Server Side Template Injection in Gorilla Mux with Hmac Signatures

Server Side Template Injection (SSTI) occurs when an attacker can control template input that is later rendered by a server-side templating engine. In a Gorilla Mux application, routes are typically defined with path variables and query parameters. When these untrusted values are passed into a template engine without proper escaping or validation, SSTI becomes possible. The use of Hmac Signatures for request authentication or integrity checks does not inherently prevent SSTI; if the Hmac verification is performed after the route is selected and the attacker-influenced data is already bound to a template, the signature can be valid while the payload remains malicious.

Consider a scenario where a route parameter or a header value is included in a template. If the application uses Go's text/template or html/template and does not autoescape content, an attacker might supply a payload like {{define "main"}}{{end}}. When this value is rendered, the template engine can execute logic or re-define blocks, leading to remote code execution or data leakage. Because Hmac Signatures are often used to ensure that a request originates from a trusted source (e.g., a webhook or a microservice), developers may mistakenly assume that content protected by a valid signature is safe to render. However, the signature does not alter the semantics of the input; it only confirms that the input has not been tampered with in transit. If the source is compromised or the signature is leaked, the malicious input retains its validity, and the template executes as intended by the attacker.

Gorilla Mux does not provide built-in template sanitization. Developers must ensure that any data used within templates is either strictly typed and validated or properly escaped. The combination of dynamic route construction and signature-based authentication can increase risk if developers conflate authentication with input safety. For example, an endpoint that accepts a user-supplied template name authenticated by Hmac may allow an attacker to enumerate or replace templates if path traversal or injection is possible. Even with a valid Hmac, the rendered output can include sensitive information or execute arbitrary logic, depending on the template engine’s capabilities and configuration.

Real-world attack patterns include injection through query parameters that influence template selection or variable injection within template contexts. The presence of Hmac Signatures might protect against tampering but does not mitigate classic injection vectors such as those catalogued in the OWASP API Top 10 and CWE-94. Proper context-aware escaping and strict allowlists for template names and variables remain essential, regardless of cryptographic integrity checks.

To remediate SSTI in a Gorilla Mux application that uses Hmac Signatures, treat authenticated data as untrusted for rendering purposes. Always validate and sanitize inputs before passing them to templates, and enforce strict allowlists for dynamic template selection. Use Go’s built-in escaping functions and ensure that the correct content type is set for each context (HTML, JS, CSS, or URL).

Below are concrete code examples for a secure implementation.

Secure Route Handling and Template Execution

import (
    "github.com/gorilla/mux"
    "html/template"
    "net/http"
    "crypto/hmac"
    "crypto/sha256"
    "encoding/hex"
)

var trustedTemplates = map[string]string{
    "home": "home.html",
    "about": "about.html",
}

func verifyHmac(payload, receivedHmac, secret string) bool {
    key := []byte(secret)
    mac := hmac.New(sha256.New, key)
    mac.Write([]byte(payload))
    expected := hex.EncodeToString(mac.Sum(nil))
    return hmac.Equal([]byte(expected), []byte(receivedHmac))
}

func renderTemplate(w http.ResponseWriter, tmpl string, data interface{}) {
    t, ok := trustedTemplates[tmpl]
    if !ok {
        http.Error(w, "template not found", http.StatusBadRequest)
        return
    }
    // Execute with autoescaping enabled for HTML
    err := template.Must(template.New(t).ParseFS(templateFS, t)).Execute(w, data)
    if err != nil {
        http.Error(w, "error rendering template", http.StatusInternalServerError)
    }
}

func handler(secret string) http.HandlerFunc {
    return func(w http.ResponseWriter, r *http.Request) {
        vars := mux.Vars(r)
        templateName := vars["name"]
        receivedHmac := r.Header.Get("X-Hmac-Signature")
        // Construct the payload used by the sender; in practice this may be a canonical representation of the request
        payload := templateName
        if !verifyHmac(payload, receivedHmac, secret) {
            http.Error(w, "invalid signature", http.StatusUnauthorized)
            return
        }
        // Strict allowlist check prevents template injection via path manipulation
        renderTemplate(w, templateName, nil)
    }
}

In this example, the Hmac is verified before any template selection, and a strict allowlist ensures only pre-approved templates can be rendered. The template content itself is loaded from a filesystem or embed, avoiding dynamic inclusion based on user input. Autoescaping is enabled by using html/template, which protects against context-specific injection when outputting variables into HTML.

For query parameters or JSON bodies that influence template variables, apply the same principle: validate, escape, and avoid direct interpolation. Do not rely on Hmac or any cryptographic signature to sanitize data for rendering contexts.