CWE-567: Unsynchronized Access to Shared Data in a Multithreaded Context
Unsynchronized Access to Shared Data in a Multithreaded Context
Weakness ID: 567 (Weakness Base)
Status: Draft
Description
Description Summary
The product does not properly synchronize shared data, such as static variables across threads, which can lead to undefined behavior and unpredictable data changes.
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.
If the shared variable contains sensitive data, it may be manipulated
or displayed in another user session. If this data is used to control
the application, its value can be manipulated to cause the application
to crash or perform poorly.
Demonstrative Examples
Example 1
The following code implements a basic counter for how many times
the page has been accesed.
(Bad Code)
Example
Language: Java
public static class Counter extends HttpServlet {
static int count = 0;
protected void doGet(HttpServletRequest in,
HttpServletResponse out)
throws ServletException, IOException {
out.setContentType("text/plain");
PrintWriter p = out.getWriter();
count++;
p.println(count + " hits so far!");
}
}
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.
Potential Mitigations
Remove the use of static variables used between servlets. If this
cannot be avoided, use synchronized access for these variables.
Description
