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docs: clarify text/template threat model and SSTI risks (#73) #112

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22 changes: 21 additions & 1 deletion src/output-encoding/cross-site-scripting.md
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Expand Up @@ -111,8 +111,28 @@ Making `param1` equal to "<h1>" will lead to `Content-Type` being sent as

![XSS while using text/template package][text-template-xss]

> **Understanding Go template threat models**
>
> The `text/template` package performs no output encoding. It is designed for generating text-based
> formats (configuration files, plain-text emails, etc.), not HTML output. When used to render HTML
> without manual escaping, user input is written verbatim — making XSS inevitable.
>
> The `html/template` package automatically contextual-escapes values, but **only when untrusted data
> is passed as template parameters** (e.g. `{{ . }}`). This safety guarantee does **not** apply when
> user input is used to build the template source string itself:
>
> ```go
> tmpl.Parse(`<div>` + userInput + `</div>`) // DANGEROUS
> ```
>
> If user-controlled data ever becomes part of the template definition, the application becomes
> vulnerable to **Server-Side Template Injection (SSTI)** — an attack that can bypass auto-escaping,
> access unexported struct fields, invoke methods, or execute arbitrary code. **User input must never
> be concatenated into the template source; it must only be supplied as a parameter value.**

By replacing the [text/template package][6] with the [html/template][2] one,
you'll be ready to proceed... safely.
you'll be ready to proceed... safely, provided untrusted data is only passed
as template parameters (never used to construct the template source).

```go
package main
Expand Down