Every developer has been there. You're mid-incident, it's late, and you need to quickly check a JWT, test a CORS header, or validate an SPF record. You search, find a tool, and then it asks you to create an account. Or worse — it uploads your data to a server you know nothing about.
This guide covers the best free online developer security tools in 2026 that run entirely in your browser. No account. No uploads. No telemetry. Your data never leaves your machine.
All 20 tools listed here are available at tools.monitorgiant.com — 100% client-side, always free.
Why "No Signup, No Upload" actually matters
When you paste a JWT token, a private key, or an internal domain name into a web tool, you are trusting that tool's infrastructure completely. Most tools send that data to their servers for processing. That means:
- ✗ Your JWT — which carries auth claims — hits a third-party server
- ✗ Your internal domain names are logged
- ✗ Your SSL certificate details are stored somewhere you don't control
Client-side tools solve this. Everything runs in your browser using Web Crypto APIs, DNS-over-HTTPS public resolvers, and JavaScript — zero server involvement for sensitive operations.
The 20 best free developer security tools — no signup required
All tools available at tools.monitorgiant.com
DNS / DKIM Inspector
The fastest way to audit your domain's email authentication posture. Fetches live A, MX, TXT, AAAA, CNAME, and NS records using Cloudflare's DNS-over-HTTPS API — no server-side proxy, directly from your browser. Validates SPF, DKIM, and DMARC records in one shot, parses DKIM selector subdomains, checks public key formatting, and flags SPF syntax errors.
When to use: Before launching a new domain for email sending. After any DNS change. When emails are landing in spam.
Open tool →Security Headers Grader
Grades any website's HTTP response headers from A+ to F. Checks Strict-Transport-Security (HSTS), Content-Security-Policy (CSP), X-Frame-Options, Referrer-Policy, Permissions-Policy, Cross-Origin-Opener-Policy (COOP), and Cross-Origin-Resource-Policy (CORP). Google uses security headers as a trust signal — missing HSTS or CSP hurts both users and SEO.
When to use: Before every deploy. Auditing production sites. After any config change.
Open tool →HTTP Headers Inspector
Shows every response header a URL returns — including cache-control directives, full redirect chains (301 → 302 → final destination), and non-security headers like X-Powered-By that leak your stack. Different from the grader — this is a full raw dump, useful for debugging CDN caching and verifying that your server isn't leaking version info.
When to use: Debugging CDN caching. Verifying stack info isn't exposed. Tracing redirect chains.
Open tool →SSL / TLS Certificate Checker
Checks your domain's SSL/TLS posture via DNS — without connecting to your server. Surfaces CAA records (which CAs are allowed to issue certificates), TLSA/DANE records, DNSSEC status, and HTTPS redirect detection. Includes a direct link to the full SSL Labs grading report.
When to use: Before renewing a cert. After a migration. Hardening against certificate mis-issuance.
Open tool →SSL Certificate Decoder
Paste any PEM-format certificate and get every field decoded — entirely in your browser. No server ever sees your certificate. Decodes Subject, Issuer, Serial Number, validity dates with days-remaining countdown, Subject Alternative Names (SANs), key algorithm and size, and all X.509 v3 extensions.
When to use: Verifying a cert before installing it. Auditing SANs. Checking expiry.
Open tool →JWT Decoder
⚠ Never paste production JWTs into server-side tools
Decodes any JSON Web Token — header, payload, and signature section — instantly, with zero server involvement. Shows all claims including iat, exp, sub, iss, aud, and expiry status (expired / valid / about to expire). This is the most security-sensitive tool on this list — client-side-only matters most here.
When to use: Debugging auth failures. Checking IdP claims. Verifying token expiry during incident response.
Open tool →CORS Policy Tester
Sends both a simple request and an OPTIONS preflight to any URL and shows every CORS header returned, with a plain-English verdict: Allowed / Blocked / Open (wildcard). CORS bugs are among the most common and confusing issues in web development — this tool ends the guessing.
When to use: CORS error and you can't tell if it's server config or request headers. Before deploying API changes.
Open tool →CVSS Score Calculator
Calculate a CVSS v3.1 Base Score by selecting values for all 8 metrics: Attack Vector, Attack Complexity, Privileges Required, User Interaction, Scope, Confidentiality, Integrity, and Availability impact. Produces a numeric score (0.0–10.0), severity badge (None / Low / Medium / High / Critical), and CVSS vector string for reports or bug trackers.
When to use: Writing a vulnerability report. Triaging a CVE. Communicating severity to non-technical stakeholders.
Open tool →Hash Generator
Generate cryptographic hashes from text or files using the browser's native Web Crypto API. Supports MD5, SHA-1, SHA-256, SHA-384, SHA-512, and HMAC variants with configurable secret keys. Output in Hex, Base64, or Base64URL. Files are hashed locally — nothing is uploaded.
When to use: File integrity checksums. HMAC webhook signatures. Verifying a downloaded file.
Open tool →Bcrypt Hash Generator
Generate bcrypt hashes with configurable cost factors (4–14) and verify plaintext strings against existing hashes — all computed in your browser using WebAssembly. Includes a bcrypt anatomy explainer (prefix, cost, salt, hash structure).
When to use: Testing password hashing in auth implementation. Verifying a legacy hash during migration.
Open tool →Password Generator
Generates cryptographically secure passwords using window.crypto.getRandomValues() — the browser's CSPRNG. Configurable length (4–256), charset (uppercase, lowercase, numbers, symbols), and bulk generation mode with download. Not Math.random().
Open tool →Password Strength Visualizer
Shows how long a modern GPU cluster would take to crack any password — based on entropy analysis, not just pattern matching. Real crack-time estimates, pattern detection (dictionary words, keyboard walks, repeated chars), and improvement suggestions.
Open tool →SPF Record Creator
Build a valid SPF TXT record by selecting email sending sources from 15+ provider presets (Google Workspace, Microsoft 365, Mailgun, SendGrid, etc.) and choosing your enforcement policy (~all or -all). Outputs a ready-to-publish DNS TXT record.
Open tool →DKIM Key Generator
Generates an RSA-2048 (or 1024-bit) DKIM key pair entirely in your browser. Gives you the DNS TXT record to publish and the PEM private key to configure in your mail server (Postfix, Exim, Haraka). The private key never touches a server.
Open tool →CSRF Token Generator
Generates cryptographically secure CSRF tokens of configurable length in Hex or Base64URL format, using the browser's Web Crypto API. Includes ready-to-use code examples for common frameworks.
Open tool →Subdomain Takeover Checker
Scans common subdomain names and checks whether any still point to decommissioned services — Heroku, GitHub Pages, Fastly, AWS S3. Flags stale CNAME records that could be hijacked, the classic subdomain takeover attack surface.
Open tool →WHOIS & Domain Age Checker
Fetches domain registration data via RDAP (the modern successor to WHOIS) alongside live DNS records. Shows registrar, creation/expiry dates, days-until-expiry countdown, name servers, DNSSEC status, and live A, MX, NS records.
Open tool →Email Address Validator
Validates email addresses at three levels: RFC format compliance, MX record existence via DNS-over-HTTPS, and disposable domain detection. Bulk-check up to 100 addresses at once — no server, no data stored.
Open tool →Is It Down for Everyone?
Tests DNS resolution, TCP connectivity, and HTTP response for any URL from MonitorGiant's servers — giving a definitive answer on whether a site is down globally or just on your local network or ISP. Includes full redirect chain trace.
Open tool →HTTP Status Reference
A complete, searchable reference for all HTTP status codes — 1xx through 5xx — with plain-English explanations, common causes, and what to do when you see each one. Bookmarkable daily reference.
Open tool →How these tools are different from everything else
Most free developer tools online fall into one of three traps:
They upload your data
You paste a JWT or a certificate, it goes to their server, gets logged, and lives there. These tools use client-side Web Crypto APIs and DNS-over-HTTPS exclusively — your data never leaves your tab.
They require an account
Even for a hash. These tools have no registration, no free tier gate, no email capture. Open the URL, use the tool, close the tab.
They're unmaintained
Half the "free tool" sites you find via search were last updated in 2019. These 20 tools reflect 2026 standards — CVSS v3.1, TLSA/DANE, RDAP instead of legacy WHOIS.
Quick reference — all 20 tools
| Tool | Best for |
|---|---|
| DNS / DKIM Inspector | Email auth audit — SPF, DKIM, DMARC |
| Security Headers Grader | HTTP security posture A+ to F grading |
| HTTP Headers Inspector | Full header dump + redirect chain trace |
| SSL / TLS Certificate Checker | TLS configuration check via DNS |
| SSL Certificate Decoder | PEM certificate field inspection |
| JWT Decoder | Token claim inspection (client-side only) |
| CORS Policy Tester | Cross-origin header debugging |
| CVSS Score Calculator | Vulnerability severity scoring v3.1 |
| Hash Generator | Checksums, HMAC, file integrity |
| Bcrypt Hash Generator | Password hashing + hash verification |
| Password Generator | Cryptographically secure credential creation |
| Password Strength Visualizer | Crack-time analysis + policy testing |
| SPF Record Creator | Build valid SPF DNS TXT records |
| DKIM Key Generator | RSA key pair for self-hosted email DKIM |
| CSRF Token Generator | Anti-CSRF token generation |
| Subdomain Takeover Checker | Stale DNS / hijack surface audit |
| WHOIS & Domain Age Checker | Domain registration + RDAP lookup |
| Email Address Validator | Bulk email list validation with MX check |
| Is It Down for Everyone? | Global vs local outage diagnosis |
| HTTP Status Reference | Complete 1xx–5xx status code reference |
What to use when — common security scenarios
Before deploying a new service
- 1 DNS / DKIM Inspector
- 2 SSL / TLS Certificate Checker
- 3 Security Headers Grader
- 4 HTTP Headers Inspector
Incident response — auth failure
- 1 JWT Decoder
- 2 CORS Policy Tester
- 3 Is It Down for Everyone?
Setting up a new sending domain
- 1 SPF Record Creator
- 2 DKIM Key Generator
- 3 DNS / DKIM Inspector (verify)
Security audit of an external domain
- 1 WHOIS & Domain Age Checker
- 2 Subdomain Takeover Checker
- 3 Security Headers Grader
- 4 HTTP Headers Inspector
Vulnerability report writing
- 1 CVSS Score Calculator
Password policy review
- 1 Password Strength Visualizer
- 2 Password Generator
89 free tools
Bookmark this, not a random Stack Overflow answer
All 20 tools live at tools.monitorgiant.com — 89 free tools across security, developer utilities, AI/LLM tools, and design. All client-side. All free. No signup.
Open the toolbox →Going further
One-time checks are a start. Continuous monitoring is the standard.
The tools above are for spot-checks — useful mid-incident or pre-deploy. For continuous SSL expiry monitoring, HTTP header drift detection, and uptime alerts the moment something breaks, that's what MonitorGiant is built for. See all monitor types →
All tools run entirely in your browser. No data is transmitted to MonitorGiant's servers during tool use. See the privacy policy for full details.
Written by
Dileep KK, MonitorGiant
LinkedIn21+ years in IT infrastructure management and observability. Built monitoring dashboards, custom alerting pipelines, and AI token-tracking systems across cloud platforms — AWS, GCP, and Azure — and for organisations spanning defence IT, IoT manufacturing, digital marketing, SaaS email, insurance broking, parliamentary digital services, and educational ERP. Active directory, SIEM, WAF, Cloudflare, MSSQL, Linux, Windows, Entra ID — operated at every layer of the stack.