Security – Simulating And Protecting Against A DoS Attack

On a recent project, I was created a web service which parsed a set of financial statements into name/value pairs from an XBRL document. The complexity of the XBRL   specification means that parsing an XBRL document takes approximately 90 seconds on mid-spec server. To avoid users having to wait for the 90 seconds for the data, the parsed data was saved to a database. However, since the service covers over 5000 companies it would need to store approximately 100,000 sets of financial statements which places a strain on database resources as each set of statements contains several thousand data points. The obvious solution is to only store the most commonly requested financial statements in the database and load the others on-demand with a message to users that the financial statements were being loaded. However this inoccuous design decision creates a major security vulnerability.

Slow Loading Pages = Security Vulnerability
One of the hardest attacks to defend against is a Denial of Service (DoS) attack which is simply a repeated set of requests for a server resource such as a web page. The repeated requests generate an excess of load on the server which is then rendered incapable of serving other legitimate site users and hence the web site is taken down.
The difficulty in launching a DoS attack is that most web pages involve very little CPU processing and hence the flood of requests necessary to bring the site down would be extremely large and hard to generate. Attackers therefore will target slow loading pages which may be consuming a lot of server resources.

Simulating a DoS Attack
To test a web application’s vulernability to DoS attacks we can use a tool to issue multiple simultaneous requests. For this demo I will use  LOIC (Low Orbit Ion Cannon) with the source code available at
LOIC comes with a simple Windows application front-end, although you could just use the source code in the XXPFlooder.cs file to programatically launch the attack (note the simplicity of the code for running DoS attacks – the XXPFlooder.cs file is a mere 89 lines).
Using LOIC Windows app I configured it to make Http requests to the /loadaccounts?id=1002 page on my development server:

This quickly overwhelmed the development server, bringing the CPU utilization close to 100%:

DoS attacks are one of the most challenging security issues as there are numerous different ways of launching a DoS attack. Typically the defense will be at the network or server level, but in the case of a resource intensive page there are several potential application level optimisations. Firstly, the resource intensive page can be configured as a separate application with a dedicated application pool. A attack will therefore be isolated to the specific page (or area in the website) and leave the rest of the site unaffected.
Next, the page itself can be protected by using a short term in-memory cache. After executing the the resource intensive operation the resulting object can be placed in memory for a short period and subsequent requests will be served from the in-memory cache. A basic outline of the code is below:

FinancialStatements financialStatements;

//First test if there is an an object in the cache
if(Cache["FinancialStatements" + id] == null)
//If there is no object in the cache, create it an then load it into the cache
financialStatements = LoadFinancialStatements(id); //This is the resouce intensive operation
CacheItemPolicy policy = new CacheItemPolicy();
policy.AbsoluteExpiration = DateTime.Now + TimeSpan.FromMinutes(1);

Cache.Add(new CacheItem("FinancialStatements" + id, financialStatements), policy);
//If the object is already in the memory, simply load it from the
//cache which is a quick low impact operation.
financialStatements = Cache["FinancialStatements" + id];

Thus the first request will run resource intensive operation, but subsequent requests made within 60 seconds will simply be loaded from the memory.

Now running the LOIC attack again results in the below CPU resouce utilisation:

Note that CPU now only reaches a manageable 31% and only one of the cores is under significant load.

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