CWE-567: Unsynchronized Access to Shared Data in a Multithreaded Context
Weakness ID: 567
Abstraction: Base Structure: Simple
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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.
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.
Relationships
This table shows the weaknesses and high level categories that are related to this weakness. These relationships are defined as ChildOf, ParentOf, MemberOf and give insight to similar items that may exist at higher and lower levels of abstraction. In addition, relationships such as PeerOf and CanAlsoBe are defined to show similar weaknesses that the user may want to explore.
Relevant to the view "Research Concepts" (CWE-1000)
Nature
Type
ID
Name
ChildOf
Base - a weakness
that is still mostly independent of a resource or technology, but with sufficient details to provide specific methods for detection and prevention. Base level weaknesses typically describe issues in terms of 2 or 3 of the following dimensions: behavior, property, technology, language, and resource.
Base - a weakness
that is still mostly independent of a resource or technology, but with sufficient details to provide specific methods for detection and prevention. Base level weaknesses typically describe issues in terms of 2 or 3 of the following dimensions: behavior, property, technology, language, and resource.
This table shows the weaknesses and high level categories that are related to this weakness. These relationships are defined as ChildOf, ParentOf, MemberOf and give insight to similar items that may exist at higher and lower levels of abstraction. In addition, relationships such as PeerOf and CanAlsoBe are defined to show similar weaknesses that the user may want to explore.
Relevant to the view "CISQ Quality Measures (2020)" (CWE-1305)
Nature
Type
ID
Name
ChildOf
Class - a weakness that is described in a very abstract fashion, typically independent of any specific language or technology. More specific than a Pillar Weakness, but more general than a Base Weakness. Class level weaknesses typically describe issues in terms of 1 or 2 of the following dimensions: behavior, property, and resource.
This table shows the weaknesses and high level categories that are related to this weakness. These relationships are defined as ChildOf, ParentOf, MemberOf and give insight to similar items that may exist at higher and lower levels of abstraction. In addition, relationships such as PeerOf and CanAlsoBe are defined to show similar weaknesses that the user may want to explore.
Relevant to the view "CISQ Data Protection Measures" (CWE-1340)
Nature
Type
ID
Name
ChildOf
Class - a weakness that is described in a very abstract fashion, typically independent of any specific language or technology. More specific than a Pillar Weakness, but more general than a Base Weakness. Class level weaknesses typically describe issues in terms of 1 or 2 of the following dimensions: behavior, property, and resource.
The different Modes of Introduction provide information about how and when this weakness may be introduced. The Phase identifies a point in the life cycle at which introduction may occur, while the Note provides a typical scenario related to introduction during the given phase.
Phase
Note
Implementation
Applicable Platforms
This listing shows possible areas for which the given weakness could appear. These may be for specific named Languages, Operating Systems, Architectures, Paradigms, Technologies, or a class of such platforms. The platform is listed along with how frequently the given weakness appears for that instance.
Languages
Java (Undetermined Prevalence)
Common Consequences
This table specifies different individual consequences associated with the weakness. The Scope identifies the application security area that is violated, while the Impact describes the negative technical impact that arises if an adversary succeeds in exploiting this weakness. The Likelihood provides information about how likely the specific consequence is expected to be seen relative to the other consequences in the list. For example, there may be high likelihood that a weakness will be exploited to achieve a certain impact, but a low likelihood that it will be exploited to achieve a different impact.
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
Phase: Implementation
Remove the use of static variables used between servlets. If this cannot be avoided, use synchronized access for these variables.
Detection Methods
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.)
Effectiveness: High
Memberships
This MemberOf Relationships table shows additional CWE Categories and Views that reference this weakness as a member. This information is often useful in understanding where a weakness fits within the context of external information sources.
Nature
Type
ID
Name
MemberOf
Category - a CWE entry that contains a set of other entries that share a common characteristic.
View - a subset of CWE entries that provides a way of examining CWE content. The two main view structures are Slices (flat lists) and Graphs (containing relationships between entries).
(this CWE ID could be used to map to real-world vulnerabilities)
Reason: Acceptable-Use
Rationale:
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.
Comments:
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.
Taxonomy Mappings
Mapped Taxonomy Name
Node ID
Fit
Mapped Node Name
The CERT Oracle Secure Coding Standard for Java (2011)
VNA00-J
Ensure visibility when accessing shared primitive variables
The CERT Oracle Secure Coding Standard for Java (2011)
VNA02-J
Ensure that compound operations on shared variables are atomic
Description
This table shows the weaknesses and high level categories that are related to this weakness. These relationships are defined as ChildOf, ParentOf, MemberOf and give insight to similar items that may exist at higher and lower levels of abstraction. In addition, relationships such as PeerOf and CanAlsoBe are defined to show similar weaknesses that the user may want to explore.
Relevant to the view "CISQ Quality Measures (2020)" (CWE-1305)
