feat: add security linter (#158) — 9.4: Security Linter

Add scripts/security_linter.py: standalone CLI that scans Python code
for common security vulnerabilities with severity ratings (CRITICAL/HIGH/
MEDIUM/LOW). Outputs JSON report by default, Markdown optional.

Checks include: eval/exec, subprocess shell=True, pickle, yaml.load,
hardcoded secrets, weak hashes, SQL injection patterns, and dynamic
imports.

Add scripts/test_security_linter.py: pytest test suite validating
core detection patterns and report generation.

This implements the smallest concrete fix to satisfy the acceptance
criteria: runs security linters, reports findings with severity,
outputs security lint report.

Closes #158

scripts/security_linter.py

@@ -0,0 +1,174 @@
+#!/usr/bin/env python3
+"""
+security_linter.py — Scan code for security vulnerabilities.
+
+Reports security findings with severity ratings (CRITICAL/HIGH/MEDIUM/LOW).
+Outputs a JSON security lint report.
+
+Usage:
+    python3 security_linter.py --path .
+    python3 security_linter.py --path . --output security_report.json
+    python3 security_linter.py --path . --format json  # default
+    python3 security_linter.py --path . --format markdown
+"""
+
+import argparse
+import json
+import re
+import sys
+from pathlib import Path
+from typing import List, Dict, Any, Optional
+
+
+SEVERITY_CRITICAL = "CRITICAL"
+SEVERITY_HIGH = "HIGH"
+SEVERITY_MEDIUM = "MEDIUM"
+SEVERITY_LOW = "LOW"
+
+
+class SecurityFinding:
+    """Represents a security finding."""
+
+    def __init__(
+        self,
+        file: str,
+        line: int,
+        issue: str,
+        severity: str,
+        cwe: Optional[str] = None,
+        recommendation: Optional[str] = None,
+    ):
+        self.file = file
+        self.line = line
+        self.issue = issue
+        self.severity = severity
+        self.cwe = cwe
+        self.recommendation = recommendation
+
+    def to_dict(self) -> Dict[str, Any]:
+        return {
+            "file": self.file,
+            "line": self.line,
+            "issue": self.issue,
+            "severity": self.severity,
+            "cwe": self.cwe,
+            "recommendation": self.recommendation,
+        }
+
+
+# Pattern entries: (pattern_regex, description, severity, cwe, recommendation)
+# Pattern strings use normal strings (not raw) to allow ['"] character classes without
+# backslash-injection issues. \s and \b are escaped to give \s and \b in the actual regex.
+SECURITY_PATTERNS = [
+    # eval/exec - arbitrary code execution
+    (r"\beval\s*\(", "Use of eval() - arbitrary code execution risk", SEVERITY_CRITICAL, "CWE-95", "Replace with ast.literal_eval() or a safer alternative"),
+    (r"\bexec\s*\(", "Use of exec() - arbitrary code execution risk", SEVERITY_CRITICAL, "CWE-95", "Refactor to avoid exec(); use functions or config files"),
+    # subprocess with shell=True
+    (r"subprocess\.(?:run|call|check_output|Popen)\s*\([^)]*shell\s*=\s*True", "subprocess with shell=True - shell injection risk", SEVERITY_HIGH, "CWE-78", "Use shell=False and pass command as a list"),
+    # pickle.loads - arbitrary code execution
+    (r"pickle\.loads?\s*\(", "Use of pickle - arbitrary code execution on untrusted data", SEVERITY_HIGH, "CWE-502", "Use json or a safe serialization format for untrusted data"),
+    # yaml.load without Loader
+    (r"yaml\.load\s*\(", "yaml.load() - unsafe deserialization", SEVERITY_HIGH, "CWE-502", "Use yaml.safe_load()"),
+    # tempfile.mktemp - insecure temp file creation
+    (r"tempfile\.mktemp\s*\(", "tempfile.mktemp() - insecure temporary file creation", SEVERITY_MEDIUM, "CWE-377", "Use tempfile.NamedTemporaryFile or TemporaryDirectory"),
+    # random module for crypto
+    (r"\brandom\.(?:random|randint|choice|shuffle)\b", "random module used for security/cryptographic purposes", SEVERITY_MEDIUM, "CWE-338", "Use secrets module for cryptographic randomness"),
+    # md5 or sha1 for security
+    (r"hashlib\.(?:md5|sha1)\s*\(", "Weak hash function (MD5/SHA1) used for security/crypto", SEVERITY_MEDIUM, "CWE-327", "Use SHA-256 or better for cryptographic purposes"),
+    # hardcoded password patterns - single or double quote char class, >=4 content chars
+    ('[\'"][^\'"]{4,}[\'"]',  "Hardcoded password detected", SEVERITY_HIGH, "CWE-259", "Use environment variables or a secrets manager"),
+    ('[\'"][^\'"]{6,}[\'"]',  "Hardcoded API key or secret detected", SEVERITY_HIGH, "CWE-798", "Use environment variables or a secrets vault"),
+    # SQL injection patterns - parentheses balanced
+    (r"cursor\.execute\s*\([^)]*\)", "Potential SQL injection - inspect query construction", SEVERITY_HIGH, "CWE-89", "Use parameterized queries with placeholders"),
+    # assert used for security validation
+    (r"\bassert\s+[^,)]*\b(?:password|token|secret|permission|auth|admin)\b", "assert used for security validation - can be disabled with -O", SEVERITY_MEDIUM, "CWE-253", "Use explicit if/raise for security checks; assert can be stripped"),
+    # __import__ dynamic
+    (r"__import__\s*\(", "Dynamic import via __import__ - potential code injection", SEVERITY_MEDIUM, "CWE-829", "Use importlib.import_module with validated module names"),
+]
+
+
+def scan_file(path: Path) -> List[SecurityFinding]:
+    findings = []
+    try:
+        with open(path, "r", encoding="utf-8", errors="ignore") as f:
+            lines = f.readlines()
+    except (OSError, UnicodeDecodeError):
+        return findings
+
+    for line_num, line in enumerate(lines, start=1):
+        for pattern, issue, severity, cwe, recommendation in SECURITY_PATTERNS:
+            if re.search(pattern, line):
+                findings.append(
+                    SecurityFinding(
+                        file=str(path),
+                        line=line_num,
+                        issue=issue,
+                        severity=severity,
+                        cwe=cwe,
+                        recommendation=recommendation,
+                    )
+                )
+    return findings
+
+
+def scan_directory(path: Path, extensions=None) -> List[SecurityFinding]:
+    if extensions is None:
+        extensions = {".py"}
+    findings = []
+    if not path.exists():
+        raise FileNotFoundError(f"Path not found: {path}")
+    for file_path in path.rglob("*"):
+        if file_path.is_file() and file_path.suffix in extensions:
+            findings.extend(scan_file(file_path))
+    return findings
+
+
+def generate_json_report(findings: List[SecurityFinding]) -> Dict[str, Any]:
+    by_severity = {SEVERITY_CRITICAL: [], SEVERITY_HIGH: [], SEVERITY_MEDIUM: [], SEVERITY_LOW: []}
+    for f in findings:
+        by_severity[f.severity].append(f.to_dict())
+    severity_counts = {s: len(v) for s, v in by_severity.items()}
+    total = sum(severity_counts.values())
+    return {"security_scan": {"total_findings": total, "by_severity": severity_counts, "findings": [f.to_dict() for f in findings]}}
+
+
+def generate_markdown_report(findings: List[SecurityFinding]) -> str:
+    by_severity = {SEVERITY_CRITICAL: [], SEVERITY_HIGH: [], SEVERITY_MEDIUM: [], SEVERITY_LOW: []}
+    for f in findings:
+        by_severity[f.severity].append(f)
+    emoji = {SEVERITY_CRITICAL: "🔴", SEVERITY_HIGH: "🟠", SEVERITY_MEDIUM: "🟡", SEVERITY_LOW: "🟢"}
+    lines = ["# Security Lint Report\n", f"Total findings: **{len(findings)}**\n\n"]
+    has_findings = False
+    for severity in [SEVERITY_CRITICAL, SEVERITY_HIGH, SEVERITY_MEDIUM, SEVERITY_LOW]:
+        flist = by_severity[severity]
+        if flist:
+            has_findings = True
+            lines.append(f"## {emoji[severity]} {severity} ({len(flist)} findings)\n")
+            for f in flist:
+                lines.append(f"- **{f.file}:{f.line}** — {f.issue}")
+            lines.append("")
+    if not has_findings:
+        lines.append("✅ No security issues found.\n")
+    return "\n".join(lines)
+
+
+def main():
+    parser = argparse.ArgumentParser(description="Scan code for security vulnerabilities")
+    parser.add_argument("--path", type=Path, default=Path("."), help="Path to scan (file or directory)")
+    parser.add_argument("--output", "-o", type=Path, default=None, help="Output file")
+    parser.add_argument("--format", choices=["json", "markdown"], default="json", help="Output format (default: json)")
+    parser.add_argument("--extensions", type=str, default=".py", help="Comma-separated file extensions (default: .py)")
+    args = parser.parse_args()
+    exts = {e.strip() for e in args.extensions.split(",")}
+    findings = scan_directory(args.path, extensions=exts)
+    output = json.dumps(generate_json_report(findings), indent=2) if args.format == "json" else generate_markdown_report(findings)
+    if args.output:
+        args.output.write_text(output, encoding="utf-8")
+    else:
+        print(output)
+    bad = sum(1 for f in findings if f.severity in (SEVERITY_CRITICAL, SEVERITY_HIGH))
+    sys.exit(1 if bad > 0 else 0)
+
+
+if __name__ == "__main__":
+    main()

scripts/test_security_linter.py

@@ -0,0 +1,95 @@
+#!/usr/bin/env python3
+"""Tests for scripts/security_linter.py — Issue #158: 9.4 Security Linter."""
+
+import sys
+import tempfile
+from pathlib import Path
+
+sys.path.insert(0, str(Path(__file__).parent.parent / "scripts"))
+
+from security_linter import (
+    scan_file,
+    scan_directory,
+    generate_json_report,
+    generate_markdown_report,
+    SEVERITY_CRITICAL,
+    SEVERITY_HIGH,
+    SEVERITY_MEDIUM,
+    SEVERITY_LOW,
+)
+
+
+def test_scan_file_detects_eval():
+    with tempfile.NamedTemporaryFile(mode="w", suffix=".py", delete=False) as f:
+        f.write("result = eval(user_input)\n")
+        f.flush()
+        findings = scan_file(Path(f.name))
+    assert len(findings) >= 1
+    assert findings[0].severity == SEVERITY_CRITICAL
+    assert "eval" in findings[0].issue.lower()
+
+
+def test_scan_file_detects_hardcoded_password():
+    with tempfile.NamedTemporaryFile(mode="w", suffix=".py", delete=False) as f:
+        f.write("password = 'supersecret123'\n")
+        f.flush()
+        findings = scan_file(Path(f.name))
+    assert any(f.severity == SEVERITY_HIGH for f in findings)
+
+
+def test_scan_file_detects_subprocess_shell_true():
+    with tempfile.NamedTemporaryFile(mode="w", suffix=".py", delete=False) as f:
+        f.write("subprocess.run(cmd, shell=True)\n")
+        f.flush()
+        findings = scan_file(Path(f.name))
+    assert any(f.severity == SEVERITY_HIGH and "shell" in f.issue.lower() for f in findings)
+
+
+def test_scan_file_detects_pickle():
+    with tempfile.NamedTemporaryFile(mode="w", suffix=".py", delete=False) as f:
+        f.write("data = pickle.loads(raw)\n")
+        f.flush()
+        findings = scan_file(Path(f.name))
+    assert any(f.severity == SEVERITY_HIGH and "pickle" in f.issue.lower() for f in findings)
+
+
+def test_scan_file_detects_yaml_load():
+    with tempfile.NamedTemporaryFile(mode="w", suffix=".py", delete=False) as f:
+        f.write("config = yaml.load(stream)\n")
+        f.flush()
+        findings = scan_file(Path(f.name))
+    assert any("yaml.load" in f.issue.lower() for f in findings)
+
+
+def test_json_report_structure():
+    from security_linter import SecurityFinding
+    findings = [
+        SecurityFinding("foo.py", 1, "eval() used", SEVERITY_CRITICAL, "CWE-95", "Use ast.literal_eval"),
+        SecurityFinding("bar.py", 10, "hardcoded password", SEVERITY_HIGH, "CWE-259", None),
+    ]
+    report = generate_json_report(findings)
+    assert "security_scan" in report
+    assert report["security_scan"]["total_findings"] == 2
+    assert report["security_scan"]["by_severity"][SEVERITY_CRITICAL] == 1
+    assert report["security_scan"]["by_severity"][SEVERITY_HIGH] == 1
+
+
+def test_markdown_report_contains_severity():
+    from security_linter import SecurityFinding
+    findings = [
+        SecurityFinding("test.py", 1, "eval() used", SEVERITY_CRITICAL, "CWE-95", "Use ast.literal_eval"),
+    ]
+    md = generate_markdown_report(findings)
+    assert "CRITICAL" in md or "🔴" in md
+    assert "eval() used" in md
+    assert "CWE-95" in md
+
+
+def test_scan_directory_empty_dir():
+    with tempfile.TemporaryDirectory() as tmpdir:
+        findings = scan_directory(Path(tmpdir))
+        assert findings == []
+
+
+def test_scan_file_no_issues():
+    safe_code =