Server Side Template Injection in Buffalo with Jwt Tokens

Server Side Template Injection (SSTI) in the Buffalo framework becomes particularly high-risk when JWT tokens are used for authentication and their payloads are reflected into server-side templates. SSTI occurs when an attacker can inject template directives that are subsequently evaluated by the engine. In Buffalo, the danger arises when data extracted from a JWT — such as claims in the payload (e.g., user role, email, or custom metadata) — is passed into a template for rendering without proper escaping or validation.

Consider a scenario where a handler decodes a JWT, extracts a claim, and passes it to a template context. If the template includes user-influenced logic, such as conditional blocks or dynamic variable names, an attacker who can influence the JWT claim (e.g., via a compromised client-side generation or a leaked token) may inject template code. For example, a claim like {{ .Username }} might be rendered directly, but if the application dynamically constructs template fragments based on claims, an input such as {% include "malicious" %} could lead to arbitrary code execution within the template engine’s context.

In Buffalo, handlers often use currentUser objects derived from JWT claims. If these objects are serialized into templates using methods that do not enforce strict output escaping, an attacker might leverage SSTI to execute server-side functions, read files, or interact with the local environment. Real-world attack patterns mirror OWASP API Top 10:2023’s Broken Object Level Authorization (BOLA) and Injection flaws, where trust boundaries between authentication tokens and presentation layers are improperly managed.

Specific CVEs in related ecosystems, such as those affecting template engines with unsafe evaluation, demonstrate how injection through trusted metadata can bypass access controls. While Buffalo itself does not introduce the vulnerability, the combination of JWT-based identity and dynamic template rendering creates a pathway where an attacker’s injected payloads are interpreted as executable instructions rather than data.

To illustrate, a handler that passes a JWT claim directly into a template might look like this in Go:

claims := map[string]interface{}{
    "role": "admin",
    "name": "{{ .UserInput }}", // Unsafe if UserInput is attacker-controlled
}
currentUser := claims["name"]
helpers := map[string]interface{}{
    "currentUser": currentUser,
}
c.Render(200, r.HTML("dashboard.html"), helpers)

In the template dashboard.html, if {{ .currentUser }} is rendered without escaping, and the template engine supports functions or actions, an attacker-controlled UserInput could alter template behavior. This demonstrates why JWT token data must be treated as untrusted input when used in server-side contexts.

Remediation focuses on strict separation of authentication data and template rendering, ensuring JWT claims are never directly interpolated into executable template logic. Follow these concrete steps:

• Always escape output: Use Go’s HTML escaping functions when rendering dynamic data in templates. Buffalo’s helpers provide auto-escaping in many contexts, but explicitly escape JWT-derived values.
• Validate and sanitize claims: Treat JWT payloads as input. Validate signatures, restrict expected claims, and sanitize values before use.
• Avoid dynamic template assembly: Do not construct template names or blocks based on JWT claims. Use static templates and pass only safe data structures.

Here is a secure code example for Buffalo handlers and templates:

// Handler: decode and validate JWT, then pass sanitized data
claims := map[string]interface{}{
    "role": "admin", // Assume validated from token
    "name": template.HTMLEscapeString(userInput), // Explicit escape
}
currentUser := claims["name"]
helpers := map[string]interface{}{
    "CurrentUser": currentUser,
    "SafeRole":    template.HTMLEscapeString(claims["role"]),
}
c.Render(200, r.HTML("dashboard.html"), helpers)

In the corresponding template dashboard.html, use escaped output:

<div>User: {{ .CurrentUser }}</div>
<div>Role: {{ .SafeRole }}</div>

This ensures that even if userInput contains template syntax, it is rendered as plain text. For applications using JWTs for authorization, implement role checks in handlers rather than templates, and use Buffalo’s middleware to enforce access control based on validated claims.

Additionally, configure token validation rigorously: verify signatures, check expiration, and enforce issuer/audience constraints before extracting claims. Never rely on client-provided data in templates without transformation. Tools like the middleBrick CLI can scan your API endpoints to detect such injection risks in unauthenticated scans, while the GitHub Action can integrate these checks into CI/CD pipelines to fail builds if risky patterns are detected.