Writing
Notes on how things here are built, and why.
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Why every tool on this site runs in your browser
Most free PDF and image tools upload your files. Here is how this site avoids that, what it costs in engineering effort, and where the approach stops working.
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Fetching user-supplied URLs safely on shared hosting
A walk through the SSRF defences behind the security-headers checker: scheme and port rules, DNS resolution checks, pinned connections and redirect re-validation.