--- name: semgrep-rule-creator description: Creates custom Semgrep rules for detecting security vulnerabilities, bug patterns, and code patterns. Use when writing Semgrep rules or building custom static analysis detections. allowed-tools: - Bash - Read - Write - Edit - Glob - Grep - WebFetch risk: unknown source: community --- # Semgrep Rule Creator Create production-quality Semgrep rules with proper testing and validation. ## When to Use **Ideal scenarios:** - Writing Semgrep rules for specific bug patterns - Writing rules to detect security vulnerabilities in your codebase - Writing taint mode rules for data flow vulnerabilities - Writing rules to enforce coding standards ## When NOT to Use Do NOT use this skill for: - Running existing Semgrep rulesets - General static analysis without custom rules (use `static-analysis` skill) ## Rationalizations to Reject When writing Semgrep rules, reject these common shortcuts: - **"The pattern looks complete"** → Still run `semgrep --test --config .yaml .` to verify. Untested rules have hidden false positives/negatives. - **"It matches the vulnerable case"** → Matching vulnerabilities is half the job. Verify safe cases don't match (false positives break trust). - **"Taint mode is overkill for this"** → If data flows from user input to a dangerous sink, taint mode gives better precision than pattern matching. - **"One test is enough"** → Include edge cases: different coding styles, sanitized inputs, safe alternatives, and boundary conditions. - **"I'll optimize the patterns first"** → Write correct patterns first, optimize after all tests pass. Premature optimization causes regressions. - **"The AST dump is too complex"** → The AST reveals exactly how Semgrep sees code. Skipping it leads to patterns that miss syntactic variations. ## Anti-Patterns **Too broad** - matches everything, useless for detection: ```yaml # BAD: Matches any function call pattern: $FUNC(...) # GOOD: Specific dangerous function pattern: eval(...) ``` **Missing safe cases in tests** - leads to undetected false positives: ```python # BAD: Only tests vulnerable case # ruleid: my-rule dangerous(user_input) # GOOD: Include safe cases to verify no false positives # ruleid: my-rule dangerous(user_input) # ok: my-rule dangerous(sanitize(user_input)) # ok: my-rule dangerous("hardcoded_safe_value") ``` **Overly specific patterns** - misses variations: ```yaml # BAD: Only matches exact format pattern: os.system("rm " + $VAR) # GOOD: Matches all os.system calls with taint tracking mode: taint pattern-sinks: - pattern: os.system(...) ``` ## Strictness Level This workflow is **strict** - do not skip steps: - **Read documentation first**: See [Documentation](#documentation) before writing Semgrep rules - **Test-first is mandatory**: Never write a rule without tests - **100% test pass is required**: "Most tests pass" is not acceptable - **Optimization comes last**: Only simplify patterns after all tests pass - **Avoid generic patterns**: Rules must be specific, not match broad patterns - **Prioritize taint mode**: For data flow vulnerabilities - **One YAML file - one Semgrep rule**: Each YAML file must contain only one Semgrep rule; don't combine multiple rules in a single file - **No generic rules**: When targeting a specific language for Semgrep rules - avoid generic pattern matching (`languages: generic`) - **Forbidden `todook` and `todoruleid` test annotations**: `todoruleid: ` and `todook: ` annotations in tests files for future rule improvements are forbidden ## Overview This skill guides creation of Semgrep rules that detect security vulnerabilities and code patterns. Rules are created iteratively: analyze the problem, write tests first, analyze AST structure, write the rule, iterate until all tests pass, optimize the rule. **Approach selection:** - **Taint mode** (prioritize): Data flow issues where untrusted input reaches dangerous sinks - **Pattern matching**: Simple syntactic patterns without data flow requirements **Why prioritize taint mode?** Pattern matching finds syntax but misses context. A pattern `eval($X)` matches both `eval(user_input)` (vulnerable) and `eval("safe_literal")` (safe). Taint mode tracks data flow, so it only alerts when untrusted data actually reaches the sink—dramatically reducing false positives for injection vulnerabilities. **Iterating between approaches:** It's okay to experiment. If you start with taint mode and it's not working well (e.g., taint doesn't propagate as expected, too many false positives/negatives), switch to pattern matching. Conversely, if pattern matching produces too many false positives on safe cases, try taint mode instead. The goal is a working rule—not rigid adherence to one approach. **Output structure** - exactly 2 files in a directory named after the rule-id: ``` / ├── .yaml # Semgrep rule └── . # Test file with ruleid/ok annotations ``` ## Quick Start ```yaml rules: - id: insecure-eval languages: [python] severity: HIGH message: User input passed to eval() allows code execution mode: taint pattern-sources: - pattern: request.args.get(...) pattern-sinks: - pattern: eval(...) ``` Test file (`insecure-eval.py`): ```python # ruleid: insecure-eval eval(request.args.get('code')) # ok: insecure-eval eval("print('safe')") ``` Run tests (from rule directory): `semgrep --test --config .yaml .` ## Quick Reference - For commands, pattern operators, and taint mode syntax, see quick-reference.md. - For detailed workflow and examples, you MUST see workflow.md ## Workflow Copy this checklist and track progress: ``` Semgrep Rule Progress: - [ ] Step 1: Analyze the Problem - [ ] Step 2: Write Tests First - [ ] Step 3: Analyze AST structure - [ ] Step 4: Write the rule - [ ] Step 5: Iterate until all tests pass (semgrep --test) - [ ] Step 6: Optimize the rule (remove redundancies, re-test) - [ ] Step 7: Final Run ``` ## Documentation **REQUIRED**: Before writing any rule, use WebFetch to read **all** of these 4 links with Semgrep documentation: 1. [Rule Syntax](https://semgrep.dev/docs/writing-rules/rule-syntax) 2. [Pattern Syntax](https://semgrep.dev/docs/writing-rules/pattern-syntax) 3. [ToB Testing Handbook - Semgrep](https://appsec.guide/docs/static-analysis/semgrep/advanced/) 4. [Constant propagation](https://semgrep.dev/docs/writing-rules/data-flow/constant-propagation) 5. [Writing Rules Index](https://github.com/semgrep/semgrep-docs/tree/main/docs/writing-rules/)

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