249+ in-depth guides on vulnerabilities, attack surfaces, secure coding, and compliance — free and open.
Explore our security guides organized by topic.
Comprehensive guides for different types of cyber attacks
Reference guides to DNS record types — what each record does, the fields it contains, and how to …
Comprehensive security guides for popular web frameworks
Security guides for programming languages — learn to find and fix vulnerabilities in Python, PHP, Java, JavaScript, Go …
What is port X? Security guides for common network ports
Guides to SSL/TLS certificates — the chain of trust, invalid-certificate errors, renewal, and monitoring.
Step-by-step tutorials on security scanning and testing
Comprehensive security vulnerability guides covering common web and network weaknesses — how each flaw works, how to detect …
Our latest security guides and analysis.
How to renew an SSL/TLS certificate: automated ACME renewal with Certbot, manual CSR renewal for commercial CAs, when …
What SSL certificate monitoring is, which signals to track (expiry, chain, hostname, ciphers), and manual checks vs continuous …
Why a browser calls an SSL certificate invalid — expired, name mismatch, self-signed, broken chain, wrong clock, revoked …
What an SSL certificate chain is, how leaf, intermediate and root certificates build trust, why a missing intermediate …
What a DNS TXT record is, how it powers SPF, DKIM, DMARC and domain verification, the 255-character limit, …
What a DNS MX record is, how mail-server priority controls delivery order, why targets must be hostnames not …
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