sica-fondt/knowledge/cyber-skills/skills/implementing-semgrep-for-custom-sast-rules/references/api-reference.md

API Reference: Semgrep Custom SAST Rules

Libraries Used

Library	Purpose
subprocess	Execute semgrep CLI scans
json	Parse semgrep JSON output
yaml	Read and write custom Semgrep rule files
pathlib	Handle source code and rule file paths

Installation

# Python package
pip install semgrep

# Homebrew (macOS)
brew install semgrep

# Docker
docker pull semgrep/semgrep:latest

CLI Reference

Core Commands

# Scan with auto-detected rules
semgrep scan --config auto --json --output results.json /path/to/code

# Scan with specific rulesets from Semgrep Registry
semgrep scan --config p/python --config p/owasp-top-ten /path/to/code

# Scan with a custom rule file
semgrep scan --config my-rules.yaml /path/to/code

# Scan with multiple configs
semgrep scan --config p/security-audit --config ./custom-rules/ /path/to/code

Key CLI Flags

Flag	Description
--config, -c	Rule source: registry key, YAML file, or directory
--json	Output results in JSON format
--sarif	Output in SARIF format (for CI/CD integration)
--output, -o	Write results to file
--severity	Filter by severity: INFO, WARNING, ERROR
--include	Only scan files matching glob pattern
--exclude	Skip files matching glob pattern
--lang	Restrict scan to specific language
--max-target-bytes	Skip files larger than N bytes
--timeout	Per-rule timeout in seconds (default: 5)
--jobs, -j	Number of parallel jobs
--verbose, -v	Show detailed scan progress
--metrics off	Disable anonymous metrics

Custom Rule Syntax

Basic Pattern Rule

rules:
  - id: hardcoded-password
    pattern: password = "..."
    message: "Hardcoded password detected — use environment variables"
    languages: [python]
    severity: ERROR
    metadata:
      cwe: ["CWE-798: Use of Hard-coded Credentials"]
      owasp: ["A07:2021 - Identification and Authentication Failures"]

Pattern Operators

rules:
  - id: sql-injection-format-string
    patterns:
      - pattern: |
          cursor.execute($QUERY % ...)
      - pattern-not: |
          cursor.execute("..." % ())
    message: "SQL injection via string formatting — use parameterized queries"
    languages: [python]
    severity: ERROR

  - id: unsafe-deserialization
    pattern-either:
      - pattern: pickle.loads(...)
      - pattern: pickle.load(...)
      - pattern: yaml.load(..., Loader=yaml.Loader)
      - pattern: yaml.unsafe_load(...)
    message: "Unsafe deserialization — may allow remote code execution"
    languages: [python]
    severity: ERROR

  - id: missing-timeout-requests
    patterns:
      - pattern: requests.$METHOD(...)
      - pattern-not: requests.$METHOD(..., timeout=..., ...)
    message: "HTTP request without timeout — may hang indefinitely"
    languages: [python]
    severity: WARNING

Metavariable Patterns

rules:
  - id: eval-user-input
    patterns:
      - pattern: |
          $INPUT = request.$METHOD(...)
          ...
          eval($INPUT)
    message: "User input passed to eval() — command injection risk"
    languages: [python]
    severity: ERROR

Python Integration

import subprocess
import json

def run_semgrep(target_path, config="auto", severity=None):
    cmd = [
        "semgrep", "scan",
        "--config", config,
        "--json",
        "--metrics", "off",
        str(target_path),
    ]
    if severity:
        cmd.extend(["--severity", severity])

    result = subprocess.run(cmd, capture_output=True, text=True, timeout=600)
    output = json.loads(result.stdout)
    return output.get("results", [])

def summarize_findings(results):
    by_severity = {"ERROR": [], "WARNING": [], "INFO": []}
    for r in results:
        sev = r.get("extra", {}).get("severity", "INFO")
        by_severity[sev].append({
            "rule": r["check_id"],
            "file": r["path"],
            "line": r["start"]["line"],
            "message": r["extra"]["message"],
        })
    return by_severity

Semgrep Registry Rule Packs

Pack	Description
p/python	Python-specific security and correctness rules
p/javascript	JavaScript/TypeScript rules
p/owasp-top-ten	OWASP Top 10 vulnerability patterns
p/security-audit	Broad security audit rules across languages
p/secrets	Secret and credential detection
p/ci	Rules optimized for CI/CD pipelines
p/docker	Dockerfile security best practices
p/terraform	Terraform IaC security rules

Output Format

{
  "results": [
    {
      "check_id": "python.lang.security.audit.eval-detected",
      "path": "app/views.py",
      "start": {"line": 42, "col": 5},
      "end": {"line": 42, "col": 28},
      "extra": {
        "message": "Detected eval() usage — avoid with untrusted input",
        "severity": "ERROR",
        "metadata": {
          "cwe": ["CWE-95"],
          "owasp": ["A03:2021 - Injection"]
        }
      }
    }
  ],
  "errors": [],
  "stats": {
    "findings": 3,
    "errors": 0,
    "total_time": 2.45
  }
}