Exposure of Data Element to Wrong Session

The product does not sufficiently enforce boundaries between the states of different sessions, causing data to be provided to, or used by, the wrong session.


Data can "bleed" from one session to another through member variables of singleton objects, such as Servlets, and objects from a shared pool.

In the case of Servlets, developers sometimes do not understand that, unless a Servlet implements the SingleThreadModel interface, the Servlet is a singleton; there is only one instance of the Servlet, and that single instance is used and re-used to handle multiple requests that are processed simultaneously by different threads. A common result is that developers use Servlet member fields in such a way that one user may inadvertently see another user's data. In other words, storing user data in Servlet member fields introduces a data access race condition.


The following examples help to illustrate the nature of this weakness and describe methods or techniques which can be used to mitigate the risk.

Note that the examples here are by no means exhaustive and any given weakness may have many subtle varieties, each of which may require different detection methods or runtime controls.

Example One

The following Servlet stores the value of a request parameter in a member field and then later echoes the parameter value to the response output stream.

public class GuestBook extends HttpServlet {

  String name;

  protected void doPost (HttpServletRequest req, HttpServletResponse res) {
    name = req.getParameter("name");
    out.println(name + ", thanks for visiting!");


While this code will work perfectly in a single-user environment, if two users access the Servlet at approximately the same time, it is possible for the two request handler threads to interleave in the following way: Thread 1: assign "Dick" to name Thread 2: assign "Jane" to name Thread 1: print "Jane, thanks for visiting!" Thread 2: print "Jane, thanks for visiting!" Thereby showing the first user the second user's name.

See Also

Comprehensive Categorization: Exposed Resource

Weaknesses in this category are related to exposed resource.

User Session Errors

Weaknesses in this category are related to session management. Frequently these deal with the information or status about each user and their access rights for the dur...

Manage User Sessions

Weaknesses in this category are related to the design and architecture of session management. Frequently these deal with the information or status about each user and ...

Comprehensive CWE Dictionary

This view (slice) covers all the elements in CWE.

Weaknesses Introduced During Implementation

This view (slice) lists weaknesses that can be introduced during implementation.

Weakness Base Elements

This view (slice) displays only weakness base elements.

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