The Stateless-Security Paradox

JWTs(JSON Web Tokens) are stateless by design, which means your authentication server doesn't maintain session state after token issuance. This scalability benefit comes with a critical trade-off: once a JWT is signed and issued, you cannot invalidate it before its expiration time without maintaining additional infrastructure. If a user logs out or their permissions change, that token remains valid for other servers consuming it until the exp claim is reached. This forces you to either accept delayed invalidation or implement a token blacklist (essentially introducing state), defeating much of the stateless advantage.

The practical implication is that you must make an architectural decision early: either accept that tokens live for their full duration, or build a separate blacklist service (typically Redis or your cache layer) that tracks revoked tokens. Many teams underestimate this cost and encounter problems when they need immediate token revocation for security incidents or permission changes.

Access Token and Refresh Token Pattern

The industry standard solution pairs short-lived access tokens with longer-lived refresh tokens. Access tokens (typically 5-15 minutes) are sent with each request and kept minimal in scope. Refresh tokens (typically days or weeks) are stored securely and used only to obtain new access tokens. This design limits the damage if an access token leaks while keeping the user experience smooth by preventing constant re-authentication.

When access tokens expire, the client uses the refresh token to get a new one without requiring the user to re-enter credentials. This balance minimizes the window of exposure while maintaining usability. Refresh tokens themselves should also be rotated on use to further limit compromise risk.

Payload Readability and Data Exposure

JWT payloads are base64url encoded, not encrypted. Anyone with access to the token can decode and read the claims inside. This means you must never include sensitive data like passwords, API keys, PII, or payment information in the JWT payload. Even though the signature guarantees the token hasn't been modified, the content is fully transparent to any party that sees the token.

Instead, use the payload only for claims necessary for authorization decisions: user ID, roles, permissions, scopes, and issue/expiration timestamps. Store sensitive user data server-side in your database. If a token is leaked, the attacker gains visibility into the user's claims but cannot forge new tokens without the signing key.

Signature Validation Vulnerabilities

A subtle but critical vulnerability emerges when servers accept multiple signature algorithms. Algorithm confusion attacks exploit servers that accept both HMAC (HS256) and RSA (RS256) signatures. An attacker can take a token signed with RS256 using the public key and re-sign it using HMAC with that same public key. If the server's validation logic doesn't enforce a specific algorithm, it accepts the attacker-controlled signature.

The fix is straightforward but easy to overlook: explicitly specify the expected algorithm in your JWT library configuration, never accepting user-supplied algorithm claims. For example, in Node.js with jsonwebtoken, always set the algorithms: ['RS256'] option in verify() rather than allowing automatic detection. Additionally, rotate your signing keys periodically and never expose private keys or use the same key for multiple purposes.

Storage Context and XSS Attack Surface

Where you store the JWT dramatically affects your XSS risk profile. Storing tokens in localStorage is convenient for single-page applications but exposes them to XSS attacks. Any malicious script injected into the page can read localStorage and steal the token. httpOnly cookies, by contrast, are inaccessible to JavaScript, preventing script-based theft. However, httpOnly cookies are vulnerable to CSRF attacks, requiring CSRF token protection.

Most modern applications use httpOnly, Secure, SameSite cookies for better default security. If you must use localStorage (for cross-domain scenarios), implement strict Content Security Policy headers and sanitize all user inputs rigorously. The choice reflects your threat model: prioritize XSS prevention with httpOnly cookies for traditional web applications, or accept the XSS risk for localStorage if your SPA requires it. There is no universally correct choice, only trade-offs based on your deployment context and security posture.