812
Language : Not Language-Specific (Prevalence = Undetermined)
Technology : ICS/OT (Prevalence = Undetermined)
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].
Likelihood Of Exploit
Medium
Common Consequences
Scope Impact Likelihood
Confidentiality Read Application Data
If the IV is not properly initialized, data that is encrypted
can be compromised and leak information.
Detection Methods
Automated Static Analysis
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.)
Effectiveness = High
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.
Demonstrative Examples
Example 1:
In the following examples, CBC mode is used when encrypting data:
Example Language: C (Bad)
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);
CWE Version 4.14
CWE-329: Generation of Predictable IV with CBC Mode
CWE-329: Generation of Predictable IV with CBC Mode