Creates custom Semgrep rules for detecting security vulnerabilities, bug patterns, and code patterns. Use when writing Semgrep rules or building custom static analysis detections.
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Creates custom Semgrep rules for detecting security vulnerabilities, bug patterns, and code patterns. Use when writing Semgrep rules or building custom static analysis detections.
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LanguagePython
LicenseCC-BY-SA-4.0
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</> SKILL.md
semgrep-rule-creator.SKILL.md
---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
---# 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 <rule-id>.yaml <rule-id>.<ext>` 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 " + $VAR)
# GOOD: Matches all os.system calls with taint tracking
mode: taint
pattern-sources:
- pattern: input(...)
pattern-sinks:
- pattern: os.system(...)
```
## Strictness Level
This workflow is **strict** - do not skip steps:
- **Read documentation first**: See [Documentation](#documentation) before writing Semgrep rules
- **Test-first is mandatory**: Never write a rule without tests
- **100% test pass is required**: "Most tests pass" is not acceptable
- **Optimization comes last**: Only simplify patterns after all tests pass
- **Avoid generic patterns**: Rules must be specific, not match broad patterns
- **Prioritize taint mode**: For data flow vulnerabilities
- **One YAML file - one Semgrep rule**: Each YAML file must contain only one Semgrep rule; don't combine multiple rules in a single file
- **No generic rules**: When targeting a specific language for Semgrep rules - avoid generic pattern matching (`languages: generic`)
- **Forbidden `todook` and `todoruleid` test annotations**: `todoruleid: <rule-id>` and `todook: <rule-id>` annotations in tests files for future rule improvements are forbidden
## Overview
This skill guides creation of Semgrep rules that detect security vulnerabilities and code patterns. Rules are created iteratively: analyze the problem, write tests first, analyze AST structure, write the rule, iterate until all tests pass, optimize the rule.
**Approach selection:**
- **Taint mode** (prioritize): Data flow issues where untrusted input reaches dangerous sinks
- **Pattern matching**: Simple syntactic patterns without data flow requirements
**Why prioritize taint mode?** Pattern matching finds syntax but misses context. A pattern `eval($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:
```
<rule-id>/
├── <rule-id>.yaml # Semgrep rule
└── <rule-id>.<ext> # 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 <rule-id>.yaml <rule-id>.<ext>`
## Quick Reference
- For commands, pattern operators, and taint mode syntax, see [quick-reference.md]({baseDir}/references/quick-reference.md).
- For detailed workflow and examples, you MUST see [workflow.md]({baseDir}/references/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)
