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.

Resultant

Implementation
REALIZATION: This weakness is caused during implementation of an architectural security tactic.

Confidentiality
Read Application Data

Automated Static Analysis
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.)
High

Architecture and Design
Protect the application's sessions from information leakage. Make sure that a session's data is not used or visible by other sessions.

Testing
Use a static analysis tool to scan the code for information leakage vulnerabilities (e.g. Singleton Member Field).

Architecture and Design
In a multithreading environment, storing user data in Servlet member fields introduces a data access race condition. Do not use member fields to store information in the Servlet.

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.

Data Leaking Between Users

Allowed
This CWE entry is at the Base level of abstraction, which is a preferred level of abstraction for mapping to the root causes of vulnerabilities.
Carefully read both the name and description to ensure that this mapping is an appropriate fit. Do not try to 'force' a mapping to a lower-level Base/Variant simply to comply with this preferred level of abstraction.

7 Pernicious Kingdoms
2006-07-19
Draft 3
2006-07-19

Eric Dalci
Cigital
2008-07-01
updated Potential_Mitigations, Time_of_Introduction

CWE Content Team
MITRE
2008-09-08
updated Description, Relationships, Other_Notes, Taxonomy_Mappings

CWE Content Team
MITRE
2009-05-27
updated Demonstrative_Examples

CWE Content Team
MITRE
2009-10-29
updated Description, Other_Notes

CWE Content Team
MITRE
2010-09-27
updated Potential_Mitigations

CWE Content Team
MITRE
2010-12-13
updated Relationships

CWE Content Team
MITRE
2011-03-29
updated Name

CWE Content Team
MITRE
2011-06-01
updated Common_Consequences

CWE Content Team
MITRE
2011-09-13
updated Relationships, Taxonomy_Mappings

CWE Content Team
MITRE
2012-05-11
updated Relationships

CWE Content Team
MITRE
2012-10-30
updated Potential_Mitigations

CWE Content Team
MITRE
2014-07-30
updated Relationships

CWE Content Team
MITRE
2017-11-08
updated Applicable_Platforms, Modes_of_Introduction, Relationships, Taxonomy_Mappings

CWE Content Team
MITRE
2020-02-24
updated References, Relationships, Type

CWE Content Team
MITRE
2021-03-15
updated Relationships

CWE Content Team
MITRE
2023-04-27
updated Detection_Factors, Relationships

CWE Content Team
MITRE
2023-06-29
updated Mapping_Notes

CWE Content Team
MITRE
2025-12-11
4.19
2025-12-11
updated Weakness_Ordinalities

Data Leaking Between Users
Data Leak Between Sessions