CWE-95 Detail

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").

This may allow an attacker to execute arbitrary code, or at least modify what code can be executed.

Modes Of Introduction

Implementation : REALIZATION: This weakness is caused during implementation of an architectural security tactic.
Implementation : This weakness is prevalent in handler/dispatch procedures that might want to invoke a large number of functions, or set a large number of variables.

Applicable Platforms

Language

Name: Java (Undetermined)
Name: JavaScript (Undetermined)
Name: Python (Undetermined)
Name: Perl (Undetermined)
Name: PHP (Undetermined)
Name: Ruby (Undetermined)
Class: Interpreted (Undetermined)

Technologies

Name: AI/ML (Undetermined)

Common Consequences

Observed Examples

Potential Mitigations

Phases : Architecture and Design // Implementation
If possible, refactor your code so that it does not need to use eval() at all.
Phases : Implementation

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 because it only contains alphanumeric characters, but it is not valid if the input is only expected to contain colors such as "red" or "blue."

Do not rely exclusively on looking for malicious or malformed inputs. This is likely to miss at least one undesirable input, especially if the code's environment changes. This can give attackers enough room to bypass the intended validation. However, denylists can be useful for detecting potential attacks or determining which inputs are so malformed that they should be rejected outright.

Phases : 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 that are allowed to contain properly-encoded dangerous content.

Phases : Implementation

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 [REF-1372], so the python documentation discourages use of ast.literal_eval() on untrusted data [REF-1373].

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

Vulnerability Mapping Notes

Rationale : This CWE entry is at the Variant level of abstraction, which is a preferred level of abstraction for mapping to the root causes of vulnerabilities.
Comments : Carefully read both the name and description to ensure that this mapping is an appropriate fit. Do not try to 'force' a mapping to a lower-level Base/Variant simply to comply with this preferred level of abstraction.

Related Attack Patterns

Notes

Factors: special character errors can play a role in increasing the variety of code that can be injected, although some vulnerabilities do not require special characters at all, e.g. when a single function without arguments can be referenced and a terminator character is not necessary.

References

REF-62

The Art of Software Security Assessment
Mark Dowd, John McDonald, Justin Schuh.

REF-1372

How ast.literal_eval can cause memory exhaustion
https://www.reddit.com/r/learnpython/comments/zmbhcf/how_astliteral_eval_can_cause_memory_exhaustion/

REF-1373

ast - Abstract Syntax Trees
https://docs.python.org/3/library/ast.html#ast.literal_eval

Submission

Modifications