Overview
SSRF (Server-Side Request Forgery) in Go applications using the Gin framework can allow an attacker to coax your server into issuing HTTP requests to internal services, cloud metadata endpoints, or other protected resources. This can lead to information disclosure, unauthorized access, or even control of internal systems if the server can interact with sensitive endpoints. In a typical Gin route that consumes a user-supplied URL and fetches it server-side, an attacker can drive requests to places the application should never reach.
Manifestation in Go (Gin) often occurs when a handler accepts a URL parameter and immediately uses http.Get or an unconfigured http.Client to fetch the target. Without validation, enforcing a strict scheme, or restricting destinations, an attacker can use your service as a proxy to internal networks or sensitive endpoints. Insecure defaults (for example, permissive proxies from environment, unlimited redirects, or lax timeouts) amplify risk in production.
Real-world impact includes discovery of internal services via DNS, access to cloud metadata endpoints to retrieve credentials, or exfiltration of tokens from secret-management endpoints. SSRF can be a foothold for broader attacks if the target resource can influence privileged operations. In Gin applications, this risk is common in endpoints that fetch remote resources specified by the client (for example, image fetchers, content fetchers, or webhook handlers) and can be exploited even with seemingly benign features.
Remediation should focus on constraining user input, using allowlists, and enforcing robust network controls. This guide demonstrates how to refactor vulnerable routes to prevent SSRF and highlights patterns that reduce risk in Go (Gin) applications.
Code Fix Example
Go (Gin) API Security Remediation
package main
import (
"io/ioutil"
"net/http"
"net/url"
"time"
"github.com/gin-gonic/gin"
)
func main() {
r := gin.Default()
r.GET("/vulnerable/fetch", vulnerableFetch)
r.GET("/fixed/fetch", fixedFetch)
r.Run()
}
func vulnerableFetch(c *gin.Context) {
target := c.Query("url")
if target == "" {
c.String(400, "missing url")
return
}
resp, err := http.Get(target)
if err != nil {
c.String(500, err.Error())
return
}
defer resp.Body.Close()
body, _ := ioutil.ReadAll(resp.Body)
c.Data(resp.StatusCode, resp.Header.Get("Content-Type"), body)
}
func fixedFetch(c *gin.Context) {
raw := c.Query("url")
if raw == "" {
c.String(400, "missing url")
return
}
u, err := url.Parse(raw)
if err != nil {
c.String(400, "invalid url")
return
}
if u.Scheme != "http" && u.Scheme != "https" {
c.String(400, "unsupported scheme")
return
}
host := u.Hostname()
allowed := map[string]bool{
"example.org": true,
"api.example.org": true,
}
if !allowed[host] {
c.String(403, "host not allowed")
return
}
client := &http.Client{Timeout: 10 * time.Second}
resp, err := client.Get(raw)
if err != nil {
c.String(502, err.Error())
return
}
defer resp.Body.Close()
body, _ := ioutil.ReadAll(resp.Body)
c.Data(resp.StatusCode, resp.Header.Get("Content-Type"), body)
}