM, see [REF-1178].

+ Functional Areas
  • Cryptography
+ Notes


As of CWE 4.5, terminology related to randomness, entropy, and predictability can vary widely. Within the developer and other communities, "randomness" is used heavily. However, within cryptography, "entropy" is distinct, typically implied as a measurement. There are no commonly-used definitions, even within standards documents and cryptography papers. Future versions of CWE will attempt to define these terms and, if necessary, distinguish between them in ways that are appropriate for different communities but do not reduce the usability of CWE for mapping, understanding, or other scenarios.
+ References
[REF-1175] Nikita Borisov, Ian Goldberg and David Wagner. "Intercepting Mobile Communications: The Insecurity of 802.11". 3. Risks of Keystream Reuse. Proceedings of the Seventh Annual International Conference on Mobile Computing And Networking. ACM. 2001-07. <>.
[REF-1175] Nikita Borisov, Ian Goldberg and David Wagner. "Intercepting Mobile Communications: The Insecurity of 802.11". Appendix C. Proceedings of the Seventh Annual International Conference on Mobile Computing And Networking. ACM. 2001-07. <>.
[REF-1176] Wikipedia. "Birthday problem". 2021-03-06. <>.
[REF-1177] Wikipedia. "Initialization Vector". 2021-03-08. <>.
[REF-1178] NIST. "Recommendation for Block Cipher Modes of Operation: Galois/Counter Mode (GCM) and GMAC". 8.2 IV Constructions. 2007-11. <>.
[REF-1179] Arxum Path Security. "CBC Mode is Malleable. Don't trust it for Authentication". 2019-10-16. <>.
+ Content History
+ Submissions
Submission DateSubmitterOrganization
2021-03-09CWE Content TeamMITRE
+ Modifications
Modification DateModifierOrganization
2021-07-20CWE Content TeamMITRE
updated Maintenance_Notes, Observed_Examples, References
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Page Last Updated: October 13, 2022