The product uses or accesses a resource that has not been initialized.
When a resource has not been properly initialized, the product may behave unexpectedly. This may lead to a crash or invalid memory access, but the consequences vary depending on the type of resource and how it is used within the product.

Primary

Resultant

Implementation

Medium

Confidentiality
Read Memory
Read Application Data
When reusing a resource such as memory or a program variable, the original contents of that resource may not be cleared before it is sent to an untrusted party.

Availability
DoS: Crash, Exit, or Restart
The uninitialized resource may contain values that cause program flow to change in ways that the programmer did not intend.

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
Explicitly initialize the resource before use. If this is performed through an API function or standard procedure, follow all required steps.

Implementation
Pay close attention to complex conditionals that affect initialization, since some branches might not perform the initialization.

Implementation
Avoid race conditions (CWE-362) during initialization routines.

Build and Compilation
Run or compile the product with settings that generate warnings about uninitialized variables or data.

Here, a boolean initiailized field is consulted to ensure that initialization tasks are only completed once. However, the field is mistakenly set to true during static initialization, so the initialization code is never reached.

private boolean initialized = true;public void someMethod() {
if (!initialized) {

// perform initialization tasks
...
initialized = true;
}

The following code intends to limit certain operations to the administrator only.

$username = GetCurrentUser();$state = GetStateData($username);if (defined($state)) {$uid = ExtractUserID($state);}

# do stuff
if ($uid == 0) {DoAdminThings();}

If the application is unable to extract the state information - say, due to a database timeout - then the $uid variable will not be explicitly set by the programmer. This will cause $uid to be regarded as equivalent to "0" in the conditional, allowing the original user to perform administrator actions. Even if the attacker cannot directly influence the state data, unexpected errors could cause incorrect privileges to be assigned to a user just by accident.

The following code intends to concatenate a string to a variable and print the string.

char str[20];strcat(str, "hello world");printf("%s", str);

This might seem innocent enough, but str was not initialized, so it contains random memory. As a result, str[0] might not contain the null terminator, so the copy might start at an offset other than 0. The consequences can vary, depending on the underlying memory.
If a null terminator is found before str[8], then some bytes of random garbage will be printed before the "hello world" string. The memory might contain sensitive information from previous uses, such as a password (which might occur as a result of CWE-14 or CWE-244). In this example, it might not be a big deal, but consider what could happen if large amounts of memory are printed out before the null terminator is found.
If a null terminator isn't found before str[8], then a buffer overflow could occur, since strcat will first look for the null terminator, then copy 12 bytes starting with that location. Alternately, a buffer over-read might occur (CWE-126) if a null terminator isn't found before the end of the memory segment is reached, leading to a segmentation fault and crash.

This example will leave test_string in an
unknown condition when i is the same value as err_val,
because test_string is not initialized
(CWE-456). Depending on where this code segment appears
(e.g. within a function body), test_string might be
random if it is stored on the heap or stack. If the
variable is declared in static memory, it might be zero
or NULL. Compiler optimization might contribute to the
unpredictability of this address.

char *test_string;
if (i != err_val)
{
test_string = "Hello World!";
}
printf("%s", test_string);

When the printf() is reached,
test_string might be an unexpected address, so the
printf might print junk strings (CWE-457).

To fix this code, there are a couple approaches to
making sure that test_string has been properly set once
it reaches the printf().
One solution would be to set test_string to an
acceptable default before the conditional:

char *test_string = "Done at the beginning";
if (i != err_val)
{
test_string = "Hello World!";
}
printf("%s", test_string);

Another solution is to ensure that each
branch of the conditional - including the default/else
branch - could ensure that test_string is set:

char *test_string;
if (i != err_val)
{
test_string = "Hello World!";
}
else {
test_string = "Done on the other side!";
}
printf("%s", test_string);

CVE-2019-9805
Chain: Creation of the packet client occurs before initialization is complete (CWE-696) resulting in a read from uninitialized memory (CWE-908), causing memory corruption.
https://www.cve.org/CVERecord?id=CVE-2019-9805

CVE-2008-4197
Use of uninitialized memory may allow code execution.
https://www.cve.org/CVERecord?id=CVE-2008-4197

CVE-2008-2934
Free of an uninitialized pointer leads to crash and possible code execution.
https://www.cve.org/CVERecord?id=CVE-2008-2934

CVE-2008-0063
Product does not clear memory contents when generating an error message, leading to information leak.
https://www.cve.org/CVERecord?id=CVE-2008-0063

CVE-2008-0062
Lack of initialization triggers NULL pointer dereference or double-free.
https://www.cve.org/CVERecord?id=CVE-2008-0062

CVE-2008-0081
Uninitialized variable leads to code execution in popular desktop application.
https://www.cve.org/CVERecord?id=CVE-2008-0081

CVE-2008-3688
Chain: Uninitialized variable leads to infinite loop.
https://www.cve.org/CVERecord?id=CVE-2008-3688

CVE-2008-3475
Chain: Improper initialization leads to memory corruption.
https://www.cve.org/CVERecord?id=CVE-2008-3475

CVE-2005-1036
Chain: Bypass of access restrictions due to improper authorization (CWE-862) of a user results from an improperly initialized (CWE-909) I/O permission bitmap
https://www.cve.org/CVERecord?id=CVE-2005-1036

CVE-2008-3597
Chain: game server can access player data structures before initialization has happened leading to NULL dereference
https://www.cve.org/CVERecord?id=CVE-2008-3597

CVE-2009-2692
Chain: Use of an unimplemented network socket operation pointing to an uninitialized handler function (CWE-456) causes a crash because of a null pointer dereference (CWE-476)
https://www.cve.org/CVERecord?id=CVE-2009-2692

CVE-2009-0949
Chain: improper initialization of memory can lead to NULL dereference
https://www.cve.org/CVERecord?id=CVE-2009-0949

CVE-2009-3620
Chain: some unprivileged ioctls do not verify that a structure has been initialized before invocation, leading to NULL dereference
https://www.cve.org/CVERecord?id=CVE-2009-3620

EXP33-C
Do not read uninitialized memory
CWE More Abstract

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.

CWE Content Team
MITRE
2012-12-21
2.4
2013-02-21
New weakness based on discussion on the CWE research list in December 2012.

CWE Content Team
MITRE
2017-11-08
updated Taxonomy_Mappings

CWE Content Team
MITRE
2019-01-03
updated Relationships

CWE Content Team
MITRE
2020-02-24
updated Description, Relationships

CWE Content Team
MITRE
2020-08-20
updated Relationships

CWE Content Team
MITRE
2020-12-10
updated Relationships

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

CWE Content Team
MITRE
2023-01-31
updated Description, Potential_Mitigations

CWE Content Team
MITRE
2023-04-27
updated Relationships

CWE Content Team
MITRE
2023-06-29
updated Mapping_Notes

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

CWE Content Team
MITRE
2025-12-11
4.19
2025-12-11
updated Detection_Factors, Observed_Examples