CWE

Common Weakness Enumeration

A Community-Developed List of Software Weakness Types

CWE/SANS Top 25 Most Dangerous Software Errors
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ID

CWE-772: Missing Release of Resource after Effective Lifetime

Weakness ID: 772
Abstraction: Base
Status: Incomplete
Presentation Filter:
+ Description

Description Summary

The software does not release a resource after its effective lifetime has ended, i.e., after the resource is no longer needed.

Extended Description

When a resource is not released after use, it can allow attackers to cause a denial of service.

+ Time of Introduction
  • Architecture and Design
  • Implementation
+ Applicable Platforms

Architectural Paradigms

Mobile Application

+ Common Consequences
ScopeEffect
Availability

Technical Impact: DoS: resource consumption (other)

When allocating resources without limits, an attacker could prevent all other processes from accessing the same type of resource.

+ Likelihood of Exploit

Medium to High

+ Demonstrative Examples

Example 1

The following code attempts to process a file by reading it in line by line until the end has been reached.

(Bad Code)
Example Language: Java 
private void processFile(string fName)
{
BufferReader in = new BufferReader(new FileReader(fName));
String line;
while ((line = in.ReadLine()) != null)
{
processLine(line);
}
}

The problem with the above code is that it never closes the file handle it opens. The Finalize() method for BufferReader eventually calls Close(), but there is no guarantee as to how long it will take before the Finalize() method is invoked. In fact, there is no guarantee that Finalize() will ever be invoked. In a busy environment, this can result in the VM using up all of its available file handles.

Example 2

The following code attempts to open a new connection to a database, process the results returned by the database, and close the allocated SqlConnection object.

(Bad Code)
Example Language: C# 
SqlConnection conn = new SqlConnection(connString);
SqlCommand cmd = new SqlCommand(queryString);
cmd.Connection = conn;
conn.Open();
SqlDataReader rdr = cmd.ExecuteReader();
HarvestResults(rdr);
conn.Connection.Close();

The problem with the above code is that if an exception occurs while executing the SQL or processing the results, the SqlConnection object is not closed. If this happens often enough, the database will run out of available cursors and not be able to execute any more SQL queries.

Example 3

The following method never closes the file handle it opens. The Finalize() method for StreamReader eventually calls Close(), but there is no guarantee as to how long it will take before the Finalize() method is invoked. In fact, there is no guarantee that Finalize() will ever be invoked. In a busy environment, this can result in the VM using up all of its available file handles.

(Bad Code)
Example Language: Java 
private void processFile(string fName) {
StreamWriter sw = new StreamWriter(fName);
string line;
while ((line = sr.ReadLine()) != null){
processLine(line);
}
}

Example 4

This code attempts to open a connection to a database and catches any exceptions that may occur.

(Bad Code)
Example Language: Java 
try {
Connection con = DriverManager.getConnection(some_connection_string);
}
catch ( Exception e ) {
log( e );
}

If an exception occurs after establishing the database connection and before the same connection closes, the pool of database connections may become exhausted. If the number of available connections is exceeded, other users cannot access this resource, effectively denying access to the application.

Example 5

Under normal conditions the following C# code executes a database query, processes the results returned by the database, and closes the allocated SqlConnection object. But if an exception occurs while executing the SQL or processing the results, the SqlConnection object is not closed. If this happens often enough, the database will run out of available cursors and not be able to execute any more SQL queries.

(Bad Code)
Example Language: C# 
...
SqlConnection conn = new SqlConnection(connString);
SqlCommand cmd = new SqlCommand(queryString);
cmd.Connection = conn;
conn.Open();
SqlDataReader rdr = cmd.ExecuteReader();
HarvestResults(rdr);
conn.Connection.Close();
...

Example 6

The following C function does not close the file handle it opens if an error occurs. If the process is long-lived, the process can run out of file handles.

(Bad Code)
Example Language:
int decodeFile(char* fName) {
char buf[BUF_SZ];
FILE* f = fopen(fName, "r");
if (!f) {
printf("cannot open %s\n", fName);
return DECODE_FAIL;
}
else {
while (fgets(buf, BUF_SZ, f)) {
if (!checkChecksum(buf)) {
return DECODE_FAIL;
}
else {
decodeBlock(buf);
}
}
}
fclose(f);
return DECODE_SUCCESS;
}
+ Observed Examples
ReferenceDescription
Chain: anti-virus product encounters a malformed file but returns from a function without closing a file descriptor (CWE-775) leading to file descriptor consumption (CWE-400) and failed scans.
Sockets not properly closed when attacker repeatedly connects and disconnects from server.
Does not shut down named pipe connections if malformed data is sent.
Chain: memory leak (CWE-404) leads to resource exhaustion.
Product allows exhaustion of file descriptors when processing a large number of TCP packets.
Port scan triggers CPU consumption with processes that attempt to read data from closed sockets.
Product allows resource exhaustion via a large number of calls that do not complete a 3-way handshake.
Return values of file/socket operations not checked, allowing resultant consumption of file descriptors.
+ Potential Mitigations

Phase: Requirements

Strategy: Language Selection

Use a language that does not allow this weakness to occur or provides constructs that make this weakness easier to avoid.

For example, languages such as Java, Ruby, and Lisp perform automatic garbage collection that releases memory for objects that have been deallocated.

Phase: Implementation

It is good practice to be responsible for freeing all resources you allocate and to be consistent with how and where you free resources in a function. If you allocate resources that you intend to free upon completion of the function, you must be sure to free the resources at all exit points for that function including error conditions.

Phases: Operation; Architecture and Design

Strategy: Limit Resource Consumption

Use resource-limiting settings provided by the operating system or environment. For example, when managing system resources in POSIX, setrlimit() can be used to set limits for certain types of resources, and getrlimit() can determine how many resources are available. However, these functions are not available on all operating systems.

When the current levels get close to the maximum that is defined for the application (see CWE-770), then limit the allocation of further resources to privileged users; alternately, begin releasing resources for less-privileged users. While this mitigation may protect the system from attack, it will not necessarily stop attackers from adversely impacting other users.

Ensure that the application performs the appropriate error checks and error handling in case resources become unavailable (CWE-703).

+ Relationships
NatureTypeIDNameView(s) this relationship pertains toView(s)
ChildOfWeakness BaseWeakness Base400Uncontrolled Resource Consumption ('Resource Exhaustion')
Research Concepts1000
ChildOfWeakness BaseWeakness Base404Improper Resource Shutdown or Release
Development Concepts (primary)699
Research Concepts (primary)1000
ChildOfCategoryCategory8082010 Top 25 - Weaknesses On the Cusp
Weaknesses in the 2010 CWE/SANS Top 25 Most Dangerous Programming Errors (primary)800
ChildOfCategoryCategory8672011 Top 25 - Weaknesses On the Cusp
Weaknesses in the 2011 CWE/SANS Top 25 Most Dangerous Software Errors (primary)900
ChildOfCategoryCategory882CERT C++ Secure Coding Section 14 - Concurrency (CON)
Weaknesses Addressed by the CERT C++ Secure Coding Standard (primary)868
ChildOfCategoryCategory982SFP Secondary Cluster: Failure to Release Resource
Software Fault Pattern (SFP) Clusters (primary)888
ParentOfWeakness BaseWeakness Base401Improper Release of Memory Before Removing Last Reference ('Memory Leak')
Research Concepts (primary)1000
ParentOfWeakness VariantWeakness Variant775Missing Release of File Descriptor or Handle after Effective Lifetime
Research Concepts (primary)1000
MemberOfViewView884CWE Cross-section
CWE Cross-section (primary)884
CanFollowWeakness BaseWeakness Base911Improper Update of Reference Count
Research Concepts1000
+ Theoretical Notes

Vulnerability theory is largely about how behaviors and resources interact. "Resource exhaustion" can be regarded as either a consequence or an attack, depending on the perspective. This entry is an attempt to reflect one of the underlying weaknesses that enable these attacks (or consequences) to take place.

+ Taxonomy Mappings
Mapped Taxonomy NameNode IDFitMapped Node Name
CERT C++ Secure CodingCON02-CPPUse lock classes for mutex management
Software Fault PatternsSFP14Failure to release resource
+ Maintenance Notes

"Resource exhaustion" (CWE-400) is currently treated as a weakness, although it is more like a category of weaknesses that all have the same type of consequence. While this entry treats CWE-400 as a parent in view 1000, the relationship is probably more appropriately described as a chain.

+ Content History
Submissions
Submission DateSubmitterOrganizationSource
2009-05-13Internal CWE Team
Modifications
Modification DateModifierOrganizationSource
2010-02-16CWE Content TeamMITREInternal
updated Demonstrative_Examples, Potential_Mitigations, Relationships
2010-04-05CWE Content TeamMITREInternal
updated Potential_Mitigations
2010-06-21CWE Content TeamMITREInternal
updated Potential_Mitigations
2011-06-01CWE Content TeamMITREInternal
updated Common_Consequences, Relationships, Taxonomy_Mappings
2011-06-27CWE Content TeamMITREInternal
updated Observed_Examples, Related_Attack_Patterns, Relationships
2011-09-13CWE Content TeamMITREInternal
updated Relationships, Taxonomy_Mappings
2012-05-11CWE Content TeamMITREInternal
updated Demonstrative_Examples, Relationships, Taxonomy_Mappings
2012-10-30CWE Content TeamMITREInternal
updated Potential_Mitigations
2013-02-21CWE Content TeamMITREInternal
updated Relationships
2014-02-18CWE Content TeamMITREInternal
updated Applicable_Platforms, Demonstrative_Examples
2014-07-30CWE Content TeamMITREInternal
updated Relationships, Taxonomy_Mappings
2017-01-19CWE Content TeamMITREInternal
updated Relationships

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Page Last Updated: May 05, 2017