Overview
SSRF can have severe real-world consequences: an attacker can trick a vulnerable service into fetching internal resources, cloud metadata endpoints, or other protected services, potentially exfiltrating data, triggering internal actions, or enabling network reconnaissance. In cloud and multi-tenant environments, SSRF may enable access to instance metadata, tokens, or credentials that would otherwise be protected behind the network perimeter. Without proper controls, such an issue can escalate from a simple proxy bug to a gateway for broader compromise.
In Go applications using the Gin framework, SSRF commonly appears when a route proxies a remote resource or fetches a URL provided by a client without strict validation. Using the standard library HTTP client (e.g., http.Get or a default client) with a URL derived from user input turns your service into an open proxy. If internal services or sensitive endpoints are reachable from the host, an attacker can leverage this to probe, access, or disrupt internal infrastructure.
This vulnerability class manifests in Go (Gin) when a handler reads a URL from request parameters and immediately issues a request to that URL, often with redirects allowed or without validating the destination. Returning the remote content directly to the client, or chaining multiple remote fetches, can magnify exposure. The root causes typically include missing URL validation, lack of destination allowlisting, and insufficient request timeouts or content-size controls.
Remediation focuses on constraining outbound fetches, validating inputs, and hardening the HTTP client usage. Implement allowlists for destinations, canonicalize and validate URLs, resolve DNS to detect and block private or reserved addresses, enforce timeouts and response size limits, and avoid proxying arbitrary content unless explicitly needed. Add robust logging and monitoring to detect SSRF attempts and keep dependencies up to date.
Code Fix Example
Go (Gin) API Security Remediation
Vulnerable pattern:
package main
import (
"io"
"net/http"
"github.com/gin-gonic/gin"
)
func vulnerableHandler(c *gin.Context) {
target := c.Query("url")
resp, err := http.Get(target)
if err != nil {
c.String(500, "error")
return
}
defer resp.Body.Close()
body, _ := io.ReadAll(resp.Body)
c.Data(resp.StatusCode, resp.Header.Get("Content-Type"), body)
}
// Fixed version:
package main
import (
"io"
"net"
"net/http"
"net/url"
"time"
"github.com/gin-gonic/gin"
)
var allowlist = map[string]struct{}{
"internal-api.local": {},
"external.example.com": {},
}
func isPrivateOrReserved(ip net.IP) bool {
if ip == nil {
return false
}
if ip4 := ip.To4(); ip4 != nil {
switch {
case ip4[0] == 10:
return true
case ip4[0] == 172 && ip4[1] >= 16 && ip4[1] <= 31:
return true
case ip4[0] == 192 && ip4[1] == 168:
return true
case ip4[0] == 127:
return true
}
} else {
if ip.IsLoopback() {
return true
}
// fc00::/7 is private range for IPv6
if ip[0] == 0xfc || ip[0] == 0xfd {
return true
}
}
return false
}
func fixedHandler(c *gin.Context) {
raw := c.Query("url")
u, err := url.ParseRequestURI(raw)
if err != nil {
c.String(400, "invalid URL")
return
}
if u.Scheme != "http" && u.Scheme != "https" {
c.String(400, "unsupported scheme")
return
}
if _, ok := allowlist[u.Hostname()]; !ok {
c.String(403, "host not allowed")
return
}
ips, err := net.LookupIP(u.Hostname())
if err != nil {
c.String(400, "invalid host")
return
}
for _, ip := range ips {
if isPrivateOrReserved(ip) {
c.String(403, "private/internal destination blocked")
return
}
}
client := http.Client{
Timeout: 5 * time.Second,
CheckRedirect: func(req *http.Request, via []*http.Request) error {
// prevent open redirects from the proxy path
return http.ErrUseLastResponse
},
}
resp, err := client.Get(u.String())
if err != nil {
c.String(502, "fetch error")
return
}
defer resp.Body.Close()
limited := io.LimitReader(resp.Body, 1024*1024) // 1MB max
body, _ := io.ReadAll(limited)
c.Data(resp.StatusCode, resp.Header.Get("Content-Type"), body)
}
func main() {
r := gin.Default()
r.GET("/vuln", vulnerableHandler)
r.GET("/fix", fixedHandler)
r.Run()
}