Server Side Template Injection in Buffalo with Basic Auth

Server Side Template Injection (SSTI) in the Buffalo framework occurs when user-controlled data is interpolated into a template in a way that allows template code execution. Buffalo uses the html/template package by default, which provides auto-escaping and structural safeguards; however, these protections can be bypassed when unsafe template actions are invoked or when developers use context-specific escaping functions incorrectly.

When Basic Authentication is used in Buffalo—typically via middleware that parses the Authorization header and sets user identity in the context or session—it can unintentionally broaden the attack surface for SSTI. For example, if a handler embeds Basic Auth–derived values (such as a username or a role claim extracted from a token or a custom header) into a template without proper sanitization, an attacker may supply payloads that leverage template actions to read files, execute functions, or probe the runtime environment. The combination is notable because Basic Auth often appears in non-standard implementations (for example, parsing credentials from custom headers) where authorization logic is less strict and template rendering may be more permissive.

Consider a scenario where a Buffalo handler retrieves a username from a custom header and passes it to a template:

// In a Buffalo handler
user := c.Request().Header.Get("X-User")
ctx := map[string]interface{}{
    "Username": user,
}
c.Render(200, r.HTML("dashboard.html", ctx))

If the template dashboard.html uses unsafe concatenation or the |safe filter, an attacker’s crafted X-User header could inject template directives. For instance, {{ (index .Username 0) }} might be leveraged to enumerate variables, and with access to restricted functions, more severe actions could follow. While the default html/template blocks most dangerous functions, misconfigurations—such as adding custom template functions that perform filesystem access or using the |safe filter—can enable path traversal or remote code execution equivalents within the template context.

Basic Auth does not inherently cause SSTI, but it can contribute when developers mistakenly trust headers that are easily manipulated and when templates are constructed with relaxed escaping. Security checks that test unauthenticated attack surfaces can still trigger SSTI findings if the endpoint reflects user input in template evaluation, even when authentication is expected elsewhere. Proper validation, strict context-aware escaping, and minimizing the use of |safe are essential to mitigate this interaction.

To prevent SSTI in Buffalo when using Basic Auth, ensure that any data derived from authentication—whether from headers, cookies, or custom middleware—is treated as untrusted and properly escaped. Avoid passing raw header values directly to templates; instead, sanitize and validate them before use.

Prefer structured user models and authorization checks rather than injecting raw credentials into templates. If you must pass values, use context-aware escaping functions and never apply the |safe filter to untrusted input. Below are concrete code examples demonstrating secure handling with Basic Auth in Buffalo.

Example 1: Safe header parsing with validation and no template injection risk

// In a Buffalo action
func UserShow(c buffalo.Context) error {
    auth := c.Request().Header.Get("Authorization")
    if auth == "" {
        return c.Render(401, r.JSON(map[string]string{"error": "unauthorized"}))
    }
    // Expecting "Basic base64(credentials)" — parse safely
    parts := strings.SplitN(auth, " ", 2)
    if len(parts) != 2 || parts[0] != "Basic" {
        return c.Render(400, r.JSON(map[string]string{"error": "bad request"}))
    }
    decoded, err := base64.StdEncoding.DecodeString(parts[1])
    if err != nil {
        return c.Render(400, r.JSON(map[string]string{"error": "bad request"}))
    }
    creds := string(decoded)
    // Validate format, avoid injection
    if !isValidCredentialFormat(creds) {
        return c.Render(401, r.JSON(map[string]string{"error": "unauthorized"}))
    }
    // Use a structured user representation, not raw credentials
    user := &User{Username: extractUsername(creds)}
    c.Set("current_user", user)
    return c.Render(200, r.HTML("dashboard.html", user))
}

Example 2: Template that relies on safe context and no |safe filters

{{/* dashboard.html */}}
<h1>Welcome, {{ .Username }}</h1>
<p>Role: {{ .Role }}</p>

In this setup, .Username and .Role are plain strings from a validated user model. No user-controlled input is concatenated into the template logic, and no custom functions that access the filesystem are registered, reducing the risk of SSTI.

Additionally, audit any custom template functions registered with Buffalo. If registration is necessary, restrict their capabilities and avoid functions that read files or execute commands. Regularly test endpoints using security scanning approaches that include authenticated and unauthenticated probes to detect misconfigurations early.