CWE-567 Base Draft

Unsynchronized Access to Shared Data in a Multithreaded Context

This vulnerability occurs when multiple threads in an application can read and modify shared data, like static variables, without proper coordination. This unsynchronized access corrupts data,…

Definition

What is CWE-567?

This vulnerability occurs when multiple threads in an application can read and modify shared data, like static variables, without proper coordination. This unsynchronized access corrupts data, causes crashes, and leads to unpredictable, often security-critical, behavior.
A common example is in Java servlet-based applications, where the framework manages multithreading. Developers might mistakenly treat static variables as safe, forgetting that all servlet threads can access them simultaneously. If an attacker can influence this shared data, one thread could inject invalid or malicious content that another thread then processes, creating a serious security flaw. This issue is not limited to servlets or J2EE. It's a fundamental concurrency flaw that can appear in any multithreaded environment when developers assume single-threaded execution for shared resources. The core problem is a mismatch: the application uses a multithreaded architecture but fails to implement the necessary safeguards, like locks or atomic operations, to protect its shared state from concurrent modification.
Real-world impact

Real-world CVEs caused by CWE-567

No public CVE references are linked to this CWE in MITRE's catalog yet.

How attackers exploit it

Step-by-step attacker path

  1. 1

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

  2. 2

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

  3. 3

    - 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.

  4. 4

    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.

  5. 5

    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.

Vulnerable code example

Vulnerable Java

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

Vulnerable 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!");
  	}
  }
Secure code example

Secure pseudo

Secure pseudo 
// Validate, sanitize, or use a safe API before reaching the sink.
function handleRequest(input) {
  const safe = validateAndEscape(input);
  return executeWithGuards(safe);
}
What changed: the unsafe sink is replaced (or the input is validated/escaped) so the same payload no longer triggers the weakness.
Prevention checklist

How to prevent CWE-567

  • Implementation Remove the use of static variables used between servlets. If this cannot be avoided, use synchronized access for these variables.
Detection signals

How to detect CWE-567

Automated Static Analysis High

Automated static analysis, commonly referred to as Static Application Security Testing (SAST), can find some instances of this weakness by analyzing source code (or binary/compiled code) without having to execute it. Typically, this is done by building a model of data flow and control flow, then searching for potentially-vulnerable patterns that connect "sources" (origins of input) with "sinks" (destinations where the data interacts with external components, a lower layer such as the OS, etc.)

Plexicus auto-fix

Plexicus auto-detects CWE-567 and opens a fix PR in under 60 seconds.

Codex Remedium scans every commit, identifies this exact weakness, and ships a reviewer-ready pull request with the patch. No tickets. No hand-offs.

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Frequently asked questions

Frequently asked questions

What is CWE-567?

This vulnerability occurs when multiple threads in an application can read and modify shared data, like static variables, without proper coordination. This unsynchronized access corrupts data, causes crashes, and leads to unpredictable, often security-critical, behavior.

How serious is CWE-567?

MITRE has not published a likelihood-of-exploit rating for this weakness. Treat it as medium-impact until your threat model proves otherwise.

What languages or platforms are affected by CWE-567?

MITRE lists the following affected platforms: Java.

How can I prevent CWE-567?

Remove the use of static variables used between servlets. If this cannot be avoided, use synchronized access for these variables.

How does Plexicus detect and fix CWE-567?

Plexicus's SAST engine matches the data-flow signature for CWE-567 on every commit. When a match is found, our Codex Remedium agent opens a fix PR with the corrected code, tests, and a one-line summary for the reviewer.

Where can I learn more about CWE-567?

MITRE publishes the canonical definition at https://cwe.mitre.org/data/definitions/567.html. You can also reference OWASP and NIST documentation for adjacent guidance.

Related weaknesses

Weaknesses related to CWE-567

CWE-820 Parent

Missing Synchronization

This vulnerability occurs when multiple parts of your application (like threads or processes) use the same resource—such as a variable,…

CWE-1096 Sibling

Singleton Class Instance Creation without Proper Locking or Synchronization

This flaw occurs when a Singleton class is implemented without proper thread-safe controls, allowing multiple instances to be created in…

CWE-543 Sibling

Use of Singleton Pattern Without Synchronization in a Multithreaded Context

This vulnerability occurs when a singleton pattern is implemented in a multithreaded application without proper synchronization,…

CWE-488 Can precede

Exposure of Data Element to Wrong Session

This vulnerability occurs when an application fails to properly isolate data between different user sessions, allowing information from…

Resources

Further reading

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