A Slow Post attack is a type of Denial of Service (DoS) attack that targets web servers by exploiting the way they handle HTTP POST requests. It involves sending a legitimate-looking HTTP POST request to a server, but the request is deliberately sent at an extremely slow rate. The purpose of this attack is to tie up server resources by keeping connections open for extended periods, preventing the server from serving legitimate users or processing other requests efficiently. It is particularly effective against web servers and application servers that rely on request handling through HTTP POST methods.
In a Slow Post attack, the attacker sends a POST request, but the data is deliberately sent very slowly, typically in small chunks. The attack works as follows:
Send Initial HTTP POST Request: The attacker initiates a normal HTTP POST request, which appears legitimate to the server.
Send Data Slowly: Instead of sending the entire body of the POST request all at once, the attacker sends it in very small chunks, deliberately slowing down the transmission.
Hold Connection Open: The server waits for the full request, consuming server resources to maintain the connection. The attacker keeps sending small parts of the request, keeping the server waiting for completion.
Exhaust Server Resources: As multiple slow POST requests are sent, the server starts to run out of available resources like threads, memory, or request processing capabilities, leading to a Denial of Service for other users.
The Slow Post attack is effective because it doesn’t require a large volume of data to overload the server. Some of its key benefits include:
Low Traffic Volume: Slow Post attacks consume very little bandwidth, making them difficult to detect. The attacker doesn’t need to flood the server with traffic, making it stealthier than other DoS attacks.
Targeted Server Resources: The attack directly targets server threads, memory, and request processing capacities by exploiting the server’s method of handling POST requests.
Bypass Detection: Unlike many other types of DDoS attacks, Slow Post attacks don’t generate noticeable spikes in traffic, making them hard to distinguish from legitimate traffic.
While the Slow Post attack can be effective, there are several challenges and considerations:
Server Configuration: Slow Post attacks primarily affect thread-based servers or servers with limited timeout configurations for incoming requests. Web servers like Apache are particularly vulnerable, while more modern, event-driven servers like Nginx may be less susceptible.
Detection Difficulty: Slow Post attacks are challenging to detect because they do not generate high volumes of traffic. The attacker sends data at a pace similar to legitimate users, so security tools might not flag the traffic as malicious.
Impact on User Experience: If successful, a Slow Post attack can degrade server performance significantly, causing slow processing of legitimate requests and timeouts for real users.
Several strategies can be employed to mitigate the risk of Slow Post attacks:
Timeout Settings: Configuring timeout settings for connections can ensure that requests not completed within a specified period are dropped. For instance, web servers can close connections after a certain time without receiving complete data.
Rate Limiting: Limiting the number of POST requests a client can make within a given timeframe can help prevent excessive slow POST attacks. Rate limiting can restrict clients to sending no more than a specific number of POST requests per second.
Web Application Firewalls (WAF): A WAF can detect and filter out incomplete or suspicious POST requests, blocking requests from known malicious sources based on behavior patterns like slow data transmission.
Intrusion Detection Systems (IDS): Implementing an IDS can help detect unusual patterns in the timing of data transmissions, such as small chunks of data being sent slowly, and alert administrators to potential Slow Post attacks.
Use Non-Threaded Servers: Switching from thread-based web servers like Apache to event-driven servers like Nginx can help mitigate the impact of Slow Post attacks, as they do not allocate a separate thread for each request.
Connection Limits: Set limits on the number of connections a single client can establish with the server over a short period to prevent attackers from overwhelming the server with slow POST connections.
The Slow Post attack is a powerful yet stealthy Denial of Service (DoS) attack that targets how web servers handle HTTP POST requests. By sending slow and incomplete POST requests, attackers can exhaust server resources and disrupt normal services. While detecting and mitigating Slow Post attacks can be challenging, applying best practices such as rate limiting, timeout configurations, and WAF usage can significantly reduce the risk of such attacks.