Overview
The CVE-2026-40602 incident shows how an untrusted templating engine can lead to code execution and data exposure when user input is treated as template code. In that case, hass-cli allowed Jinja2 templates to run in an unrestricted environment, rendering user-supplied input with access to Python's internals and bypassing intended sandboxing. This is classified under CWE-94 (Code Injection) and CWE-1336 (Template Injection), highlighting the risk of evaluating user-controlled templates without strict isolation. Although this CVE concerns a Python/Jinja2 context, the core vulnerability pattern-untrusted input being executed as template code-transfers to Go (Gin) when templates are rendered from client-supplied content with dangerous capabilities exposed via a function map or by loading arbitrary templates. Go applications can replicate this class of risk if they allow dynamic templates to be supplied by clients and expose powerful helper functions in the template execution context. The fix requires disabling or tightly sandboxing dynamic templates, and only allowing templates that are pre-validated and whitelisted. The guidance here maps that risk to Go (Gin) implementations and provides concrete Go code to illustrate both the vulnerable pattern and the secure alternative.
Code Fix Example
Go (Gin) API Security Remediation
// Vulnerable pattern (Go with Gin, using text/template and exposing dangerous functions to templates)
package main
import (
"bytes"
"net/http"
"os"
"os/exec"
"text/template"
"github.com/gin-gonic/gin"
)
type Payload struct {
User string
Message string
}
func main() {
r := gin.Default()
// Vulnerable: user-provided template string is parsed and executed with dangerous FuncMap
r.GET("/render-vuln", func(c *gin.Context) {
tmplStr := c.Query("tmpl") // attacker supplies template
funcs := template.FuncMap{
// Dangerous: allows template to access environment variables
"env": func(key string) string { return os.Getenv(key) },
// Dangerous: allows executing system commands via the template
"exec": func(cmd string) string {
out, _ := exec.Command("sh", "-c", cmd).Output()
return string(out)
},
}
t, err := template.New("userTmpl").Funcs(funcs).Parse(tmplStr)
if err != nil {
c.String(http.StatusBadRequest, "template parse error: %v", err)
return
}
var buf bytes.Buffer
_ = t.Execute(&buf, Payload{User: "guest", Message: "Hello"})
c.String(http.StatusOK, buf.String())
})
// Fixed: do not expose dangerous functions; use a whitelisted, static template
r.GET("/render-fixed", func(c *gin.Context) {
tmplName := c.Query("tmplName")
// Whitelist of allowed templates; no dynamic user-provided code
allowed := map[string]string{
"greeting": "Hello {{.User}}, {{.Message}}",
"alert": "Alert for {{.User}}: {{.Message}}",
}
tmplStr, ok := allowed[tmplName]
if !ok {
c.String(http.StatusBadRequest, "template not allowed")
return
}
t, err := template.New("safeTmpl").Parse(tmplStr)
if err != nil {
c.String(http.StatusBadRequest, "template parse error: %v", err)
return
}
var buf bytes.Buffer
_ = t.Execute(&buf, Payload{User: "guest", Message: "Hello"})
c.String(http.StatusOK, buf.String())
})
r.Run()
}