Prevention of Content-Length field manipulation and resource exhaustion
# vulnerable_controller.rb
class VulnerableController < ApplicationController
def create
# This is where the vulnerability lies.
# The server will read the Content-Length from the request header and wait for that amount of data to arrive.
# If an attacker sends a very high value in the Content-Length field, the server will keep waiting for the data to arrive, thus keeping the connection open and potentially causing a denial of service.
content_length = request.headers['Content-Length'].to_i
data = request.body.read(content_length)
# process the data...
end
end
The code above represents a controller in a Ruby on Rails application. The
create
method is vulnerable to an asymmetric denial of service attack via the Content-Length field in the request header.
When a POST request is made to this endpoint, the server reads the Content-Length from the request header and waits for that amount of data to arrive before it starts processing the request. This is done by the line
content_length = request.headers['Content-Length'].to_i
and
data = request.body.read(content_length)
.
An attacker can exploit this vulnerability by sending a very high value in the Content-Length field. The server will then keep waiting for the data to arrive, thus keeping the connection open. If the attacker does this with a large number of connections, it can cause a denial of service.
The vulnerability arises from the fact that the server trusts the Content-Length value in the request header without any validation or limit checks. This allows an attacker to trick the server into waiting for more data than it should, thus tying up resources and potentially causing a denial of service.
# vulnerable_controller.rb
class VulnerableController < ApplicationController
MAX_CONTENT_LENGTH = 1000000 # Set a reasonable maximum limit for the content length
TIMEOUT = 10 # Set a reasonable timeout value
def create
content_length = request.headers['Content-Length'].to_i
# Validate the Content-Length value
if content_length > MAX_CONTENT_LENGTH
render json: { error: 'Content-Length is too large' }, status: 413
return
end
# Set a timeout for the request processing
Timeout.timeout(TIMEOUT) do
data = request.body.read(content_length)
# process the data...
end
rescue Timeout::Error
render json: { error: 'Request timed out' }, status: 408
end
end
The updated code includes several measures to mitigate the risk of an asymmetric denial of service attack via the Content-Length field.
Firstly, a maximum limit for the Content-Length is set (
MAX_CONTENT_LENGTH
). If a request comes in with a Content-Length value that exceeds this limit, the request is immediately rejected with a 413 (Payload Too Large) status code.
Secondly, a timeout (
TIMEOUT
) is set for the request processing. If the data does not arrive within this time, the connection is terminated and a 408 (Request Timeout) status code is returned.
These measures help to ensure that an attacker cannot keep a connection open indefinitely by sending a very high value in the Content-Length field and then sending the data very slowly. They also help to prevent the server from being overwhelmed by excessively large amounts of data.
In addition to these measures, it would also be beneficial to implement rate limiting, request size limits, request validation, and monitoring and logging mechanisms, as well as to keep server configurations up to date and educate developers about secure coding practices and common vulnerabilities.