detecting-eval-exec-usage

name	detecting-eval-exec-usage
description	Scan a source tree for dynamic-code-execution APIs that an attacker can hijack: Python eval / exec / compile, JavaScript eval / Function() / setTimeout(string), Ruby eval / instance_eval / class_eval, Java ScriptEngine, PHP eval / assert($str), .NET Activator.CreateInstance / Reflection.Emit with dynamic input. Use when: pre-commit gate on any application that parses user-uploaded code (rule engines, formula evaluators, plugin systems), or post-bug-report when "we run user-supplied expressions." Threshold: any call to eval / exec / Function / similar where the argument is not a string literal. Trigger with: "scan eval", "find dynamic exec", "audit eval calls", "code injection patterns".
allowed-tools	["Read","Bash(python3:*)","Glob","Grep"]
disallowed-tools	["Bash(rm:*)","Bash(curl:*)"]
version	3.0.0-dev
author	Jeremy Longshore <[e-mail]>
license	MIT
compatibility	Designed for Claude Code
tags	["security","static-analysis","code-injection","pentest"]

Detecting eval / exec Usage

Overview

Dynamic-code-execution APIs (CWE-95 Eval Injection) let an application interpret a string as code at runtime. If the string contains anything user-controllable, the application has handed the attacker arbitrary code execution.

The defensive posture: don't use these APIs. The exceptions are narrow: rule engines, formula evaluators (spreadsheet = formulas), plugin systems with explicit sandboxing. For everything else, there's almost always a safer alternative.

When the skill produces findings

Finding	Severity	Threshold	Affected control
Python eval(...) with non-literal	CRITICAL	argument contains var ref	CWE-95
Python exec(...) with non-literal	CRITICAL	argument contains var ref	CWE-95
Python compile(...) with non-literal	HIGH	source string contains var	CWE-95
Python __import__(var)	HIGH	dynamic module loading	CWE-95
JS eval(...)	CRITICAL	any	CWE-95
JS new Function(str)	CRITICAL	any non-literal	CWE-95
JS setTimeout/setInterval(string)	HIGH	string instead of function	CWE-95
Ruby eval(...)/instance_eval(...)/class_eval(...)	CRITICAL	non-literal	CWE-95
PHP eval(...)	CRITICAL	always	CWE-95
PHP assert($str)	CRITICAL	(legacy code-eval form)	CWE-95
PHP create_function	CRITICAL	deprecated, eval-equivalent	CWE-95
Java ScriptEngineManager + eval	HIGH	dynamic script execution	CWE-95
C# Activator.CreateInstance(Type.GetType(str))	HIGH	type loading from string	CWE-95

Prerequisites

• Python 3.9+
• Source tree on local filesystem

Instructions

Run

python3 ${CLAUDE_PLUGIN_ROOT}/skills/detecting-eval-exec-usage/scripts/scan_eval.py /path/to/repo

Options: --output FILE, --format json|jsonl|markdown, --min-severity, --include-tests, --languages LIST.

Interpret

CRITICAL = direct RCE vector. Replace the dynamic execution with explicit logic (lookup table, switch statement) or a sandboxed expression library (Python simpleeval, JavaScript expr-eval, Ruby Dentaku).

Remediation

See references/PLAYBOOK.md.

Examples

Pre-commit

python3 ${CLAUDE_PLUGIN_ROOT}/skills/detecting-eval-exec-usage/scripts/scan_eval.py \
    --min-severity high $(git diff --name-only main...HEAD | tr '\n' ' ')

CI

- run: |
    python3 plugins/security/penetration-tester/skills/detecting-eval-exec-usage/scripts/scan_eval.py \
        . --min-severity high

Output

JSON / JSONL / Markdown. Exit codes: 0 / 1 / 2.

Error Handling

False positive on eval("'literal'") — the value is a constant string. Verify the regex match by reading the source line.

Resources

• references/THEORY.md — Why dynamic-code execution is the highest-impact injection class, sandbox limits, the formula-evaluator design pattern
• references/PLAYBOOK.md — Per-language safe alternatives (Python simpleeval / ast.literal_eval, JS expression-eval libraries, Ruby Dentaku, Java scripting sandboxes)