Automated static analysis, commonly referred to as Static Application Security Testing (SAST), can find some instances of this weakness by analyzing source code (or binary/compiled code) without having to execute it. Typically, this is done by building a model of data flow and control flow, then searching for potentially-vulnerable patterns that connect "sources" (origins of input) with "sinks" (destinations where the data interacts with external components, a lower layer such as the OS, etc.)
CWE-329 Variant Draft
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 encryption. Predictable IVs undermine the security of CBC,…
Definition
What is CWE-329?
This vulnerability occurs when software uses a predictable or reused Initialization Vector (IV) with Cipher Block Chaining (CBC) mode encryption. Predictable IVs undermine the security of CBC, making encrypted data vulnerable to dictionary and chosen-plaintext attacks, especially when the same key is used multiple times.
CBC mode improves upon basic ECB encryption by ensuring identical plaintext blocks encrypt to different ciphertext blocks. It achieves this by XOR-ing the first plaintext block with a unique, random Initialization Vector (IV) before encryption, and then chaining each subsequent block with the previous ciphertext. This process breaks patterns in the plaintext. However, if the IV is predictable or reused, this core security mechanism fails.
When an attacker can guess or calculate the IV, they can launch practical attacks. Even if IVs aren't identical, predictable sequences allow attackers to perform chosen-plaintext attacks (CPA), where they analyze differences between known inputs and resulting ciphertexts to deduce the encryption key or decrypt data. Always generate IVs using a cryptographically secure random number generator for each encryption operation to maintain CBC's security guarantees.
Real-world impact
Real-world CVEs caused by CWE-329
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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
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messages for a door-unlocking product use a fixed IV in CBC mode, which is the same after each restart
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application uses AES in CBC mode, but the pseudo-random secret and IV are generated using math.random, which is not cryptographically strong.
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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.
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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.
How attackers exploit it
Step-by-step attacker path
- 1
Identify a code path that handles untrusted input without validation.
- 2
Craft a payload that exercises the unsafe behavior — injection, traversal, overflow, or logic abuse.
- 3
Deliver the payload through a normal request and observe the application's reaction.
- 4
Iterate until the response leaks data, executes attacker code, or escalates privileges.
Vulnerable code example
Vulnerable C
In the following examples, CBC mode is used when encrypting data:
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
// 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-329
- 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.
Detection signals
How to detect CWE-329
CWE-329
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Frequently asked questions What is CWE-329?
How serious is CWE-329?
What languages or platforms are affected by CWE-329?
How can I prevent CWE-329?
How does Plexicus detect and fix CWE-329?
Where can I learn more about CWE-329?
Frequently asked questions
What is CWE-329?
This vulnerability occurs when software uses a predictable or reused Initialization Vector (IV) with Cipher Block Chaining (CBC) mode encryption. Predictable IVs undermine the security of CBC, making encrypted data vulnerable to dictionary and chosen-plaintext attacks, especially when the same key is used multiple times.
How serious is CWE-329?
MITRE rates the likelihood of exploit as Medium — exploitation is realistic but typically requires specific conditions.
What languages or platforms are affected by CWE-329?
MITRE lists the following affected platforms: ICS/OT.
How can I prevent CWE-329?
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.
How does Plexicus detect and fix CWE-329?
Plexicus's SAST engine matches the data-flow signature for CWE-329 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-329?
MITRE publishes the canonical definition at https://cwe.mitre.org/data/definitions/329.html. You can also reference OWASP and NIST documentation for adjacent guidance.
Related weaknesses
Weaknesses related to CWE-329
Resources
Further reading
- MITRE — official CWE-329 https://cwe.mitre.org/data/definitions/329.html
- The CLASP Application Security Process https://cwe.mitre.org/documents/sources/TheCLASPApplicationSecurityProcess.pdf
- Why IND-CPA implies randomized encryption https://blog.cryptographyengineering.com/why-ind-cpa-implies-randomized-encryption/
- Recommendation for Block Cipher Modes of Operation https://nvlpubs.nist.gov/nistpubs/Legacy/SP/nistspecialpublication800-38a.pdf
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