The product does not check the return value from a method or function, which can prevent it from detecting unexpected states and conditions.
Two common programmer assumptions are "this function call can never fail" and "it doesn't matter if this function call fails". If an attacker can force the function to fail or otherwise return a value that is not expected, then the subsequent program logic could lead to a vulnerability, because the product is not in a state that the programmer assumes. For example, if the program calls a function to drop privileges but does not check the return code to ensure that privileges were successfully dropped, then the program will continue to operate with the higher privileges.

Primary

Many functions will return some value about the success of their actions. This will alert the program whether or not to handle any errors caused by that function.

Implementation

Low

Availability
Integrity
Unexpected State
DoS: Crash, Exit, or Restart
An unexpected return value could place the system in a state that could lead to a crash or other unintended behaviors.

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

Implementation
Check the results of all functions that return a value and verify that the value is expected.
High
Checking the return value of the function will typically be sufficient, however beware of race conditions (CWE-362) in a concurrent environment.

Implementation
For any pointers that could have been modified or provided from a function that can return NULL, check the pointer for NULL before use. When working with a multithreaded or otherwise asynchronous environment, ensure that proper locking APIs are used to lock before the check, and unlock when it has finished [REF-1484].

Implementation
Ensure that you account for all possible return values from the function.

Implementation
When designing a function, make sure you return a value or throw an exception in case of an error.

Consider the following code segment:

char buf[10], cp_buf[10];fgets(buf, 10, stdin);strcpy(cp_buf, buf);

The programmer expects that when fgets() returns, buf will contain a null-terminated string of length 9 or less. But if an I/O error occurs, fgets() will not null-terminate buf. Furthermore, if the end of the file is reached before any characters are read, fgets() returns without writing anything to buf. In both of these situations, fgets() signals that something unusual has happened by returning NULL, but in this code, the warning will not be noticed. The lack of a null terminator in buf can result in a buffer overflow in the subsequent call to strcpy().

In the following example, it is possible to request that memcpy move a much larger segment of memory than assumed:

int returnChunkSize(void *) {

/* if chunk info is valid, return the size of usable memory,

* else, return -1 to indicate an error

*/
...
}int main() {...memcpy(destBuf, srcBuf, (returnChunkSize(destBuf)-1));...}

If returnChunkSize() happens to encounter an error it will return -1. Notice that the return value is not checked before the memcpy operation (CWE-252), so -1 can be passed as the size argument to memcpy() (CWE-805). Because memcpy() assumes that the value is unsigned, it will be interpreted as MAXINT-1 (CWE-195), and therefore will copy far more memory than is likely available to the destination buffer (CWE-787, CWE-788).

The following code does not check to see if memory allocation succeeded before attempting to use the pointer returned by malloc().

buf = (char*) malloc(req_size);strncpy(buf, xfer, req_size);

The traditional defense of this coding error is: "If my program runs out of memory, it will fail. It doesn't matter whether I handle the error or allow the program to die with a segmentation fault when it tries to dereference the null pointer." This argument ignores three important considerations:

Depending upon the type and size of the application, it may be possible to free memory that is being used elsewhere so that execution can continue.

It is impossible for the program to perform a graceful exit if required. If the program is performing an atomic operation, it can leave the system in an inconsistent state.

The programmer has lost the opportunity to record diagnostic information. Did the call to malloc() fail because req_size was too large or because there were too many requests being handled at the same time? Or was it caused by a memory leak that has built up over time? Without handling the error, there is no way to know.

The following examples read a file into a byte array.

char[] byteArray = new char[1024];for (IEnumerator i=users.GetEnumerator(); i.MoveNext() ;i.Current()) {String userName = (String) i.Current();String pFileName = PFILE_ROOT + "/" + userName;StreamReader sr = new StreamReader(pFileName);sr.Read(byteArray,0,1024);//the file is always 1k bytessr.Close();processPFile(userName, byteArray);}

FileInputStream fis;byte[] byteArray = new byte[1024];for (Iterator i=users.iterator(); i.hasNext();) {
String userName = (String) i.next();String pFileName = PFILE_ROOT + "/" + userName;FileInputStream fis = new FileInputStream(pFileName);fis.read(byteArray); // the file is always 1k bytesfis.close();processPFile(userName, byteArray);

The code loops through a set of users, reading a private data file for each user. The programmer assumes that the files are always 1 kilobyte in size and therefore ignores the return value from Read(). If an attacker can create a smaller file, the program will recycle the remainder of the data from the previous user and treat it as though it belongs to the attacker.

The following code does not check to see if the string returned by getParameter() is null before calling the member function compareTo(), potentially causing a NULL dereference.

String itemName = request.getParameter(ITEM_NAME);if (itemName.compareTo(IMPORTANT_ITEM) == 0) {...}...

The following code does not check to see if the string returned by the Item property is null before calling the member function Equals(), potentially causing a NULL dereference.

String itemName = request.Item(ITEM_NAME);if (itemName.Equals(IMPORTANT_ITEM)) {...}...

The traditional defense of this coding error is: "I know the requested value will always exist because.... If it does not exist, the program cannot perform the desired behavior so it doesn't matter whether I handle the error or allow the program to die dereferencing a null value." But attackers are skilled at finding unexpected paths through programs, particularly when exceptions are involved.

The following code shows a system property that is set to null and later dereferenced by a programmer who mistakenly assumes it will always be defined.

System.clearProperty("os.name");...String os = System.getProperty("os.name");if (os.equalsIgnoreCase("Windows 95")) System.out.println("Not supported");

The traditional defense of this coding error is: "I know the requested value will always exist because.... If it does not exist, the program cannot perform the desired behavior so it doesn't matter whether I handle the error or allow the program to die dereferencing a null value." But attackers are skilled at finding unexpected paths through programs, particularly when exceptions are involved.

The following VB.NET code does not check to make sure that it has read 50 bytes from myfile.txt. This can cause DoDangerousOperation() to operate on an unexpected value.

Dim MyFile As New FileStream("myfile.txt", FileMode.Open, FileAccess.Read, FileShare.Read)Dim MyArray(50) As ByteMyFile.Read(MyArray, 0, 50)DoDangerousOperation(MyArray(20))

In .NET, it is not uncommon for programmers to misunderstand Read() and related methods that are part of many System.IO classes. The stream and reader classes do not consider it to be unusual or exceptional if only a small amount of data becomes available. These classes simply add the small amount of data to the return buffer, and set the return value to the number of bytes or characters read. There is no guarantee that the amount of data returned is equal to the amount of data requested.

It is not uncommon for Java programmers to misunderstand read() and related methods that are part of many java.io classes. Most errors and unusual events in Java result in an exception being thrown. But the stream and reader classes do not consider it unusual or exceptional if only a small amount of data becomes available. These classes simply add the small amount of data to the return buffer, and set the return value to the number of bytes or characters read. There is no guarantee that the amount of data returned is equal to the amount of data requested. This behavior makes it important for programmers to examine the return value from read() and other IO methods to ensure that they receive the amount of data they expect.

This example takes an IP address from a user, verifies that it is well formed and then looks up the hostname and copies it into a buffer.

void host_lookup(char *user_supplied_addr){
struct hostent *hp;in_addr_t *addr;char hostname[64];in_addr_t inet_addr(const char *cp);

/*routine that ensures user_supplied_addr is in the right format for conversion */

validate_addr_form(user_supplied_addr);addr = inet_addr(user_supplied_addr);hp = gethostbyaddr( addr, sizeof(struct in_addr), AF_INET);strcpy(hostname, hp->h_name);
}

If an attacker provides an address that appears to be well-formed, but the address does not resolve to a hostname, then the call to gethostbyaddr() will return NULL. Since the code does not check the return value from gethostbyaddr (CWE-252), a NULL pointer dereference
(CWE-476) would then occur in the call to strcpy().
Note that this code is also vulnerable to a buffer overflow (CWE-119).

The following function attempts to acquire a lock in order to perform operations on a shared resource.

void f(pthread_mutex_t *mutex) {
pthread_mutex_lock(mutex);

/* access shared resource */

pthread_mutex_unlock(mutex);
}

However, the code does not check the value returned by pthread_mutex_lock() for errors. If pthread_mutex_lock() cannot acquire the mutex for any reason, the function may introduce a race condition into the program and result in undefined behavior.
In order to avoid data races, correctly written programs must check the result of thread synchronization functions and appropriately handle all errors, either by attempting to recover from them or reporting them to higher levels.

int f(pthread_mutex_t *mutex) {
int result;
result = pthread_mutex_lock(mutex);if (0 != result)return result;

/* access shared resource */

return pthread_mutex_unlock(mutex);
}

CVE-2020-17533
Chain: unchecked return value (CWE-252) of some functions for policy enforcement leads to authorization bypass (CWE-862)
https://www.cve.org/CVERecord?id=CVE-2020-17533

CVE-2020-6078
Chain: The return value of a function returning a pointer is not checked for success (CWE-252) resulting in the later use of an uninitialized variable (CWE-456) and a null pointer dereference (CWE-476)
https://www.cve.org/CVERecord?id=CVE-2020-6078

CVE-2019-15900
Chain: sscanf() call is used to check if a username and group exists, but the return value of sscanf() call is not checked (CWE-252), causing an uninitialized variable to be checked (CWE-457), returning success to allow authorization bypass for executing a privileged (CWE-863).
https://www.cve.org/CVERecord?id=CVE-2019-15900

CVE-2007-3798
Unchecked return value leads to resultant integer overflow and code execution.
https://www.cve.org/CVERecord?id=CVE-2007-3798

CVE-2006-4447
Program does not check return value when invoking functions to drop privileges, which could leave users with higher privileges than expected by forcing those functions to fail.
https://www.cve.org/CVERecord?id=CVE-2006-4447

CVE-2006-2916
Program does not check return value when invoking functions to drop privileges, which could leave users with higher privileges than expected by forcing those functions to fail.
https://www.cve.org/CVERecord?id=CVE-2006-2916

CVE-2008-5183
chain: unchecked return value can lead to NULL dereference
https://www.cve.org/CVERecord?id=CVE-2008-5183

CVE-2010-0211
chain: unchecked return value (CWE-252) leads to free of invalid, uninitialized pointer (CWE-824).
https://www.cve.org/CVERecord?id=CVE-2010-0211

CVE-2017-6964
Linux-based device mapper encryption program does not check the return value of setuid and setgid allowing attackers to execute code with unintended privileges.
https://www.cve.org/CVERecord?id=CVE-2017-6964

CVE-2002-1372
Chain: Return values of file/socket operations are not checked (CWE-252), allowing resultant consumption of file descriptors (CWE-772).
https://www.cve.org/CVERecord?id=CVE-2002-1372

Unchecked Return Value

Ignored function return value

A7
Improper Error Handling
CWE More Specific

ERR33-C
Detect and handle standard library errors
Imprecise

POS54-C
Detect and handle POSIX library errors
Imprecise

EXP00-J
Do not ignore values returned by methods

EXP32-PL
Do not ignore function return values
Exact

SFP4
Unchecked Status Condition

ASCSM-CWE-252-resource

ASCRM-CWE-252-data

ASCRM-CWE-252-resource

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

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

CWE Content Team
MITRE
2008-11-24
updated Relationships, Taxonomy_Mappings

CWE Content Team
MITRE
2009-01-12
updated Background_Details, Demonstrative_Examples, Description, Observed_Examples, Other_Notes, Potential_Mitigations

CWE Content Team
MITRE
2009-03-10
updated Relationships

CWE Content Team
MITRE
2009-05-27
updated Demonstrative_Examples

CWE Content Team
MITRE
2009-07-27
updated Demonstrative_Examples

CWE Content Team
MITRE
2009-12-28
updated Common_Consequences, Demonstrative_Examples, References

CWE Content Team
MITRE
2010-02-16
updated Demonstrative_Examples, Potential_Mitigations, References

CWE Content Team
MITRE
2010-04-05
updated Demonstrative_Examples

CWE Content Team
MITRE
2010-06-21
updated Demonstrative_Examples, References

CWE Content Team
MITRE
2010-09-27
updated Observed_Examples

CWE Content Team
MITRE
2010-12-13
updated Demonstrative_Examples

CWE Content Team
MITRE
2011-06-01
updated Common_Consequences, Demonstrative_Examples, Relationships, Taxonomy_Mappings

CWE Content Team
MITRE
2011-06-27
updated Common_Consequences

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

CWE Content Team
MITRE
2012-05-11
updated Common_Consequences, References, Relationships

CWE Content Team
MITRE
2014-06-23
updated Demonstrative_Examples, Potential_Mitigations

CWE Content Team
MITRE
2014-07-30
updated Demonstrative_Examples, Relationships, Taxonomy_Mappings

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

CWE Content Team
MITRE
2018-03-27
updated References

CWE Content Team
MITRE
2019-01-03
updated References, Relationships, Taxonomy_Mappings

CWE Content Team
MITRE
2019-06-20
updated Relationships

CWE Content Team
MITRE
2020-02-24
updated References

CWE Content Team
MITRE
2020-06-25
updated Observed_Examples

CWE Content Team
MITRE
2020-08-20
updated Relationships

CWE Content Team
MITRE
2021-03-15
updated Demonstrative_Examples, Observed_Examples, Relationships, Weakness_Ordinalities

CWE Content Team
MITRE
2021-07-20
updated Observed_Examples

CWE Content Team
MITRE
2023-01-31
updated Description

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

CWE Content Team
MITRE
2023-06-29
updated Mapping_Notes, Relationships

CWE Content Team
MITRE
2023-10-26
updated Observed_Examples

CWE Content Team
MITRE
2025-09-09
4.18
2025-09-09
updated Potential_Mitigations, References

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

Martin Sebor
Cisco Systems, Inc.
2010-04-30
Provided Demonstrative Example and suggested CERT reference