Overview
A real-world SSRF example is CVE-2026-31017, where ERPNext's Print Format feature would render user-submitted HTML into PDFs and fetch resources referenced by iframes from the server. The PDF rendering engine could be coaxed to request internal services or metadata endpoints, leaking sensitive information. This vulnerability demonstrates how server-side rendering of user-controlled content can trigger unintended outbound requests from the hosting server. In Go (Gin) apps, a similar pattern can occur when the server accepts user-controlled HTML or URLs and feeds them into a server-side renderer or HTTP fetcher that pulls remote resources during processing. The vulnerability mirrors the ERPNext scenario and shows that SSRF can arise even in modern Go web services if you expose a rendering path that touches external resources driven by user input. Remediation requires both input sanitization and rendering configuration to prevent server-initiated requests to internal or external endpoints. The guidance below demonstrates a concrete pattern in Go (Gin) and how to harden it against CVE-2026-31017-like risk.
Code Fix Example
Go (Gin) API Security Remediation
package main
import (
"bytes"
"fmt"
"os/exec"
"strings"
"github.com/gin-gonic/gin"
"github.com/microcosm-cc/bluemonday"
)
type HTMLPayload struct {
HTML string `json:"html"`
}
func main() {
r := gin.Default()
r.POST("/vuln/render", vulnerableRender)
r.POST("/fix/render", fixedRender)
r.Run(":8080")
}
func vulnerableRender(c *gin.Context) {
var p HTMLPayload
if err := c.BindJSON(&p); err != nil {
c.JSON(400, gin.H{"error": err.Error()})
return
}
if err := renderPDFVulnerable(p.HTML); err != nil {
c.JSON(500, gin.H{"error": err.Error()})
return
}
c.JSON(200, gin.H{"status": "ok"})
}
func fixedRender(c *gin.Context) {
var p HTMLPayload
if err := c.BindJSON(&p); err != nil {
c.JSON(400, gin.H{"error": err.Error()})
return
}
if err := renderPDFFixed(p.HTML); err != nil {
c.JSON(500, gin.H{"error": err.Error()})
return
}
c.JSON(200, gin.H{"status": "ok"})
}
func renderPDFVulnerable(html string) error {
// Vulnerable path: render user HTML via PDF tool reading from stdin.
// Assumes wkhtmltopdf is installed and accessible.
return callWKHTMLTopdf(html, "vulnerable.pdf", false)
}
func renderPDFFixed(html string) error {
// Fixed path: sanitize input to remove external resources before rendering.
safe := sanitizeHTML(html)
return callWKHTMLTopdf(safe, "fixed.pdf", true)
}
func sanitizeHTML(input string) string {
p := bluemonday.UGCPolicy()
// bluemonday defaults already disallow script/iframe; this ensures consistent sanitization.
return p.Sanitize(input)
}
func callWKHTMLTopdf(input string, outPath string, disable bool) error {
args := []string{ "-q" }
if disable {
// Mitigate SSRF by restricting rendering workflow: disable external links and local file access where possible.
args = append(args, "--disable-external-links", "--disable-local-file-access")
}
// Read HTML from stdin and write PDF to outPath
cmd := exec.Command("wkhtmltopdf", append(args, "-", outPath)...)
cmd.Stdin = bytes.NewBufferString(input)
var stderr bytes.Buffer
cmd.Stderr = &stderr
if err := cmd.Run(); err != nil {
return fmt.Errorf("wkhtmltopdf failed: %v; stderror: %s", err, stderr.String())
}
return nil
}