Server Side Template Injection in Gin with Api Keys

Server Side Template Injection (SSTI) in the Gin framework occurs when user-controlled data is passed into a template without proper escaping or sandboxing, enabling an attacker to inject template logic. When API keys are handled through Gin templates—such as rendering keys in debug pages, status responses, or developer-facing endpoints—the combination can expose sensitive logic or allow template code execution. For example, if an endpoint accepts an apikey query parameter and directly interpolates it into a Go template, an attacker may leverage template delimiters to read files or execute commands.

Consider a Gin handler that renders a user profile including an API key:

c.HTML(http.StatusOK, "profile", gin.H{
    "APIKey": c.Query("apikey"),
})

If the profile template uses unsafe string concatenation or the template engine allows execution (e.g., via the .Execute pipeline with user input), an attacker can supply a value like {{ (index (printf "%s" "127.0.0.1") 0) }} to probe internal behavior. While Gin’s default html/template escapes variables, developers sometimes disable escaping or use custom delimiters, which can enable SSTI. In API key contexts, this becomes risky when keys are rendered into HTML/JS or when template logic is used to format responses conditionally based on key metadata.

Additionally, if API keys are used to gate middleware that selects which template functions or data sources to expose, an injected template may traverse internal structures (e.g., via dict or urlquery lookups) and reach sensitive configuration values. The risk is higher when templates are built dynamically or when developer convenience endpoints expose key-like values without proper validation.

To mitigate SSTI risks when handling API keys in Gin, ensure user input is never directly embedded into templates. Use strict context-aware escaping and avoid passing raw keys into template rendering. Instead, treat API keys as opaque strings and only pass non-sensitive metadata to templates.

Safe handler example:

c.HTML(http.StatusOK, "profile", gin.H{
    "ShowKey": false, // never pass the raw key to the template
})
// In profile template (profile.tmpl):
{{ if .ShowKey }}API Key: {{ .APIKey }}{{ end }}

Better yet, avoid rendering API keys in HTML/JS entirely. If you must display masked key fragments, sanitize and transform them in Go before templating:

maskedKey := "****"
if len(apiKey) > 4 {
    maskedKey = "****" + apiKey[len(apiKey)-4:]
}
c.HTML(http.StatusOK, "profile", gin.H{
    "MaskedKey": maskedKey,
})

Custom template functions with strict allowlists: If you require dynamic formatting, define safe helper functions and set them via FuncMap, avoiding direct evaluation of user input:

funcMap := template.FuncMap{
    "mask": func(s string) string {
        if len(s) <= 4 {
            return "****"
        }
        return "****" + s[len(s)-4:]
    },
}
tmpl := template.Must(template.New("profile").Funcs(funcMap).ParseFiles("templates/profile.tmpl"))
c.HTML(http.StatusOK, "profile", gin.H{
    "APIKey": c.Query("apikey"),
})
// In template: {{ mask .APIKey }}

Disable template actions for untrusted contexts: When keys are used only for authentication, avoid rendering them in responses that are ever processed as templates. Use JSON responses for machine-to-machine communication:

c.JSON(http.StatusOK, gin.H{
    "masked_key": maskedKey,
})

Finally, audit middleware that uses API keys to select template paths or functions. Ensure such selection is based on a strict allowlist and never derived from user-controlled input to prevent template injection via indirect paths.