Web applications are particularly susceptible to
denial of service attacks. Note that network denial of service
attacks, such as SYN floods, are a separate problem that is outside
the scope of this document.
A web application can't easily tell the
difference between an attack and ordinary traffic. There are many
factors that contribute to this difficulty, but one of the most
important is that, for a number of reasons, IP addresses are not
useful as an identification credential. Because there is no reliable
way to tell where an HTTP request is from, it is very difficult to
filter out malicious traffic. For distributed attacks, how would an
application tell the difference between a true attack, multiple users
all hitting reload at the same time (which might happen if there is a
temporary problem with the site), or getting 'slashdotted'?
Most web servers can handle several hundred
concurrent users under normal use. A single attacker can generate
enough traffic from a single host to swamp many applications. Load
balancing can make these attacks more difficult, but far from
impossible, especially if sessions are tied to a particular server.
This is a good reason to make an application's session data as
small as possible and to make it somewhat difficult to start a new
Once an attacker can consume all of some required
resource, they can prevent legitimate users from using the system.
Some resources that are limited include bandwidth, database
connections, disk storage, CPU, memory, threads, or application
specific resources. All of these resources can be consumed by
attacks that target them. For example, a site that allows
unauthenticated users to request message board traffic may start many
database queries for each HTTP request they receive. An attacker can
‚easily send so many requests that the database connection pool will
get used up and there will be none left to service legitimate users.
Other variants of these attacks target system
resources related to a particular user. For example, an attacker
might be able to lock out a legitimate user by sending invalid
credentials until the system locks out the account. Or the attacker
might request a new password for a user, forcing them to access their
email account to regain access. Alternatively, if the system locks
any resources for a single user, then an attacker could potentially
tie them up so others could not use them.
Some web applications are even susceptible to
attacks that will take them offline immediately. Applications that do
not properly handle errors can even take down the web application
container. These attacks are particularly devastating because they
instantly prevent all other users from using the application.
There are a wide variety of these attacks, most of
which can be easily launched with a few lines of perl code from a low
powered computer. While there are no perfect defenses to these
attacks, it is possible to make it more difficult for them to
A9.2 Environments Affected
All web servers, application servers, and web
application environments are susceptible to denial of service
A9.3 Examples and References
A9.4 How to Determine If You Are Vulnerable
One of the hardest parts of denial of service
attacks is determining whether you are vulnerable. Load testing
tools, such as JMeter can generate web traffic so that you can test
certain aspects of how your site performs under heavy load. Certainly
one important test is how many requests per second your application
ącan field. Testing from a single IP address is useful as it will give
you an idea of how many requests an attacker will have to generate in
order to damage your site.
To determine if any resources can be used to
create a denial of service, you should analyze each one to see if
there is a way to exhaust it. You should particularly focus on what
an unauthenticated user can do, but unless you trust all of your
users, you should examine what an authenticated user can do as well.
A9.5 How to Protect Yourself
Defending against denial of service attacks is
difficult, as there is no way to protect against these attacks
As a general rule, you should limit the resources
allocated to any user to a bare minimum. For authenticated users, it
is possible to establish quotas so that you can limit the amount of
load a particular user can put on your system. In particular, you
might consider only handling one request per user at a time by
synchronizing on the user's session. You might also consider
dropping any requests that you are currently processing for a user
when another request from that user arrives.
For unauthenticated users, you should avoid any
unnecessary access to databases or other expensive resources. Try to
architect the flow of your site so that an unauthenticated user will
not be able to invoke any expensive operations. You might consider
caching the content received by unauthenticated users instead of
generating it or accessing databases to retrieve it.
You should also check your error handling scheme
to ensure that an error cannot affect the overall operation of the