Server Side Template Injection in Gorilla Mux with Api Keys

Server Side Template Injection (SSTI) in a Gorilla Mux router can be triggered through API key handling when keys are dynamically interpolated into response templates without proper escaping or validation. Gorilla Mux is a widely used HTTP router and matcher for building Go web services; it does not provide built-in templating, so developers often compose HTML, JavaScript, or configuration fragments directly in handlers or pass user-controlled data (such as API key metadata) into a template engine.

In this scenario, an API key—typically a bearer token or opaque string provided by a client—may be logged, echoed, or rendered in a diagnostic or admin interface. If that key is placed into a template string that is later parsed and executed (for example, using text/template or html/template), and the template relies on contextual autoescaping that is not enforced, an attacker who can influence the key value may inject template actions. A malicious API key like {{.Request}} or {{urlquery "x"}} can cause the template engine to execute actions, access internal variables, or leak sensitive information depending on the template’s execution context.

The combination of Gorilla Mux route variables and API key material increases risk because keys often appear in logs, error messages, or user-facing JSON responses that are later embedded in templates. If a developer uses mux.Vars to extract an identifier from the route (e.g., /keys/{keyId}) and then fetches the corresponding key value from a data store, they might pass the raw key string into a template for display. Without strict type constraints, escaping, or sandboxing, this enables SSTI. Moreover, if the API key is used to gate middleware that conditionally renders different templates, an attacker who can guess or brute-force key patterns may probe for template injection surfaces across authenticated flows, effectively turning API key handling into an injection channel.

Real-world patterns mirror known OWASP API Top 10 categories such as Broken Object Level Authorization (BOLA) and Injection. For example, if an endpoint like /admin/report?apikey=TOKEN uses TOKEN to select a report template and the template engine processes user-controlled strings, an attacker can supply crafted keys to enumerate internal functions or environment variables. This is especially dangerous when the same service exposes both public endpoints and administrative interfaces, because a leaked or weak API key can serve as a pivot for template injection.

Because middleBrick scans unauthenticated attack surfaces and tests input validation and data exposure, it can detect situations where API key values are reflected in responses or documentation and indicate whether they are improperly handled by templating logic. Findings include evidence of unsafe reflection, missing context-aware escaping, and routes where key parameters are concatenated into template-like strings, helping developers identify and isolate these vectors before they are weaponized.

Remediation focuses on strict separation of data and template logic, context-aware escaping, and avoiding the direct inclusion of API key material in any template that can be influenced by an attacker. Below are concrete, safe patterns for handling API keys in Gorilla Mux services.

• Do not pass API keys to templates. Treat keys as sensitive configuration, not user input. If you must display masked key metadata (for example, the last four characters), compute that representation in Go before passing it as a plain string, and never concatenate raw keys into template text.
• Use explicit data structures and autoescaping. For HTML templates, use html/template and define a struct that excludes raw keys:

package main

import (
    "html/template"
    "net/http"

    "github.com/gorilla/mux"
)

type ReportData struct {
    ReportTitle string
    Owner       string
    LastFour    string // masked representation, e.g., "****-****-****-A1B2"
}

func reportHandler(w http.ResponseWriter, r *http.Request) {
    vars := mux.Vars(r)
    keyID := vars["keyId"]
    // Fetch key metadata securely; do not expose the raw key.
    masked, err := fetchMaskedKeyLastFour(keyID)
    if err != nil {
        http.Error(w, "invalid key", http.StatusBadRequest)
        return
    }
    data := ReportData{
        ReportTitle: "Key Usage Summary",
        Owner:       "service-account",
        LastFour:    masked,
    }
    // Parse once, typically at startup; ensure templates use autoescaping.
    tmpl := template.Must(template.New("report").Parse(reportTemplate))
    _ = tmpl.Execute(w, data)
}

const reportTemplate = `
<h1>{{.ReportTitle}}</h1>
<p>Owner: {{.Owner}}</p>
<p>Key: {{.LastFour}}</p>
`

• If you must accept user input that will be rendered in a non-HTML context (e.g., JSON or plain text), validate and sanitize strictly. Use allowlists for acceptable characters and reject any input that contains template-like constructs such as {{ or {% when those constructs have no business meaning.
• For configuration or code generation use cases where templates are processed server-side, isolate the template execution environment. Do not allow API key strings to participate in template syntax. Instead, pass only safe metadata and keep keys confined to secure configuration stores.

Implementing these patterns reduces the attack surface for SSTI and aligns with secure handling of credentials in web services. middleBrick can validate whether API key parameters are reflected in responses and whether templates exhibit unsafe concatenation, providing prioritized remediation guidance mapped to relevant compliance frameworks.