Weakness · Medium
CWE-329: Generation of Predictable IV with CBC Mode
What it is
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.
- Severity (our rating)
- Medium
- Likelihood of exploit
- Medium
- Introduced during
- Implementation
- Exploited in the wild
- No CVE in CISA's catalog
In more detail
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 an attacker can do
- Read Application Data (Confidentiality)
How to prevent it
Implementation
- NIST recommends two methods of generating unpredictable IVs for CBC mode. 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 to find it
- 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.)
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Start freeSource: MITRE Common Weakness Enumeration, CWE version 4.19.1. CWE is a trademark of The MITRE Corporation.