Server Side Template Injection in Buffalo with Bearer Tokens

Server Side Template Injection (SSTI) in the Buffalo framework occurs when user-controlled data is interpolated into a template without proper escaping or validation, enabling an attacker to inject template directives. When an API endpoint relies on Bearer Tokens for authorization but fails to validate or sanitize values derived from the token claims (or token metadata) before rendering them in templates, the attack surface expands. For example, if a handler decodes a Bearer Token, extracts a claim such as name or scope, and passes it directly to a template without escaping, an attacker who presents a crafted token can inject template commands.

In Buffalo, templates are typically rendered with the html/template package, which provides automatic escaping for HTML contexts. However, if developers use the .Set method to pass data into templates and later execute actions like {{.UserInput}}, unsanitized input can lead to SSTI when the template executes injected logic. Consider a scenario where the Bearer Token contains a custom claim used to personalize UI elements; if this claim is bound into the template context without escaping, an attacker could supply a token with a payload such as {{urlquery "cmd"}} to probe or manipulate internal behavior. While Buffalo does not execute arbitrary Go code from templates by default, unsafe use of template actions or third-party template functions can still enable injection paths.

The combination of Bearer Tokens and SSTI is particularly risky in APIs that return rendered HTML or partial templates to clients. If the token is accepted as a bearer credential and its associated claims are used to drive dynamic content, an attacker can probe for injection via crafted HTTP requests with modified Authorization headers. For instance, a token containing a malicious string like {{ .Release.Version }} could expose internal data if the template logic does not enforce strict context-aware escaping. This pattern aligns with OWASP API Top 10 API5:2023 – Injection, where injection vectors appear in API processing pipelines. Tools like middleBrick can detect such exposure by scanning unauthenticated endpoints and analyzing OpenAPI specs alongside runtime behavior, highlighting places where token-derived data reaches template execution paths.

Concrete example in Buffalo: A handler retrieves a claim from the Bearer Token and assigns it to the template context. If the claim is not sanitized, an attacker-controlled token can lead to template injection. Consider the following vulnerable handler snippet:

func MyHandler(c buffalo.Context) error {
    token := c.Request().Header.Get("Authorization")
    // Assume extractClaim is a custom function that extracts a claim from the Bearer Token
    claim, _ := extractClaim(token)
    c.Set("userData", claim)
    return c.Render(200, r.H{"data": c.Map{"input": c.Params().Get("input")}})
}

If the template uses {{.userData}} without escaping, and the token contains template syntax, SSTI becomes feasible. Developers should treat Bearer Token claims as untrusted input, apply context-sensitive escaping, and avoid direct interpolation into templates.

Remediation focuses on ensuring Bearer Token claims are never directly interpolated into templates and that all dynamic data is escaped according to the output context. In Buffalo, use the built-in escaping functions provided by html/template and validate token claims before use.

• Sanitize claims before setting them in the context: Always treat token claims as untrusted. Use functions like template.HTMLEscapeString or context-aware escaping when preparing data for templates.
• Avoid passing raw token data to templates: Instead of injecting claims directly, transform them into safe representations or omit them from the template context entirely.

Example of a secure handler that processes a Bearer Token safely:

import (
    "html/template"
    "strings"
)

func SecureHandler(c buffalo.Context) error {
    authHeader := c.Request().Header.Get("Authorization")
    if authHeader == "" {
        return c.Render(401, r.JSON{"error": "unauthorized"})
    }
    parts := strings.Split(authHeader, " ")
    if len(parts) != 2 || parts[0] != "Bearer" {
        return c.Render(400, r.JSON{"error": "bad request"})
    }
    tokenValue := parts[1]
    // Perform validation or extraction without directly exposing raw claims in templates
    claim, err := extractAndValidateClaim(tokenValue)
    if err != nil {
        return c.Render(401, r.JSON{"error": "invalid token"})
    }
    // Use escaped output when rendering
    safeData := template.HTMLEscapeString(claim)
    c.Set("safeClaim", safeData)
    return c.Render(200, r.H{"status": "ok"})
}

func extractAndValidateClaim(token string) (string, error) {
    // Placeholder for actual JWT or token parsing logic
    // Ensure claims are validated and sanitized
    return "verified-claim", nil
}

In templates, always rely on automatic escaping provided by html/template. Do not use the safe filter unless you fully trust the source:

{{.safeClaim}}  

If using custom template functions, ensure they do not introduce new injection vectors. For APIs that return JSON rather than HTML, set appropriate content types and avoid embedding user-controlled strings in executable contexts. middleBrick scans can help identify endpoints where Bearer Token claims influence template rendering and highlight missing escaping.