CWE-95: Improper Neutralization of Directives in Dynamically Evaluated Code ('Eval Injection')
Description
The product receives input from an upstream component, but it does not neutralize or incorrectly neutralizes code syntax before using the input in a dynamic evaluation call (e.g. eval).
- Abstraction
- VariantA weakness tied to a specific language, technology or resource.
- Likelihood of exploit
- Medium
- Status
- Incomplete
Common consequences
- Confidentiality
Read Files or Directories; Read Application Data
The injected code could access restricted data / files.
- Access Control
Bypass Protection Mechanism
In some cases, injectable code controls authentication; this may lead to a remote vulnerability.
- Access Control
Gain Privileges or Assume Identity
Injected code can access resources that the attacker is directly prevented from accessing.
- IntegrityConfidentialityAvailabilityOther
Execute Unauthorized Code or Commands
Code injection attacks can lead to loss of data integrity in nearly all cases as the control-plane data injected is always incidental to data recall or writing. Additionally, code injection can often result in the execution of arbitrary code or at least modify what code can be executed.
- Non-Repudiation
Hide Activities
Often the actions performed by injected control code are unlogged.
Potential mitigations
- Architecture and DesignImplementationRefactoring
If possible, refactor your code so that it does not need to use eval() at all.
- ImplementationInput Validation
Assume all input is malicious. Use an accept known good input validation strategy, i.e., use a list of acceptable inputs that strictly conform to specifications. Reject any input that does not strictly conform to specifications, or transform it into something that does. When performing input validation, consider all potentially relevant properties, including length, type of input, the full range of acceptable values, missing or extra inputs, syntax, consistency across related fields, and conformance to business rules. As an example of business rule logic, boat may be syntactically valid...
- Implementation
Inputs should be decoded and canonicalized to the application's current internal representation before being validated (CWE-180, CWE-181). Make sure that your application does not inadvertently decode the same input twice (CWE-174). Such errors could be used to bypass allowlist schemes by introducing dangerous inputs after they have been checked. Use libraries such as the OWASP ESAPI Canonicalization control. Consider performing repeated canonicalization until your input does not change any more. This will avoid double-decoding and similar scenarios, but it might inadvertently modify inputs...
- ImplementationEffectiveness: Discouraged Common Practice
For Python programs, it is frequently encouraged to use the ast.literal_eval() function instead of eval, since it is intentionally designed to avoid executing code. However, an adversary could still cause excessive memory or stack consumption via deeply nested structures, so the python documentation discourages use of ast.literal_eval() on untrusted data.