Store authentication tokens securely
SkillFiles & storagetoken-storage-security is a skill that guides an AI agent through checking that login tokens are stored safely so attackers cannot steal them via browser scripts. It is used when reviewing an authentication implementation, setting up a new auth system, or evaluating whether the current token storage approach exposes the application to XSS-based token theft.
Available today. Use it from your connected AI after setup.
No other account needed.
Have the authentication code or design you want reviewed available to the agent.
Then ask your AI: use the Store authentication tokens securely skill
What your AI can do with it
- Review an existing authentication implementation for token storage issues
- Guide setup of a new auth system with safe token storage
- Evaluate whether current token storage exposes the app to XSS-based token theft
- Check that login tokens cannot be stolen via browser scripts
Getting started
- Have the authentication code or design you want reviewed available to the agent.
- Add the token-storage-security skill to the agent's available skills.
- Ask the agent to review the auth implementation or evaluate the token storage approach for XSS-based token theft.
What this skill tells your AI
The instructions your AI receives, as published by thedaviddias/front-end-checklist in skills/token-storage-security/SKILL.md and read by ahel’s review.
localStorage is accessible to any JavaScript running on the page. A single XSS vulnerability — including one in a third-party script — can exfiltrate all tokens silently. httpOnly cookies are completely invisible to JavaScript; even if an attacker executes arbitrary code on the page, they cannot read the cookie. This single architectural choice eliminates the most common token theft vector.
Quick Reference
- Store session tokens and JWTs in httpOnly cookies — not localStorage
- localStorage is readable by any JavaScript on the page, including injected XSS code
- Combine httpOnly with Secure and SameSite=Strict (or Lax) cookie flags
- Use short-lived access tokens and rotate refresh tokens on each use
- Protect every state-changing request with CSRF defenses, not cookie flags alone
Check
Check whether authentication tokens are stored in httpOnly cookies or in JavaScript-accessible storage like localStorage. Verify that POST, PUT, PATCH, and DELETE requests also require a CSRF token or an equivalent anti-forgery control.
Fix
Move token storage from localStorage/sessionStorage to httpOnly cookies set by the server, and ensure the cookies have Secure and SameSite flags. Add CSRF protection to every state-changing route that relies on browser-sent credentials.
Explain
Explain why localStorage is vulnerable to XSS token theft and how httpOnly cookies mitigate this attack vector.
Code Review
Review authentication flows, token storage calls, and API client code. Flag any use of localStorage.setItem, sessionStorage.setItem, or document.cookie for storing access tokens, JWTs, or session identifiers. Also flag credentialed mutations that lack a CSRF token, Origin check, or same-site enforcement.
For full implementation details, code examples, and framework-specific guidance,
see references/rule.md.
Rule page: https://frontendchecklist.io/en/rules/security/token-storage-security
Signals
- GitHub stars
- 74k
- Forks
- 7k
- Last commit
- Aug 2026
Questions
- When should this skill be used?
- When reviewing an authentication implementation, setting up a new auth system, or evaluating whether the current token storage approach exposes the application to XSS-based token theft.
- What does the skill actually check?
- It guides the agent through checking that login tokens are stored safely so attackers cannot steal them via browser scripts.
Advanced
- Item type
- skill
- Key
token-storage-security- Source
- github.com/thedaviddias/front-end-checklist