Denial of Service (DoS)

Description

A Denial of Service (DoS) attack aims to render a network or application resource unavailable to its intended users. Attackers typically overwhelm the target with excessive requests, resource-intensive tasks, or exploit a bottleneck in the system's design, causing partial or complete service interruption. This can result in significant downtime, financial losses, and damage to an organization's reputation.

DoS attacks often exploit insufficient resource management or concurrency controls. A single endpoint that triggers an expensive database query, or a file upload function lacking size restrictions, can become a bottleneck when abused by an attacker. In more severe cases, a Distributed Denial of Service (DDoS) employs multiple hosts to send massive traffic simultaneously, making it harder to distinguish legitimate traffic from malicious overload attempts.

Examples

Volumetric Flooding

Attackers generate a high volume of traffic (e.g., HTTP GET requests) to saturate a server's network bandwidth or processing capacity. Without proper rate limiting or filtering, the server becomes overwhelmed and unable to handle legitimate requests.

Resource-Intensive Endpoints

Some requests—such as complex database queries, file compression, or image resizing—require significant CPU or memory. Attackers can exploit these endpoints by sending repeated or large requests, causing the system to run out of resources.

Slowloris (Slow HTTP Attacks)

Attackers keep many connections open by sending partial HTTP requests slowly, preventing the server from closing these connections. Over time, the server runs out of available connections, denying new incoming legitimate requests.

Application Logic Loops

If an application has a poorly designed workflow (e.g., redirect loops or nested operations triggered by a single request), attackers can craft requests that repeatedly trigger resource-heavy processes, resulting in denial of service.

Remediation

  1. Rate Limiting and Throttling

    • Enforce limits on how many requests an IP or user can make within a specific time window.
    • Configure backoff algorithms or request queuing to balance incoming traffic.

  2. Use a Content Delivery Network (CDN)

    • Offload static content (images, scripts, styles) to CDN nodes, reducing the load on your origin server.
    • Many CDNs also provide DDoS protection, filtering out malicious traffic before it reaches your server.

  3. Implement Resource Constraints

    • Configure maximum file upload sizes, limit recursion or loop depth in server-side code, and ensure timeouts for long-running requests.
    • Use defensive measures like circuit breakers or graceful degradation to keep the system responsive under heavy load.

  4. Apply Web Application Firewall (WAF) and Intrusion Detection

    • Deploy WAF rules to identify and block known DoS patterns or suspicious traffic spikes.
    • Use Intrusion Detection Systems (IDS) or Intrusion Prevention Systems (IPS) to monitor and mitigate threats in real time.

  5. Scalable Infrastructure

    • Design your application to scale horizontally, adding more servers or containers as traffic grows.
    • Use load balancers that distribute requests evenly and detect overloaded instances.