Overview
This guide explains an SSRF-like elevation of privilege vulnerability described for ASP.NET Core (C#) in CVE-2017-0249, tied to CWE-20 (Improper Input Validation). The root cause is failure to sanitize or validate certain web requests, allowing attacker-controlled inputs to influence server-side calls. In practice, this can enable an attacker to trick the server into requesting internal resources or services, potentially bypassing network boundaries and escalating privileges depending on what internal endpoints are reached. The vulnerability highlights how insufficient validation of user-supplied URLs in server-side request scenarios can create significant risk in ASP.NET Core applications. The guidance here references CVE-2017-0249 explicitly and emphasizes the importance of proper input validation to prevent SSRF-like behavior and privilege escalation.
Code Fix Example
ASP.NET Core API Security Remediation
Vulnerable pattern:
using System.Net.Http;
using System.Threading.Tasks;
using Microsoft.AspNetCore.Http;
public class SsrfDemo
{
public async Task<string> GetExternalContentUnsafe(HttpContext httpContext)
{
var url = httpContext.Request.Query[\"url\"].ToString();
using (var http = new HttpClient())
{
var resp = await http.GetAsync(url);
return await resp.Content.ReadAsStringAsync();
}
}
}
Fixed pattern:
using System;
using System.Net.Http;
using System.Threading.Tasks;
using Microsoft.AspNetCore.Http;
using System.Collections.Generic;
public class SsrfDemo
{
private static readonly HashSet<string> AllowedHosts = new HashSet<string>(StringComparer.OrdinalIgnoreCase)
{
\"trusted.example.com\",
\"api.internal.local\"
};
public async Task<string> GetExternalContentSafe(HttpContext httpContext)
{
var urlString = httpContext.Request.Query[\"url\"].ToString();
if (!Uri.IsWellFormedUriString(urlString, UriKind.Absolute))
{
throw new ArgumentException(\"Invalid URL\");
}
var uri = new Uri(urlString);
if (!AllowedHosts.Contains(uri.Host))
{
throw new UnauthorizedAccessException(\"Host not allowed\");
}
using (var http = new HttpClient())
{
var resp = await http.GetAsync(uri);
return await resp.Content.ReadAsStringAsync();
}
}
}