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frontend-security

Frontend & websites

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Updated 10-16 08:45

About this skill

Frontend Security Audit Skill

Perform security audits of frontend-adjacent codebases by mapping trust boundaries, tracing untrusted data into dangerous sinks, and then using pattern searches as supporting evidence. This skill covers browser/frontend concerns first, but also routes related Node.js, npm, auth/session, and upload findings to the correct references when they affect frontend security.

Audit Process

  1. Map the application context - Identify routes, rendering models, API boundaries, authentication/session mechanisms, third-party scripts, uploads, package-manager entry points, and deployment/runtime assumptions.
  2. Trace data flow - List untrusted sources such as URL parameters, forms, storage, postMessage, server responses, uploaded files, cookies, headers, and package metadata. Follow them to DOM, navigation, template, command, filesystem, database, network, and log sinks.
  3. Scan for evidence - Use rg searches, framework-aware checks, and AST or Semgrep rules where useful. Treat matches as leads to review in context, not proof of a vulnerability by themselves.
  4. Review defenses - Check output encoding, sanitization, CSP, CSRF controls, token storage, input validation, upload handling, dependency controls, and framework-specific escape hatches.
  5. Rate risk in context - Assign severity from exploitability, impact, reachability, privileges required, user interaction, data sensitivity, and compensating controls.
  6. Report findings - Include evidence, affected trust boundary, exploit scenario, severity rationale, remediation, and the most relevant reference.

Scope Routing

Use the main workflow for browser, template, and frontend framework issues. Load the narrower references when the audit touches one of these areas:

Finding area Route to
Browser XSS, unsafe URLs, tabnabbing, HTML injection references/xss-prevention.md
DOM sinks, DOM clobbering, postMessage, client-side storage references/dom-security.md
CSP headers, nonce/hash policies, Trusted Types hardening references/csp-configuration.md
Forms, cookie-based auth, state-changing requests references/csrf-protection.md
URL, number, date, schema, and structured input parsing references/input-validation.md
React, Astro, Twig, SSR, hydration, and template patterns references/framework-patterns.md
File uploads, generated downloads, archive handling references/file-upload-security.md
JWT lifecycle, token storage, refresh flows references/jwt-security.md
Node runtime, npm supply chain, scripts, command execution references/nodejs-npm-security.md

Name the boundary when a finding is outside pure frontend code. For example, "browser-to-API CSRF", "frontend-triggered upload processing", or "package-script supply-chain risk" is clearer than treating every issue as a frontend vulnerability.

State scope limits in the report. This skill can identify frontend-triggered server/API, upload, auth, or supply-chain risks, but deep backend authorization, database, infrastructure, malware, or incident-response review should be routed to the appropriate specialist workflow.

Evidence Searches

Start with the broad extension set below, then narrow it to the project. Prefer AST-aware tools or Semgrep for final confirmation when syntax or multiline structure matters.

FRONTEND_GLOBS='*.{js,jsx,ts,tsx,mjs,cjs,mts,cts,astro,vue,svelte,html,htm,twig,html.twig,njk,ejs,hbs,mustache,liquid,mdx}'

XSS, DOM, and Navigation Leads

# React and JSX escape hatches
rg -n "dangerouslySetInnerHTML|suppressHydrationWarning" --glob "$FRONTEND_GLOBS"

# Direct HTML parsing and DOM mutation
rg -n "\\.(innerHTML|outerHTML)\\s*=|insertAdjacentHTML|document\\.write|DOMParser" --glob "$FRONTEND_GLOBS"
rg -n "\\.(srcdoc|text|html)\\s*=|\\$\\([^)]*\\)\\.html\\(" --glob "$FRONTEND_GLOBS"

# URL, redirect, and new-window sinks
rg -n "location\\.(href|assign|replace)\\s*=|window\\.open\\(|target=[\"']_blank" --glob "$FRONTEND_GLOBS"
rg -n "javascript:|data:text/html|blob:" --glob "$FRONTEND_GLOBS"

# Dynamic code execution
rg -n "\\beval\\s*\\(|new Function\\s*\\(|set(?:Timeout|Interval)\\s*\\(\\s*['\"]" --glob "$FRONTEND_GLOBS"

# Template bypasses
rg -n "\\|raw|autoescape\\s+false|unsafeHTML|html\\s*`" --glob "$FRONTEND_GLOBS"

CSRF, Auth, and API Leads

# Forms and state-changing requests
rg -n "<form|method=[\"']post|fetch\\(|axios\\.|ky\\.|got\\." --glob "$FRONTEND_GLOBS"
rg -nUP "fetch\\s*\\([^)]*method\\s*:\\s*['\"](?:POST|PUT|PATCH|DELETE)['\"]" --glob "$FRONTEND_GLOBS"
rg -n "\\b(?:post|put|patch|delete)\\s*\\(" --glob "$FRONTEND_GLOBS"

# Token and session storage
rg -n "(localStorage|sessionStorage|indexedDB|document\\.cookie)\\b" --glob "$FRONTEND_GLOBS"
rg -n "(Authorization|Bearer|refresh[_-]?token|access[_-]?token|id[_-]?token|jwt)" --glob "$FRONTEND_GLOBS"

Secret, Upload, and Runtime Leads

# Hardcoded secrets and environment exposure
rg -n -i "(api[_-]?key|client[_-]?secret|password|private[_-]?key|token)\\s*[:=]" --glob "$FRONTEND_GLOBS" --glob "*.env*"

# Upload and download paths
rg -n "(multer|busboy|formidable|FileReader|URL\\.createObjectURL|Content-Disposition|extractAllTo|adm-zip|yauzl)" --glob "$FRONTEND_GLOBS" --glob "package.json"

# npm and command-execution risk
rg -n "\"(preinstall|install|postinstall|prepare)\"|child_process|exec\\(|execFile\\(|spawn\\(" --glob "package.json" --glob "*.{js,ts,mjs,cjs,mts,cts,sh,bash,zsh,yml,yaml,toml,json}" --glob "Dockerfile" --glob "*.dockerfile" --glob ".github/workflows/*"

When a search has many matches, sample representative paths first, then follow the source-to-sink chain for code that handles untrusted input or sensitive state. Do not assign severity from the search pattern alone.

Reference Documentation

Load these references based on findings:

  • XSS vulnerabilities found: See references/xss-prevention.md
  • CSRF concerns: See references/csrf-protection.md
  • DOM manipulation issues: See references/dom-security.md
  • CSP review needed: See references/csp-configuration.md
  • Input handling issues: See references/input-validation.md
  • Node.js/NPM audit: See references/nodejs-npm-security.md
  • Framework-specific patterns: See references/framework-patterns.md
  • File upload handling: See references/file-upload-security.md
  • JWT implementation: See references/jwt-security.md

Risk-Based Severity

Use vulnerability class as a starting clue, then rate the specific finding:

  • Critical - Likely exploit path with high impact, such as unauthenticated account takeover, privileged stored XSS, exposed secrets that grant production access, or remote code execution in reachable build/runtime paths.
  • High - Plausible exploit with meaningful user, data, integrity, or supply chain impact, especially when reachable by ordinary users or low-privilege attackers.
  • Medium - Real weakness with limited reach, lower sensitivity, meaningful prerequisites, or compensating controls that reduce immediate impact.
  • Low - Hard-to-exploit issue, defense-in-depth gap, informational exposure, deprecated pattern, or hygiene improvement with weak immediate impact.

Record the severity rationale using these factors:

  • exploitability and required attacker capability;
  • user interaction and privileges required;
  • reachability from production code paths;
  • data sensitivity and integrity impact;
  • persistence, lateral movement, and blast radius;
  • existing controls such as CSP, SameSite cookies, validation, sandboxing, authorization, monitoring, and dependency pinning.

Report Format

## Security Audit Report

### Summary

- Critical: X findings
- High: X findings
- Medium: X findings
- Low: X findings

### Critical Findings

#### [CRITICAL-001] Title

- **Location**: file:line
- **Trust boundary**: Source to sink being crossed
- **Evidence**: Code snippet or search result
- **Exploit scenario**: How an attacker reaches the issue
- **Risk**: Description of the vulnerability
- **Severity rationale**: Why this is Critical/High/Medium/Low in this project
- **Remediation**: How to fix
- **Reference**: OWASP link

### High Findings

[...]

OWASP Reference Links

For comprehensive guidance, consult these OWASP cheatsheets directly: