🍋
Menu
Troubleshooting Beginner 1 min read 263 words

Troubleshooting JWT Token Issues

JSON Web Tokens are widely used for authentication but can be frustrating to debug. This guide covers common JWT problems including expiration errors, signature mismatches, and payload decoding issues.

Key Takeaways

  • A JWT consists of three Base64URL-encoded parts separated by dots: header, payload, and signature.
  • Symptom: `TokenExpiredError` or `jwt expired` error.
  • This is also a known security vulnerability — never trust the `alg` claim from untrusted tokens.
  • Never let the JWT itself dictate which algorithm to use.

JWT Structure

A JWT consists of three Base64URL-encoded parts separated by dots: header, payload, and signature. Understanding this structure is key to debugging.

Common Issues

Token Expired

Symptom: TokenExpiredError or jwt expired error.

Cause: The exp (expiration) claim in the payload is in the past. This can happen due to clock skew between servers, or because the token's lifetime is too short.

Fix: Check the exp claim by decoding the payload (without verification). Compare with the current time. Add a clock tolerance (leeway) of 30-60 seconds to account for clock differences.

Invalid Signature

Symptom: JsonWebTokenError: invalid signature.

Cause: The signing key used to verify doesn't match the key used to create the token. This happens when keys are rotated, when using the wrong key for the algorithm, or when the token was tampered with.

Fix: Ensure you're using the correct signing key. For RS256, verify you're using the public key (not private) for verification. Check the kid (key ID) claim in the header.

Algorithm Confusion

Symptom: Signature verification fails despite correct keys.

Cause: The alg header says RS256 but verification uses HS256, or vice versa. This is also a known security vulnerability — never trust the alg claim from untrusted tokens.

Fix: Always specify the expected algorithm explicitly in your verification code. Never let the JWT itself dictate which algorithm to use.

Debugging Steps

  1. Decode the header and payload (Base64URL) without verification.
  2. Check exp, iat, and nbf timestamps.
  3. Verify the iss (issuer) and aud (audience) claims.
  4. Confirm the algorithm matches your verification setup.

Связанные инструменты

Связанные форматы

Связанные руководства

JSON vs YAML vs TOML: Choosing a Configuration Format

Configuration files are the backbone of modern applications. JSON, YAML, and TOML each offer different trade-offs between readability, complexity, and tooling support that affect your development workflow.

How to Format and Validate JSON Data

Malformed JSON causes silent failures in APIs and configuration files. Learn how to format, validate, and debug JSON documents to prevent integration errors and improve readability.

Base64 Encoding: How It Works and When to Use It

Base64 converts binary data into ASCII text, making it safe for transmission through text-based systems. Learn when Base64 is the right choice and when alternatives like hex encoding or URL encoding are more appropriate.

Best Practices for Working with Unix Timestamps

Unix timestamps provide a language-agnostic way to represent points in time, but they come with pitfalls around time zones, precision, and the 2038 problem. This guide covers best practices for storing and converting timestamps.

Hash Functions Compared: MD5, SHA-1, SHA-256, and Beyond

Hash functions are used for file integrity, password storage, and digital signatures. This comparison covers the most common algorithms, their security status, and when to use each one in modern applications.

How to Debug JWT Tokens Step by Step

JWT tokens contain encoded claims that can be decoded and inspected. Learn how to decode, verify, and troubleshoot JWT authentication issues.

How to Format and Validate JSON Data

Minified JSON is unreadable and errors are hard to find. Learn how to pretty-print, validate, and transform JSON for debugging and development.

How to Generate UUIDs and Unique Identifiers

Unique identifiers are fundamental to distributed systems. Learn the differences between UUID v4, v7, ULID, and other ID formats and when to use each.

How to Encode and Decode Base64 Data

Base64 converts binary data to ASCII text for safe transmission. Learn how Base64 encoding works and its common uses in web development.

How to Convert Between Number Systems (Binary, Hex, Octal)

Developers regularly work with binary, hexadecimal, and octal numbers. Learn conversion techniques and when each number system is used.

REST API Testing: Best Practices for Developers

Testing APIs thoroughly prevents bugs and ensures reliability. Learn strategies for testing endpoints, handling edge cases, and validating responses.

JSON vs XML vs Protocol Buffers: Data Serialization Compared

Choosing the right data serialization format affects performance, readability, and development speed. Compare JSON, XML, and Protocol Buffers for your use case.

SHA-256 vs SHA-3 vs BLAKE3: Hash Function Comparison

Hash functions are fundamental to security and data integrity. Compare the performance, security, and use cases of modern hash algorithms.

Troubleshooting Base64 Encoding Errors

Base64 encoding and decoding can fail in subtle ways. Learn how to diagnose padding errors, charset issues, and corrupted data.

Troubleshooting Unix Timestamp Conversion Issues

Unix timestamps can be confusing — seconds vs milliseconds, timezone handling, and the Year 2038 problem. Learn to diagnose and fix timestamp issues.

URL Encoding Best Practices for Web Developers

Proper URL encoding prevents broken links and security vulnerabilities. Learn which characters must be encoded and how to handle international URLs.

How to Debug API Requests and Responses

Inspect HTTP headers, request bodies, response codes, and timing to diagnose API integration issues.

Epoch and Unix Timestamp Conversion Guide

Convert between Unix timestamps, ISO 8601 dates, and human-readable formats across timezones.

Cron Expression Syntax Guide

Write and debug cron schedule expressions for task automation, with examples for common scheduling patterns.

HTTP Header Security Best Practices

Configure security headers including CSP, HSTS, X-Frame-Options, and permissions policy for web applications.

How to Design RESTful API URL Structures

Well-designed API URLs are intuitive, consistent, and follow REST conventions. Learn patterns for resource naming, versioning, filtering, and pagination.

How to Generate Secure API Keys

Create cryptographically secure API keys with proper entropy, formatting, and management practices.

Git Hooks for Automated Code Quality

Git hooks run scripts automatically before or after Git events. Set up pre-commit hooks for linting, formatting, and testing to catch issues before they reach your repository.

YAML vs JSON vs TOML for Configuration Files

Compare configuration file formats for developer tools, CI/CD pipelines, and application settings.

Encoding Explained: UTF-8, ASCII, Base64, and URL Encoding

Understand character encodings, binary-to-text encoding, and URL encoding to prevent data corruption and bugs.

Environment Variable Management Best Practices

Environment variables separate configuration from code, enabling the same application to run in development, staging, and production. Learn secure patterns for managing them.

API Testing and Debugging with Developer Tools

Test REST APIs effectively using browser DevTools, curl, and dedicated API testing tools.

Troubleshooting CORS Errors in Web Applications

CORS errors are among the most confusing web development issues. Learn how Cross-Origin Resource Sharing works, why browsers block requests, and how to fix common misconfigurations.

Docker vs Virtual Machines for Development Environments

Compare Docker containers and VMs for creating consistent development environments.

HTTP Status Codes: A Developer's Quick Reference

Choosing the right HTTP status code communicates your API's intent clearly. Learn when to use each status code family and the most important codes in each range.

Diff Algorithms: Understanding How Text Comparison Works

Text comparison tools use sophisticated algorithms to detect additions, deletions, and modifications between two documents. Learn how Myers, patience, and histogram diff algorithms work.

Git Branching Strategies Compared

Compare GitFlow, trunk-based, and GitHub Flow branching strategies for different team sizes.

Performance Optimization for Browser-Based Processing Tools

Client-side tools run in the browser with limited resources. Learn how to keep tools responsive using Web Workers, chunked processing, and efficient memory management.

API Rate Limiting: Strategies and Implementation

Rate limiting protects APIs from abuse and ensures fair resource allocation. Learn common algorithms, response headers, and client-side handling strategies.