Api Rate Abuse in Chi with Saml

Rate abuse in Chi when SAML is used for authentication can occur when an attacker sends many authentication requests or token validation calls that exceed the intended request limits. Without proper rate controls around SAML endpoints, an unauthenticated or low-privilege actor can trigger repeated parsing, validation, or redirection logic tied to SAML responses. This may amplify risk around Authentication and BOLA/IDOR checks, because each request may involve user identity resolution or session creation.

Chi is a lightweight web framework for .NET where endpoints are typically defined as routes with minimal middleware. When SAML is integrated—often via a handler or middleware for SAML POST bindings—the framework may process incoming assertions on a route such as /saml/acs. If this route does not enforce per-identity or per-client rate limits, an attacker can replay or flood SAML responses, causing high CPU usage, session table growth, or enumeration of user identities through timing differences.

Because middleBrick scans the unauthenticated attack surface, it will flag missing rate controls on SAML endpoints under Rate Limiting and Authentication checks. Findings may reference general API Top 10 risks like Broken Object Level Authorization when identity lookups follow token validation, and Injection if SAML responses are not strictly validated. The scan’s cross-referencing of OpenAPI specs and runtime behavior can reveal whether SAML-related routes are missing protections that exist for other routes, highlighting inconsistent security posture across Chi endpoints.

To remediate rate abuse involving SAML in Chi, apply explicit rate limiting to SAML endpoints and enforce per-identity throttling. Below is a minimal, syntactically correct example for a Chi app using SAML with cookie-based session creation after assertion validation.

using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Http;
using Sustainsys.Saml2;
using Sustainsys.Saml2.AspNetCore2;

var builder = WebApplication.CreateBuilder(args);

// Configure SAML with defaults; adjust entityId and SPOptions as needed
builder.Services.AddAuthentication(Saml2Defaults.AuthenticationScheme)
    .AddSaml2(options =>
    {
        options.SPOptions.EntityId = new EntityId("https://app.example.com/saml");
        options.IdentityProviders.Add(new IdentityProvider(
            new EntityId("https://idp.example.com/saml"), options.SPOptions)
        {
            MetadataLocation = "https://idp.example.com/saml/metadata"
        });
    });

var app = builder.Build();

// Apply per-route rate limiting for SAML endpoints
// Use a fixed window or sliding store; here we use in-memory with a policy
builder.Services.AddRateLimiter(options =>
{
    options.GlobalLimiter = PartitionedRateLimiter.Create(context =>
    {
        // Partition by SAML endpoint and, if available, NameID or session key
        var userKey = context.User.Identity?.IsAuthenticated == true
            ? context.User.Identity.Name
            : context.Request.Path.ToString();
        return RateLimitPartition.GetFixedWindowLimiter(
            partitionKey: userKey,
            factory: _ => new FixedWindowRateLimiterOptions
            {
                PermitLimit = 10,
                Window = TimeSpan.FromMinutes(1)
            });
    });
});

app.UseAuthentication();
app.UseAuthorization();

// Apply the rate limiter to the SAML ACS route
app.MapPost("/saml/acs", async (HttpContext context) =>
{
    // SAML middleware processes the response; rate limiter runs first
    await context.ChallengeAsync(Saml2Defaults.AuthenticationScheme);
})
.WithRateLimiter();

// Optionally protect other identity-related endpoints
app.MapGet("/api/profile", (ClaimsPrincipal user) =>
{
    if (!user.Identity.IsAuthenticated) return Results.Unauthorized();
    return Results.Ok(new { user.Identity.Name });
}).RequireRateLimiting("global");

app.Run();

In this example, the rate limiter partitions by user identity (NameID) or request path to prevent a single entity from overwhelming the SAML endpoint. You can tune PermitLimit and Window based on expected traffic patterns. For distributed deployments, replace in-memory storage with a persistent store such as Redis using StackExchange.RateLimiting.StackExchangeRedis.

Additionally, validate SAML responses strictly and avoid leaking user information in error paths to reduce enumeration risks. Combine this with broader protections—such as authentication checks and input validation—since middleBrick will still flag remaining issues under Authentication and Input Validation if SAML assertions are not properly constrained.