Server Side Template Injection in Buffalo

Server Side Template Injection (SSTI) occurs when an attacker injects malicious template syntax into user-controlled input that is later processed by a template engine. In Buffalo, a Go web framework, this vulnerability commonly arises when developers directly embed request data into template rendering without proper sanitization. Buffalo uses the standard Go html/template package by default, which provides strong auto-escaping for HTML contexts but can be bypassed when developers misuse template.HTML or use custom template functions that lack proper context awareness.

A typical vulnerable pattern in Buffalo handlers involves passing untrusted data directly into template execution. For example, consider a handler that renders a user profile page where the username is taken from a URL parameter and inserted into the template context without validation:

func ShowUser(c buffalo.Context) error {
    username := c.Param("username")
    // Vulnerable: username is passed directly as template data
    return c.Render(200, r.HTML("users/show.html", map[string]interface{}{
        "Username": username, // Attacker controls this
    }))
}

While html/template escapes HTML by default, SSTI in Buffalo can still occur through:

• Custom template functions: If developers register unsafe functions (e.g., one that executes shell commands or evaluates expressions), attackers can invoke them via template syntax like {{ unsafeFunc "rm -rf /" }}.
• Misuse of template.HTML: Casting user input to template.HTML tells the template engine to trust the input as safe HTML, enabling injection if the input contains template directives.
• Nested templates: Using template.Must with user-controlled template names or paths could allow loading arbitrary templates.

Real-world parallels include CVE-2016-4977 in Apache Commons Text (though not Go-specific), where expression language injection led to remote code execution. In Buffalo, the risk is highest when developers extend template functionality without sandboxing user input, turning what should be a presentation layer into an attack surface.

Detecting SSTI in Buffalo applications requires analyzing both static code patterns and dynamic behavior. Since Buffalo applications are compiled Go binaries, static analysis alone may miss runtime template function registrations. middleBrick identifies SSTI vulnerabilities through black-box testing by injecting template payloads into request parameters, headers, and body content, then observing whether the template syntax is evaluated rather than treated as plain text.

When scanning a Buffalo endpoint, middleBrick sends payloads designed to trigger expression evaluation in common template contexts. For the html/template engine, payloads like {{7*7}} or {{printf "%s" "test"}} are used. If the response contains 49 or the evaluated result, it indicates successful template injection. middleBrick also tests for out-of-band interactions (e.g., via DNS or HTTP callbacks) to detect blind SSTI where output is not directly reflected.

Specifically for Buffalo, middleBrick checks:

• Whether custom template functions are inadvertently exposed (e.g., via c.Set("funcName", unsafeFunc) in handlers).
• If any endpoint reflects user input in template-rendered responses without adequate context escaping.
• Whether error messages leak template syntax (e.g., "illegal character in template") that could aid attackers in refining payloads.

For example, a vulnerable endpoint might return a 200 OK with 49 when injected with {{7*7}} in a parameter, indicating successful code execution within the template context. middleBrick flags this as a high-severity finding, providing the exact payload and request/response details for reproduction. The scanner does not require credentials or agents—it tests the unauthenticated attack surface by submitting the base URL and discovering endpoints through passive observation and active probing.

Fixing SSTI in Buffalo applications centers on eliminating the injection of untrusted data into template execution contexts while leveraging Buffalo’s built-in security practices. The primary defense is ensuring all data passed to templates is properly sanitized and that no unsafe template functions are registered unless absolutely necessary—and even then, only with strict input validation.

Replace patterns that pass raw user input directly into template contexts. Instead, validate and sanitize data before inclusion. For instance, the vulnerable ShowUser handler should be fixed as follows:

func ShowUser(c buffalo.Context) error {
    username := c.Param("username")
    // Validate: ensure username contains only expected characters
    if matched, _ := regexp.MatchString(`^[a-zA-Z0-9_]+$`, username); !matched {
        return c.Error(400, errors.New("invalid username"))
    }
    // Safe: username is validated and treated as plain text
    return c.Render(200, r.HTML("users/show.html", map[string]interface{}{
        "Username": username,
    }))
}

Additional Buffalo-specific measures include:

• Avoiding template.HTML for user-controlled data: If HTML output is truly needed, use Buffalo’s sanitize.HTML helper (from github.com/gobuffalo/buffalo/render) which runs input through an HTML sanitizer before marking it as safe.
• Restricting custom template functions: When registering functions via c.Set("funcName", f) or in buffalo.App setup, audit each function to ensure it cannot be abused. Prefer using Buffalo’s built-in helpers (link_to, path_for) which are designed to be safe.
• Using Buffalo’s parameter validation: Leverage c.Bind() with struct tags and validation (e.g., validate package) to ensure input conforms to expected formats before it reaches template rendering.
• Leveraging middleware: Implement request-scoped middleware that validates or sanitizes common input sources (params, query, body) based on route groups.

After fixes, rescan with middleBrick to confirm the SSTI vulnerability is resolved. The scanner will no longer detect template expression evaluation in responses, confirming that user input is properly contained within data contexts rather than being interpreted as template code. Remember: middleBrick detects and reports—it does not apply fixes. Remediation must be implemented in your Buffalo application code following these patterns.