Overview
CVE-2017-0249 describes an elevation of privilege vulnerability in ASP.NET Core that arises when the framework fails to properly sanitize web requests, enabling attackers to bypass function-level authorization. This is a CWE-20 Improper Input Validation issue where untrusted input influences access decisions. In practice, attackers could craft requests that manipulate input used for authorization, potentially gaining access to resources or actions that should be restricted. The vulnerability highlights the risk of trusting user-supplied data for security decisions and the need for robust input handling in ASP.NET Core applications.
In real-world ASP.NET Core apps, this manifests when endpoints perform authorization based on user-provided data (for example, query parameters, headers, or path segments) rather than relying on the authenticated identity and explicit resource ownership checks. An attacker could construct requests that appear to originate from an authorized user to access privileged resources. This class of vulnerability is commonly referred to as Broken Function Level Authorization, where function-level access controls are not consistently enforced across endpoints due to improper input handling and insufficient validation.
The remediation is to stop trusting user-controlled input for authorization, enforce proper endpoint authorization using ASP.NET Core's policy-based authorization, verify resource ownership in the data store, and remove ad-hoc checks that depend on untrusted input. Upgrading to patched ASP.NET Core versions that include the fixes, auditing endpoints for function-level authorization gaps, and adding security tests to verify authorization behavior are critical steps in mitigating CVE-2017-0249.
Code Fix Example
ASP.NET Core API Security Remediation
using Microsoft.AspNetCore.Authorization;
using Microsoft.AspNetCore.Mvc;
using System.Collections.Generic;
using System.Linq;
using System.Security.Claims;
namespace VulnerabilityDemo
{
public class DemoResource { public int Id; public string OwnerId; public string Data; }
[ApiController]
[Route("api/[controller]")]
public class DemoController : ControllerBase
{
private static readonly List<DemoResource> Resources = new List<DemoResource>
{
new DemoResource { Id = 1, OwnerId = "alice", Data = "Alice Secret" },
new DemoResource { Id = 2, OwnerId = "bob", Data = "Bob Secret" }
};
// Vulnerable pattern: relies on a user-supplied value for authorization
[HttpGet("vul/{id}")]
public IActionResult GetResourceVulnerable(int id, string requestedOwner)
{
var r = Resources.FirstOrDefault(x => x.Id == id);
if (r == null) return NotFound();
// Vulnerable: authorization depends on user-supplied input
if (requestedOwner != User.Identity.Name)
{
return Forbid();
}
return Ok(r.Data);
}
// Fixed pattern: enforce ownership using authenticated identity
[HttpGet("fix/{id}")]
[Authorize]
public IActionResult GetResourceFixed(int id)
{
var r = Resources.FirstOrDefault(x => x.Id == id);
if (r == null) return NotFound();
var userId = User.FindFirst(ClaimTypes.NameIdentifier)?.Value;
if (r.OwnerId != userId) return Forbid();
return Ok(r.Data);
}
}
}