CWE-1204 Base Incomplete

Generation of Weak Initialization Vector (IV)

This vulnerability occurs when software uses a weak or predictable Initialization Vector (IV) for cryptographic operations. Many encryption algorithms require IVs to be both unique and unpredictable…

Definition

What is CWE-1204?

This vulnerability occurs when software uses a weak or predictable Initialization Vector (IV) for cryptographic operations. Many encryption algorithms require IVs to be both unique and unpredictable to ensure security, and failing to meet these requirements can compromise the entire encryption process.
Certain encryption methods, like block ciphers in specific modes, rely heavily on strong Initialization Vectors. The IV must be both unique (never reused with the same key) and unpredictable (random) to prevent attackers from deducing patterns or recovering plaintext. If the IV generation is flawed—due to a bug, a poor random source, or a reused value—the cryptographic protection can be significantly weakened. In practice, attacking a weak IV is often easier than breaking the core cipher. Attackers can exploit predictable or repeated IVs to perform decryption, reveal data patterns, or bypass security entirely. Therefore, developers must ensure their IV generation adheres strictly to the requirements of the specific cryptographic primitive being used.
Real-world impact

Real-world CVEs caused by CWE-1204

  • CVE-2020-1472

    ZeroLogon vulnerability - use of a static IV of all zeroes in AES-CFB8 mode

  • 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.

  • CVE-2001-0161

    wireless router does not use 6 of the 24 bits for WEP encryption, making it easier for attackers to decrypt traffic

  • CVE-2001-0160

    WEP card generates predictable IV values, making it easier for attackers to decrypt traffic

  • CVE-2017-3225

    device bootloader uses a zero initialization vector during AES-CBC

  • CVE-2016-6485

    crypto framework uses PHP rand function - which is not cryptographically secure - for an initialization vector

  • CVE-2014-5386

    encryption routine does not seed the random number generator, causing the same initialization vector to be generated repeatedly

  • 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

How attackers exploit it

Step-by-step attacker path

  1. 1

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

  2. 2

    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.

  3. 3

    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].

Vulnerable code example

Vulnerable C

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

Vulnerable 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);
Secure code example

Secure pseudo

Secure pseudo 
// Validate, sanitize, or use a safe API before reaching the sink.
function handleRequest(input) {
  const safe = validateAndEscape(input);
  return executeWithGuards(safe);
}
What changed: the unsafe sink is replaced (or the input is validated/escaped) so the same payload no longer triggers the weakness.
Prevention checklist

How to prevent CWE-1204

  • 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].
Detection signals

How to detect CWE-1204

SAST High

Run static analysis (SAST) on the codebase looking for the unsafe pattern in the data flow.

DAST Moderate

Run dynamic application security testing against the live endpoint.

Runtime Moderate

Watch runtime logs for unusual exception traces, malformed input, or authorization bypass attempts.

Code review Moderate

Code review: flag any new code that handles input from this surface without using the validated framework helpers.

Plexicus auto-fix

Plexicus auto-detects CWE-1204 and opens a fix PR in under 60 seconds.

Codex Remedium scans every commit, identifies this exact weakness, and ships a reviewer-ready pull request with the patch. No tickets. No hand-offs.

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Frequently asked questions

Frequently asked questions

What is CWE-1204?

This vulnerability occurs when software uses a weak or predictable Initialization Vector (IV) for cryptographic operations. Many encryption algorithms require IVs to be both unique and unpredictable to ensure security, and failing to meet these requirements can compromise the entire encryption process.

How serious is CWE-1204?

MITRE has not published a likelihood-of-exploit rating for this weakness. Treat it as medium-impact until your threat model proves otherwise.

What languages or platforms are affected by CWE-1204?

MITRE has not specified affected platforms for this CWE — it can apply across most application stacks.

How can I prevent CWE-1204?

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…

How does Plexicus detect and fix CWE-1204?

Plexicus's SAST engine matches the data-flow signature for CWE-1204 on every commit. When a match is found, our Codex Remedium agent opens a fix PR with the corrected code, tests, and a one-line summary for the reviewer.

Where can I learn more about CWE-1204?

MITRE publishes the canonical definition at https://cwe.mitre.org/data/definitions/1204.html. You can also reference OWASP and NIST documentation for adjacent guidance.

Related weaknesses

Weaknesses related to CWE-1204

CWE-330 Parent

Use of Insufficiently Random Values

This vulnerability occurs when an application uses random values that are not sufficiently unpredictable in security-sensitive operations,…

CWE-1241 Sibling

Use of Predictable Algorithm in Random Number Generator

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CWE-331 Sibling

Insufficient Entropy

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CWE-334 Sibling

Small Space of Random Values

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CWE-335 Sibling

Incorrect Usage of Seeds in Pseudo-Random Number Generator (PRNG)

This vulnerability occurs when a Pseudo-Random Number Generator (PRNG) is used, but its initial seed value is not handled securely or…

CWE-338 Sibling

Use of Cryptographically Weak Pseudo-Random Number Generator (PRNG)

This vulnerability occurs when software uses a pseudo-random number generator (PRNG) that is not cryptographically strong for…

CWE-340 Sibling

Generation of Predictable Numbers or Identifiers

This vulnerability occurs when a system creates numbers or identifiers that are too easy to guess, undermining security mechanisms that…

CWE-344 Sibling

Use of Invariant Value in Dynamically Changing Context

This vulnerability occurs when code uses a fixed, unchanging value (like a hardcoded string, number, or reference) in a situation where…

CWE-329 Child

Generation of Predictable IV with CBC Mode

This vulnerability occurs when software uses a predictable or reused Initialization Vector (IV) with Cipher Block Chaining (CBC) mode…

Resources

Further reading

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