CWE-567: Unsynchronized Access to Shared Data in a Multithreaded Context

Description

The product does not properly synchronize shared data, such as static variables across threads, which can lead to undefined behavior and unpredictable data changes.

Submission Date :

Dec. 15, 2006, midnight

Modification Date :

2023-06-29 00:00:00+00:00

Organization :

MITRE

Extended Description

Within servlets, shared static variables are not protected from concurrent access, but servlets are multithreaded. This is a typical programming mistake in J2EE applications, since the multithreading is handled by the framework. When a shared variable can be influenced by an attacker, one thread could wind up modifying the variable to contain data that is not valid for a different thread that is also using the data within the variable.

Note that this weakness is not unique to servlets.

Example Vulnerable Codes

Example - 1

The following code implements a basic counter for how many times the page has been accesed.



out.setContentType("text/plain");PrintWriter p = out.getWriter();count++;p.println(count + " hits so far!");static int count = 0;protected void doGet(HttpServletRequest in, HttpServletResponse out)throws ServletException, IOException {}public static class Counter extends HttpServlet {}

Consider when two separate threads, Thread A and Thread B, concurrently handle two different requests:

Assume this is the first occurrence of doGet, so the value of count is 0.doGet() is called within Thread A.The execution of doGet() in Thread A continues to the point AFTER the value of the count variable is read, then incremented, but BEFORE it is saved back to count. At this stage, the incremented value is 1, but the value of count is 0.doGet() is called within Thread B, and due to a higher thread priority, Thread B progresses to the point where the count variable is accessed (where it is still 0), incremented, and saved. After the save, count is 1.Thread A continues. It saves the intermediate, incremented value to the count variable - but the incremented value is 1, so count is "re-saved" to 1.

At this point, both Thread A and Thread B print that one hit has been seen, even though two separate requests have been processed. The value of count should be 2, not 1.

While this example does not have any real serious implications, if the shared variable in question is used for resource tracking, then resource consumption could occur. Other scenarios exist.

Related Weaknesses

This table shows the weaknesses and high level categories that are related to this weakness. These relationships are defined to give an overview of the different insight to similar items that may exist at higher and lower levels of abstraction.

Visit http://cwe.mitre.org/ for more details.