3
For example, if session identifiers were provided in ascending order, there would be close to zero
bits of entropy in the session identifier no matter the identifier's length. Assuming that the session
identifiers are being generated using a good source of random numbers, we will estimate the
number of bits of entropy in a session identifier to be half the total number of bits in the session
identifier. For realistic identifier lengths this is possible, though perhaps optimistic.
Common Consequences
Scope Impact Likelihood
Access Control Gain Privileges or Assume Identity
If an attacker can guess an authenticated user's session
identifier, they can take over the user's session.
Potential Mitigations
Phase: Implementation
Session identifiers should be at least 128 bits long to prevent brute-force session guessing. A
shorter session identifier leaves the application open to brute-force session guessing attacks.
Phase: Implementation
A lower bound on the number of valid session identifiers that are available to be guessed is the
number of users that are active on a site at any given moment. However, any users that abandon
their sessions without logging out will increase this number. (This is one of many good reasons to
have a short inactive session timeout.) With a 64 bit session identifier, assume 32 bits of entropy.
For a large web site, assume that the attacker can try 1,000 guesses per second and that there
are 10,000 valid session identifiers at any given moment. Given these assumptions, the expected
time for an attacker to successfully guess a valid session identifier is less than 4 minutes. Now
assume a 128 bit session identifier that provides 64 bits of entropy. With a very large web site, an
attacker might try 10,000 guesses per second with 100,000 valid session identifiers available to
be guessed. Given these assumptions, the expected time for an attacker to successfully guess a
valid session identifier is greater than 292 years.
Demonstrative Examples
Example 1:
The following XML example code is a deployment descriptor for a Java web application deployed
on a Sun Java Application Server. This deployment descriptor includes a session configuration
property for configuring the session ID length.
Example Language: XML (Bad)
<sun-web-app>
...
<session-config>
<session-properties>
<property name="idLengthBytes" value="8">
<description>The number of bytes in this web module's session ID.</description>
</property>
</session-properties>
</session-config>
...
</sun-web-app>
This deployment descriptor has set the session ID length for this Java web application to 8 bytes
(or 64 bits). The session ID length for Java web applications should be set to 16 bytes (128 bits) to
prevent attackers from guessing and/or stealing a session ID and taking over a user's session.
Note for most application servers including the Sun Java Application Server the session ID length is
by default set to 128 bits and should not be changed. And for many application servers the session
ID length cannot be changed from this default setting. Check your application server documentation
CWE Version 4.14
CWE-7: J2EE Misconfiguration: Missing Custom Error Page
CWE-7: J2EE Misconfiguration: Missing Custom Error Page