Stack Overflow in Aspnet with Bearer Tokens

A common pattern in ASP.NET APIs is to accept Bearer tokens in the Authorization header and to use claims from the token for authorization decisions. When this pattern is combined with behaviors that lead to a Stack Overflow, the API surface can expose information leaks or crash under load, which may be surfaced by security checks such as BFLA/Privilege Escalation and Unsafe Consumption. A Stack Overflow can arise from unbounded recursion or deeply nested object graphs during model binding or from processing large or malicious payloads in endpoints that deserialize JSON without constraints.

Consider an endpoint that reads a token, extracts a user identifier, and then recursively traverses organizational units or permissions. If the data structure contains cycles or an attacker provides deeply nested JSON that causes the model binder or business logic to recurse without limits, the runtime can exhaust the thread stack. In parallel, if the endpoint trusts claims from the Bearer token without validating the token scope or audience, it might perform insecure direct object references (IDOR) by assuming the identifier in the token always maps to the correct resource.

For example, an endpoint like the following can become problematic when the input is not bounded and the authorization logic is tightly coupled to the token claims:

// Example: vulnerable pattern with Bearer token and recursive traversal
[ApiController]
[Route("api/[controller]")]
public class OrgController : ControllerBase
{
    private readonly IOrgRepository _repo;
    public OrgController(IOrgRepository repo) => _repo = repo;

    [HttpGet("{id}")]
    public IActionResult Get(int id)
    {
        // Bearer token may contain "sub" or custom claims used directly
        var userClaim = User.FindFirst("sub")?.Value;
        if (string.IsNullOrEmpty(userClaim))
        {
            return Unauthorized();
        }

        // Potential BFLA: no check whether user can access this specific org node
        var orgNode = _repo.GetByIdWithChildren(id); // recursive traversal
        return Ok(orgNode);
    }
}

In this scenario, an attacker could provide an id that triggers deep recursion, leading to a Stack Overflow. Additionally, the Bearer token’s claims may be used to infer relationships without proper checks, enabling IDOR if the token’s subject does not imply access to the targeted resource. Insecure Consumption is highlighted when the API deserializes untrusted input without size or depth limits, and BFLA/Privilege Escalation occurs when the API elevates behavior based on token claims without verifying whether the token scope includes the requested privilege.

ASP.NET Core’s built-in mechanisms can mitigate these risks. Use model validation to limit nesting depth and size, apply consistent authorization checks that do not rely solely on token claims, and enforce rate limiting to reduce the impact of resource exhaustion. MiddleBrick’s LLM/AI Security checks and BFLA/Privilege Escalation tests are designed to surface such patterns by correlating runtime behavior with OpenAPI specifications and authentication methods like Bearer tokens.

Remediation focuses on three areas: input validation to prevent Stack Overflow, robust authorization that does not overtrust Bearer token claims, and safe consumption of request payloads. Below are concrete, working examples for ASP.NET Core that address these concerns.

1) Prevent Stack Overflow with model validation and recursion limits

Configure JSON options to limit nesting depth and object graph cycles. Use DataAnnotations or FluentValidation to enforce reasonable constraints on incoming models.

// Program.cs or Startup.cs
builder.Services.AddControllers()
    .AddJsonOptions(options =>
    {
        options.JsonSerializerOptions.MaxDepth = 32; // prevents deep nesting
        options.JsonSerializerOptions.ReferenceHandler = System.Text.Json.Serialization.ReferenceHandler.IgnoreCycles;
    });

// Example DTO with validation
public class OrgRequest
{
    [Required]
    [Range(1, int.MaxValue, ErrorMessage = "Id must be greater than 0")]
    public int Id { get; set; }

    [MaxLength(200, ErrorMessage = "Name cannot exceed 200 characters")]
    public string Name { get; set; }
}

In the controller, use model state validation and avoid recursive traversal unless necessary and bounded.

[ApiController]
[Route("api/[controller]")]
public class OrgController : ControllerBase
{
    private readonly IOrgRepository _repo;
    public OrgController(IOrgRepository repo) => _repo = repo;

    [HttpGet("{id}")]
    public IActionResult Get(int id)
    {
        if (!ModelState.IsValid)
        {
            return BadRequest(ModelState);
        }

        // Apply authorization using policy or explicit checks, not only token claims
        var orgNode = _repo.GetByIdWithChildrenBounded(id, maxDepth: 5);
        if (orgNode == null)
        {
            return NotFound();
        }

        // Ensure the token’s subject has explicit access to this resource
        var userId = User.FindFirst("sub")?.Value;
        if (string.IsNullOrEmpty(userId) || !_repo.UserCanAccessOrg(userId, id))
        {
            return Forbid();
        }

        return Ok(orgNode);
    }
}

2) Apply least-privilege authorization independent of token claims

Do not assume that a Bearer token’s claims grant access to a specific resource. Use policy-based authorization and verify ownership or roles against the data store.

// Authorization policy example in Program.cs
builder.Services.AddAuthorization(options =>
{
    options.AddPolicy("OrgAccess", policy =>
        policy.RequireAssertion(context =>
            context.User.HasClaim(c => c.Type == "scope" && c.Value == "orgs.read")
                || context.User.IsInRole("Admin")));
});

// Usage in controller
[Authorize(Policy = "OrgAccess")]
[HttpGet("{id}")]
public IActionResult Get(int id)
{
    // safe logic here
}

3) Secure Bearer token handling and input sanitization

Always validate token audiences and issuers using standard mechanisms, and avoid using raw token claims for object ownership without checks.

// Configure JWT Bearer authentication with strict validation
builder.Services.AddAuthentication(JwtBearerDefaults.AuthenticationScheme)
    .AddJwtBearer(options =>
    {
        options.Authority = "https://auth.example.com";
        options.Audience = "api1";
        options.TokenValidationParameters = new TokenValidationParameters
        {
            ValidateIssuer = true,
            ValidateAudience = true,
            ValidateLifetime = true,
            ValidateIssuerSigningKey = true
        };
    });

// Example of safe usage: read token for auditing, not for access control alone
[HttpGet("{id}")]
public IActionResult Get(int id)
{
    var subject = User.FindFirst(JwtRegisteredClaimNames.Sub)?.Value;
    // Log subject for audit, but authorize via explicit service checks
    _audit.Log(subject, id);
    return Ok(_repo.GetWithPermissions(id, subject));
}

These patterns reduce the risk of Stack Overflow via input size and cycles, limit privilege escalation by decoupling token claims from authorization, and ensure safe consumption of requests. MiddleBrick’s CLI can scan your endpoints with middlebrick scan <url> to detect insecure patterns, and the Pro plan provides continuous monitoring to catch regressions as APIs evolve.