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template-injection

User input reaching a template compiler rather than its context: the source-versus-values distinction, double-render pipelines, template names chosen by the caller, why a template sandbox is a mitigation and not a boundary, minimizing what the render context exposes, and template-driven resource exhaustion. Use when rendering templates with user-influenced values, when a template string or template name is assembled from input, or when a product lets users author templates.

skills/template-injection/SKILL.md

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Aug 14, 2026
License
MIT

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# Template Injection (SSTI)

## Rules (for AI agents)

### ALWAYS
- Pass user data as **bound values**, never into the template **source** string. This
  is the whole skill in one line: `render(tmpl, {"name": name})` is safe,
  `render("Hello " + name)` is SSTI. The compiler treats its source as code, and the
  context as data — so the only question that matters about any user value is which of
  the two it reaches.
- Keep the set of template sources **static and trusted**, loaded from files or
  constants. A template body read from a database column, an API response, or a
  request field is attacker-supplied code, whatever the column is called.
- Choose the template **by name from an allowlist**, never by a name the caller