Server Side Template Injection in Buffalo with Hmac Signatures

Server Side Template Injection (SSTI) in Buffalo can be triggered when user-controlled data is interpolated into templates without proper escaping or validation. Buffalo uses the built-in html/template package, which provides automatic escaping for HTML contexts when templates are used correctly. However, when developers use Hmac Signatures to bind request parameters or tokens to a template context, they may inadvertently pass raw or minimally validated data into the template layer, creating a path for SSTI if the data includes malicious template syntax.

Consider a scenario where a Hmac Signature is generated from user input (e.g., an identifier or a redirect URL) and then that input is placed into the template without escaping. If an attacker crafts a payload such as {{ .UserInput }} and the application reflects this into the rendered output without proper sanitization, the template engine may execute the embedded Go template actions. This can lead to unauthorized variable access, information disclosure, or even remote code execution if the template context is misconfigured or additional unsafe functions are registered.

The risk is compounded when Hmac Signatures are used for integrity checks but the underlying data fed into the template remains untrusted. For example, a developer might sign a URL parameter containing a template-like string and later evaluate it in a template, assuming the signature guarantees safety. However, the signature only ensures the value has not been tampered with; it does not neutralize template syntax. An attacker can still inject template directives that access internal struct fields, call functions, or iterate over collections, leading to sensitive data exposure or logic manipulation.

Real-world impact aligns with OWASP API Top 10 and general web template injection categories. Instances of SSTI in Buffalo applications using Hmac Signatures can expose application internals, enable phishing via crafted URLs, or serve as a pivot for further attacks. Because Buffalo encourages rapid prototyping, developers might skip input validation when Hmac Signatures are present, mistakenly believing cryptographic integrity replaces sanitization. This misconception creates a window for attackers to exploit template parsing logic through seemingly protected parameters.

To detect this during a scan, middleBrick tests unauthenticated endpoints that accept or reflect user input in template contexts, checking for reflected template syntax in responses and attempting controlled injections. In scenarios involving Hmac Signatures, it validates whether signature verification is decoupled from template rendering and whether untrusted data is escaped or isolated from template evaluation.

Remediation centers on strict input validation, contextual output escaping, and ensuring Hmac Signatures are used solely for integrity, not trust. Always treat data originating from the request as untrusted, even if it carries a valid signature. Buffalo provides helpers for HTML escaping and template-safe rendering that must be applied consistently.

First, validate and sanitize all inputs before using them in templates. Use explicit allowlists for expected formats and reject any input containing template-like patterns (e.g., {{ or {%) unless absolutely necessary. Never concatenate raw user input into template strings.

Second, use Buffalo’s built-in escaping mechanisms when rendering dynamic content. In HTML templates, the {{ .Value }} syntax automatically escapes content. For contexts requiring different escaping (e.g., JavaScript or URLs), use the appropriate context-specific functions such as js or urlquery filters.

Third, keep Hmac Signatures separate from template data. Compute the signature over canonical, minimal data and verify it before any template rendering. Do not embed user input inside the signed payload in a way that would require later interpolation into templates.

Example of a secure approach in a Buffalo handler:

import (
    "github.com/gobuffalo/buffalo"
    "github.com/gobuffalo/buffalo/middleware/forcessl"
    "github.com/gobuffalo/packr/v2"
    "net/http"
    "strings"
)

func safeHandler(c buffalo.Context) error {
    userValue := c.Param("value")
    // Reject obviously malicious patterns
    if strings.Contains(userValue, "{{") || strings.Contains(userValue, "}}") {
        return c.Render(400, r.Text("invalid input"))
    }
    // Use HTML-escaped rendering
    return c.Render(200, r.HTML("page.html", struct {
        SafeValue string
    }{
        SafeValue: userValue, // auto-escaped in HTML template
    }))
}

Example of Hmac verification without template interpolation of raw input:

import (
    "crypto/hmac"
    "crypto/sha256"
    "encoding/hex"
    "net/http"
    "strings"
)

func verifyHmac(next http.Handler) http.Handler {
    secret := []byte("your-secure-secret")
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        token := r.URL.Query().Get("token")
        expected := r.URL.Query().Get("expected")
        mac := hmac.New(sha256.New, secret)
        mac.Write([]byte(token))
        computed := hex.EncodeToString(mac.Sum(nil))
        if !hmac.Equal([]byte(computed), []byte(expected)) {
            http.Error(w, "invalid signature", http.StatusForbidden)
            return
        }
        // At this point, token is verified; do not directly inject token into templates
        // Instead, extract only safe, validated parts
        safeToken := strings.TrimPrefix(token, "prefix_")
        // Proceed with business logic, then render with escaping
        c := buffalo.Wrap(w, r)
        c.Render(200, r.JSON(map[string]string{"token": safeToken}))
    })
}

These examples emphasize that Hmac Signatures protect integrity but do not sanitize input. Developers must still apply output encoding and avoid passing raw user data into template contexts.