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-908: Use of Uninitialized Resource

Weakness ID: 908
Abstraction: Base
Status: Incomplete
Presentation Filter:
+ Description

Description Summary

The software uses a resource that has not been properly initialized.

Extended Description

This can have security implications when the associated resource is expected to have certain properties or values.

+ Time of Introduction
  • Implementation
+ Applicable Platforms

Languages

Language-independent

+ Common Consequences
ScopeEffect
Confidentiality

Technical Impact: 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

Technical Impact: DoS: crash / exit / restart

The uninitialized resource may contain values that cause program flow to change in ways that the programmer did not intend.

+ Likelihood of Exploit

Medium

+ Demonstrative Examples

Example 1

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.

(Bad Code)
Example Language: Java 
private boolean initialized = true;
public void someMethod() {
if (!initialized) {
// perform initialization tasks
...

initialized = true;
}

Example 2

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

(Bad Code)
Example Language: Perl 
$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.

Example 3

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

(Bad Code)
Example Language:
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.

+ Observed Examples
ReferenceDescription
Use of uninitialized memory may allow code execution.
Free of an uninitialized pointer leads to crash and possible code execution.
Product does not clear memory contents when generating an error message, leading to information leak.
Lack of initialization triggers NULL pointer dereference or double-free.
Uninitialized variable leads to code execution in popular desktop application.
chain: Uninitialized variable leads to infinite loop.
chain: Improper initialization leads to memory corruption.
Permission bitmap is not properly initialized, leading to resultant privilege elevation or DoS.
chain: game server can access player data structures before initialization has happened leading to NULL dereference
chain: uninitialized function pointers can be dereferenced allowing code execution
chain: improper initialization of memory can lead to NULL dereference
chain: some unprivileged ioctls do not verify that a structure has been initialized before invocation, leading to NULL dereference
+ Potential Mitigations

Phase: Implementation

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

Phase: Implementation

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

Phase: Implementation

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

Phase: Build and Compilation

Run or compile the software with settings that generate warnings about uninitialized variables or data.

+ Weakness Ordinalities
OrdinalityDescription
Primary
(where the weakness exists independent of other weaknesses)
Resultant
(where the weakness is typically related to the presence of some other weaknesses)
+ Relationships
NatureTypeIDNameView(s) this relationship pertains toView(s)
ChildOfCategoryCategory452Initialization and Cleanup Errors
Development Concepts (primary)699
ChildOfWeakness ClassWeakness Class664Improper Control of a Resource Through its Lifetime
Research Concepts (primary)1000
CanFollowWeakness BaseWeakness Base909Missing Initialization of Resource
Research Concepts1000
+ References
mercy. "Exploiting Uninitialized Data". Jan 2006. < http://www.felinemenace.org/~mercy/papers/UBehavior/UBehavior.zip>.
+ Content History
Submissions
Submission DateSubmitterOrganizationSource
2012-12-21MITREInternal CWE Team
New weakness based on discussion on the CWE research list in December 2012.

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Page Last Updated: January 18, 2017