> ## Documentation Index > Fetch the complete documentation index at: https://mintlify.com/trailofbits/skills/llms.txt > Use this file to discover all available pages before exploring further. # Semgrep Rule Variant Creator > Port existing Semgrep rules to new target languages with applicability analysis and test-driven validation Port existing Semgrep rules to new target languages with proper applicability analysis and independent test-driven validation for each language variant. ## Overview The Semgrep Rule Variant Creator plugin takes an existing Semgrep rule and one or more target languages, then generates independent rule variants for each applicable language. Each variant goes through a complete 4-phase cycle ensuring quality and correctness. **Key capabilities:** * Applicability analysis before porting * Independent 4-phase cycle per language * Test-first methodology for each variant * Language-specific idiom adaptation * Proper validation before proceeding ## Installation ```bash theme={null} /plugin install trailofbits/skills/plugins/semgrep-rule-variant-creator ``` ## Prerequisites * [Semgrep](https://semgrep.dev/docs/getting-started/) installed and available in PATH * Existing Semgrep rule to port (in YAML format) * Target languages specified ## When to Use Use this plugin when you need to: * Port an existing Semgrep rule to one or more target languages * Create language-specific variants of a universal vulnerability pattern * Expand rule coverage across a polyglot codebase * Translate rules between languages with equivalent constructs ## When NOT to Use Do NOT use this plugin for: * Creating a new Semgrep rule from scratch (use `semgrep-rule-creator` instead) * Running existing rules against code * Languages where the vulnerability pattern fundamentally doesn't apply * Minor syntax variations within the same language ## Input Specification This plugin requires: 1. **Existing Semgrep rule** - YAML file path or YAML rule content 2. **Target languages** - One or more languages to port to Example invocations: ``` Port the sql-injection.yaml Semgrep rule to Go and Java ``` ``` Create Semgrep rule variants of my-rule.yaml for TypeScript, Rust, and C# ``` ``` Port this Semgrep rule to Golang ``` ## Output Structure For each applicable target language, produces: ``` -/ ├── -.yaml # Ported rule └── -. # Test file ``` ### Example Output **Input:** * Rule: `python-command-injection.yaml` * Target languages: Go, Java **Output:** ``` python-command-injection-golang/ ├── python-command-injection-golang.yaml └── python-command-injection-golang.go python-command-injection-java/ ├── python-command-injection-java.yaml └── python-command-injection-java.java ``` ## Four-Phase Workflow Each target language goes through an independent 4-phase cycle: Determine if the vulnerability pattern applies to the target language before proceeding. **Analysis criteria:** * Does the vulnerability class exist in the target language? * Does an equivalent construct exist (function, pattern, library)? * Are the semantics similar enough for meaningful detection? **Verdict options:** * `APPLICABLE` → Proceed with variant creation * `APPLICABLE_WITH_ADAPTATION` → Proceed but significant changes needed * `NOT_APPLICABLE` → Skip this language, document why Write tests BEFORE the rule using target language idioms. Create test file with: * Minimum 2 vulnerable cases (`ruleid:`) * Minimum 2 safe cases (`ok:`) * Language-specific edge cases ```go theme={null} // ruleid: sql-injection-golang db.Query("SELECT * FROM users WHERE id = " + userInput) // ok: sql-injection-golang db.Query("SELECT * FROM users WHERE id = ?", userInput) ``` Translate the original rule to the target language. 1. **Analyze AST**: `semgrep --dump-ast -l test-file` 2. **Translate patterns** to target language syntax 3. **Update metadata**: language key, message, rule ID 4. **Adapt for idioms**: Handle language-specific constructs Validate YAML and run tests to ensure correctness. ```bash theme={null} # Validate YAML semgrep --validate --config rule.yaml # Run tests semgrep --test --config rule.yaml test-file ``` **Checkpoint**: Output MUST show `All tests passed`. For taint rule debugging: ```bash theme={null} semgrep --dataflow-traces -f rule.yaml test-file ``` Complete the full 4-phase cycle for each language before moving to the next. Do not batch languages together. ## Applicability Analysis Details Before porting, perform thorough analysis: ### Does the Vulnerability Class Exist? Examples: * **Buffer overflow**: Applies to C/C++, may apply to Rust (in unsafe blocks), does NOT apply to Python/Java * **SQL injection**: Applies to any language with database access * **XSS**: Applies to any language generating HTML output ### Does an Equivalent Construct Exist? Parse the original rule to identify: * **Sinks**: What dangerous functions/methods does it detect? * **Sources**: Where does tainted data originate? * **Pattern type**: Is it taint-mode or pattern-matching? Then research the target language: * What are the equivalent dangerous functions? * What are the common source patterns? * Are there language-specific idioms to consider? ### Example Analysis ``` Original: Python os.system(user_input) Target: Go exec.Command(user_input) VERDICT: APPLICABLE REASONING: Both execute shell commands with user input. Vulnerability is identical (command injection). Detection logic (taint from input to exec) translates directly. ``` ## Example Variant Creation Let's port a Python SQL injection rule to Go: ```yaml python-sql-injection.yaml (Original) theme={null} rules: - id: sql-injection languages: [python] severity: ERROR message: SQL query constructed from user input mode: taint pattern-sources: - pattern: request.args.get(...) pattern-sinks: - pattern: cursor.execute($QUERY, ...) ``` ```yaml sql-injection-golang.yaml (Ported) theme={null} rules: - id: sql-injection-golang languages: [go] severity: ERROR message: SQL query constructed from user input mode: taint pattern-sources: - pattern: r.URL.Query().Get(...) - pattern: r.FormValue(...) pattern-sinks: - pattern: $DB.Query($QUERY, ...) - pattern: $DB.Exec($QUERY, ...) ``` ```go sql-injection-golang.go (Tests) theme={null} package main import ( "database/sql" "net/http" ) func handler1(w http.ResponseWriter, r *http.Request, db *sql.DB) { // ruleid: sql-injection-golang id := r.URL.Query().Get("id") db.Query("SELECT * FROM users WHERE id = " + id) } func handler2(w http.ResponseWriter, r *http.Request, db *sql.DB) { // ruleid: sql-injection-golang id := r.FormValue("id") db.Exec("DELETE FROM users WHERE id = " + id) } func handler3(w http.ResponseWriter, r *http.Request, db *sql.DB) { // ok: sql-injection-golang id := r.URL.Query().Get("id") db.Query("SELECT * FROM users WHERE id = ?", id) } func handler4(w http.ResponseWriter, r *http.Request, db *sql.DB) { // ok: sql-injection-golang db.Query("SELECT * FROM users WHERE id = 1") } ``` ## Key Differences from Rule Creator | Aspect | semgrep-rule-creator | semgrep-rule-variant-creator | | -------- | ----------------------- | -------------------------------- | | Input | Bug pattern description | Existing rule + target languages | | Output | Single rule+test | Multiple rule+test directories | | Workflow | Single creation cycle | Independent cycle per language | | Phase 1 | Problem analysis | Applicability analysis | ## Rationalizations to Reject When porting Semgrep rules, reject these common shortcuts: | Rationalization | Why It Fails | Correct Approach | | ----------------------------------------------- | ------------------------------------ | ------------------------------------- | | "Pattern structure is identical" | Different ASTs across languages | Always dump AST for target language | | "Same vulnerability, same detection" | Data flow differs between languages | Analyze target language idioms | | "Rule doesn't need tests since original worked" | Language edge cases differ | Write NEW test cases for target | | "Skip applicability - it obviously applies" | Some patterns are language-specific | Complete applicability analysis first | | "I'll create all variants then test" | Errors compound, hard to debug | Complete full cycle per language | | "Library equivalent is close enough" | Surface similarity hides differences | Verify API semantics match | | "Just translate the syntax 1:1" | Languages have different idioms | Research target language patterns | ## Strictness Principles **Non-negotiable requirements:** * Applicability analysis is mandatory: Don't assume patterns translate * Each language is independent: Complete full cycle before moving to next * Test-first for each variant: Never write a rule without test cases * 100% test pass required: "Most tests pass" is not acceptable ## Commands | Task | Command | | ---------------- | --------------------------------------------- | | Run tests | `semgrep --test --config rule.yaml test-file` | | Validate YAML | `semgrep --validate --config rule.yaml` | | Dump AST | `semgrep --dump-ast -l ` | | Debug taint flow | `semgrep --dataflow-traces -f rule.yaml file` | ## Foundational Knowledge The `semgrep-rule-creator` plugin is the authoritative reference for Semgrep rule creation fundamentals. Consult it for guidance on: * When to use taint mode vs pattern matching * Test-first methodology * Anti-patterns to avoid * Iterating until tests pass * Rule optimization When porting a rule, you're applying these same principles in a new language context. ## Related Plugins * **semgrep-rule-creator** - Create new Semgrep rules from scratch * **static-analysis** - Run existing Semgrep rules against code * **variant-analysis** - Find similar vulnerabilities across codebases ## Additional Resources * [Semgrep Pattern Examples](https://semgrep.dev/docs/writing-rules/pattern-examples) - Per-language pattern references * [Semgrep Testing Rules](https://semgrep.dev/docs/writing-rules/testing-rules) - Testing annotations * [Trail of Bits Testing Handbook](https://appsec.guide/docs/static-analysis/semgrep/advanced/) - Advanced patterns Maciej Domanski ([opensource@trailofbits.com](mailto:opensource@trailofbits.com))