Unsynchronized Access to Shared Data in a Multithreaded Context

Draft Base

Structure: Simple

Description

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.

Extended Description

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.

Common Consequences 1

Scope: ConfidentialityIntegrityAvailability

Impact: Read Application DataModify Application DataDoS: InstabilityDoS: Crash, Exit, or Restart

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.

Detection Methods 1

Automated Static AnalysisHigh

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.)

Potential Mitigations 1

Phase: Implementation

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

Demonstrative Examples 1

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

Code Example:Bad
Java
java

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.

Applicable Platforms

Languages:

Java : Undetermined

Modes of Introduction

Implementation

Related Attack Patterns