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ssrf-prevention

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Server-Side Request Forgery: allowlisting the destination of a server-side fetch, when re-resolution between check and connect matters, redirect and scheme bypasses, parsers that fetch on their own (XXE, SVG, HTML-to-PDF), cloud metadata, and keeping the response from becoming an oracle. Use when fetching a client-supplied URL, wiring webhooks, image proxies, PDF or preview renderers, or reviewing any HTTP-client wrapper.

skills/ssrf-prevention/SKILL.md

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Aug 14, 2026
License
MIT

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# SSRF Prevention

## Rules (for AI agents)

### ALWAYS
- Constrain the destination with an **allowlist of hosts you intend to reach**, not a
  denylist of addresses you intend to avoid. A denylist has to enumerate every spelling
  of every internal address — decimal and octal IPv4, IPv6 and IPv4-mapped IPv6, a
  hostname whose A record is private, a public wildcard resolver like `nip.io` that
  encodes any address into a name — and it only has to be wrong once.
- Know which of the two controls you built, because it decides whether re-resolution
  matters. An **allowlist of hostnames** survives DNS rebinding: the attacker controls
  DNS only for names they own, and those names are not on the list. An **IP-range
  check** does not: the name resolves to a public address when you validate and a
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