Overview
CVE-2017-0249 describes an elevation of privilege vulnerability in ASP.NET Core where the framework fails to properly sanitize web requests, enabling attackers to influence authorization and access controls. This is aligned with CWE-20 (Improper Input Validation), as untrusted input may be used to alter the application's security state.
In real-world scenarios, an attacker could craft requests that mutate the currently authenticated identity or role claims by injecting or altering query parameters, headers, or other input that the application uses to decide permissions. If an application relies on input-derived claims without proper validation or without binding to a trusted identity source (e.g., ASP.NET Core Identity or a trusted token), an adversary could bypass authorization checks and gain elevated access.
A minimal ASP.NET Core pattern could read user-supplied data and, if it matches a privileged value, injects a corresponding claim into HttpContext.User. Such behavior effectively elevates privileges based on untrusted input, bypassing server-side access controls and undermining defense-in-depth strategies. This class of vulnerability manifests in ASP.NET Core when input validation is neglected and authorization decisions are made using tainted data.
Remediation focuses on ensuring authentication sources are trusted and authorization is enforced by the framework rather than by input-derived state. Do not mutate HttpContext.User based on user input. Rely on robust identity management (ASP.NET Core Identity or JWT/OIDC tokens), apply [Authorize] attributes or policy-based authorization, validate and sanitize inputs, and implement explicit allow/deny logic at the authorization boundary. Regular security testing and threat modeling should be used to verify that privilege escalation via request data is not possible.
Code Fix Example
ASP.NET Core API Security Remediation
// Vulnerable pattern: role can be set from an untrusted query parameter
using System;
using System.Collections.Generic;
using System.Security.Claims;
using Microsoft.AspNetCore.Authorization;
using Microsoft.AspNetCore.Mvc;
namespace VulnerabilityDemo
{
[ApiController]
[Route("[controller]")]
public class DemoController : ControllerBase
{
// Vulnerable: allows privilege escalation bySupplying ?role=admin
[HttpGet("set-role")]
public IActionResult SetRoleFromQuery()
{
var role = HttpContext.Request.Query["role"].ToString();
if (string.Equals(role, "admin", StringComparison.OrdinalIgnoreCase))
{
var claims = new List<Claim> { new Claim(ClaimTypes.Role, "Admin") };
HttpContext.User = new ClaimsPrincipal(new ClaimsIdentity(claims, "Query"));
}
bool isAdmin = HttpContext.User.IsInRole("Admin");
return Ok(new { IsAdmin = isAdmin });
}
// Fixed: rely on proper authentication/authorization
[HttpGet("secure")]
[Authorize(Roles = "Admin")]
public IActionResult SecureEndpoint()
{
return Ok("You are admin");
}
}
}