The product uses untrusted input when calculating or using an array index, but the product does not validate or incorrectly validates the index to ensure the index references a valid position within the array.

Resultant
The most common condition situation leading to an out-of-bounds array index is the use of loop index variables as buffer indexes. If the end condition for the loop is subject to a flaw, the index can grow or shrink unbounded, therefore causing a buffer overflow or underflow. Another common situation leading to this condition is the use of a function's return value, or the resulting value of a calculation directly as an index in to a buffer.

out-of-bounds array index

index-out-of-range

array index underflow

Implementation

High

Integrity
Availability
DoS: Crash, Exit, or Restart
Use of an index that is outside the bounds of an array will very likely result in the corruption of relevant memory and perhaps instructions, leading to a crash, if the values are outside of the valid memory area.

Integrity
Modify Memory
If the memory corrupted is data, rather than instructions, the system will continue to function with improper values.

Confidentiality
Integrity
Modify Memory
Read Memory
Use of an index that is outside the bounds of an array can also trigger out-of-bounds read or write operations, or operations on the wrong objects; i.e., "buffer overflows" are not always the result. This may result in the exposure or modification of sensitive data.

Integrity
Confidentiality
Availability
Execute Unauthorized Code or Commands
If the memory accessible by the attacker can be effectively controlled, it may be possible to execute arbitrary code, as with a standard buffer overflow and possibly without the use of large inputs if a precise index can be controlled.

Integrity
Availability
Confidentiality
DoS: Crash, Exit, or Restart
Execute Unauthorized Code or Commands
Read Memory
Modify Memory
A single fault could allow either an overflow (CWE-788) or underflow (CWE-786) of the array index. What happens next will depend on the type of operation being performed out of bounds, but can expose sensitive information, cause a system crash, or possibly lead to arbitrary code execution.

Automated Static Analysis

This weakness can often be detected using automated static analysis tools. Many modern tools use data flow analysis or constraint-based techniques to minimize the number of false positives.
Automated static analysis generally does not account for environmental considerations when reporting out-of-bounds memory operations. This can make it difficult for users to determine which warnings should be investigated first. For example, an analysis tool might report array index errors that originate from command line arguments in a program that is not expected to run with setuid or other special privileges.

High
This is not a perfect solution, since 100% accuracy and coverage are not feasible.

Automated Dynamic Analysis
This weakness can be detected using dynamic tools and techniques that interact with the software using large test suites with many diverse inputs, such as fuzz testing (fuzzing), robustness testing, and fault injection. The software's operation may slow down, but it should not become unstable, crash, or generate incorrect results.

Automated Dynamic Analysis
Use tools that are integrated during
compilation to insert runtime error-checking mechanisms
related to memory safety errors, such as AddressSanitizer
(ASan) for C/C++ [REF-1518].
Moderate
Crafted inputs are necessary to
reach the code containing the error, such as generated
by fuzzers. Also, these tools may reduce performance,
and they only report the error condition - not the
original mistake that led to the
error.

Black Box
Black box methods might not get the needed code coverage within limited time constraints, and a dynamic test might not produce any noticeable side effects even if it is successful.

Architecture and Design
Input Validation
Use an input validation framework such as Struts or the OWASP ESAPI Validation API. Note that using a framework does not automatically address all input validation problems; be mindful of weaknesses that could arise from misusing the framework itself (CWE-1173).

Architecture and Design

For any security checks that are performed on the client side, ensure that these checks are duplicated on the server side, in order to avoid CWE-602. Attackers can bypass the client-side checks by modifying values after the checks have been performed, or by changing the client to remove the client-side checks entirely. Then, these modified values would be submitted to the server.
Even though client-side checks provide minimal benefits with respect to server-side security, they are still useful. First, they can support intrusion detection. If the server receives input that should have been rejected by the client, then it may be an indication of an attack. Second, client-side error-checking can provide helpful feedback to the user about the expectations for valid input. Third, there may be a reduction in server-side processing time for accidental input errors, although this is typically a small savings.

Requirements
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, Ada allows the programmer to constrain the values of a variable and languages such as Java and Ruby will allow the programmer to handle exceptions when an out-of-bounds index is accessed.

Operation
Build and Compilation
Environment Hardening

Run or compile the software using features or extensions that randomly arrange the positions of a program's executable and libraries in memory. Because this makes the addresses unpredictable, it can prevent an attacker from reliably jumping to exploitable code.
Examples include Address Space Layout Randomization (ASLR) [REF-58] [REF-60] and Position-Independent Executables (PIE) [REF-64]. Imported modules may be similarly realigned if their default memory addresses conflict with other modules, in a process known as "rebasing" (for Windows) and "prelinking" (for Linux) [REF-1332] using randomly generated addresses. ASLR for libraries cannot be used in conjunction with prelink since it would require relocating the libraries at run-time, defeating the whole purpose of prelinking.
For more information on these techniques see D3-SAOR (Segment Address Offset Randomization) from D3FEND [REF-1335].
Defense in Depth
These techniques do not provide a complete solution. For instance, exploits frequently use a bug that discloses memory addresses in order to maximize reliability of code execution [REF-1337]. It has also been shown that a side-channel attack can bypass ASLR [REF-1333].

Operation
Environment Hardening

Use a CPU and operating system that offers Data Execution Protection (using hardware NX or XD bits) or the equivalent techniques that simulate this feature in software, such as PaX [REF-60] [REF-61]. These techniques ensure that any instruction executed is exclusively at a memory address that is part of the code segment.
For more information on these techniques see D3-PSEP (Process Segment Execution Prevention) from D3FEND [REF-1336].

Defense in Depth
This is not a complete solution, since buffer overflows could be used to overwrite nearby variables to modify the software's state in dangerous ways. In addition, it cannot be used in cases in which self-modifying code is required. Finally, an attack could still cause a denial of service, since the typical response is to exit the application.

Implementation
Input Validation

Assume all input is malicious. Use an "accept known good" input validation strategy, i.e., use a list of acceptable inputs that strictly conform to specifications. Reject any input that does not strictly conform to specifications, or transform it into something that does.
When performing input validation, consider all potentially relevant properties, including length, type of input, the full range of acceptable values, missing or extra inputs, syntax, consistency across related fields, and conformance to business rules. As an example of business rule logic, "boat" may be syntactically valid because it only contains alphanumeric characters, but it is not valid if the input is only expected to contain colors such as "red" or "blue."
Do not rely exclusively on looking for malicious or malformed inputs. This is likely to miss at least one undesirable input, especially if the code's environment changes. This can give attackers enough room to bypass the intended validation. However, denylists can be useful for detecting potential attacks or determining which inputs are so malformed that they should be rejected outright.
When accessing a user-controlled array index, use a stringent range of values that are within the target array. Make sure that you do not allow negative values to be used. That is, verify the minimum as well as the maximum of the range of acceptable values.

Implementation
Be especially careful to validate all input when invoking code that crosses language boundaries, such as from an interpreted language to native code. This could create an unexpected interaction between the language boundaries. Ensure that you are not violating any of the expectations of the language with which you are interfacing. For example, even though Java may not be susceptible to buffer overflows, providing a large argument in a call to native code might trigger an overflow.

Architecture and Design
Operation
Environment Hardening
Run your code using the lowest privileges that are required to accomplish the necessary tasks [REF-76]. If possible, create isolated accounts with limited privileges that are only used for a single task. That way, a successful attack will not immediately give the attacker access to the rest of the software or its environment. For example, database applications rarely need to run as the database administrator, especially in day-to-day operations.

Architecture and Design
Operation
Sandbox or Jail

Run the code in a "jail" or similar sandbox environment that enforces strict boundaries between the process and the operating system. This may effectively restrict which files can be accessed in a particular directory or which commands can be executed by the software.
OS-level examples include the Unix chroot jail, AppArmor, and SELinux. In general, managed code may provide some protection. For example, java.io.FilePermission in the Java SecurityManager allows the software to specify restrictions on file operations.
This may not be a feasible solution, and it only limits the impact to the operating system; the rest of the application may still be subject to compromise.
Be careful to avoid CWE-243 and other weaknesses related to jails.

Limited
The effectiveness of this mitigation depends on the prevention capabilities of the specific sandbox or jail being used and might only help to reduce the scope of an attack, such as restricting the attacker to certain system calls or limiting the portion of the file system that can be accessed.

In the code snippet below, an untrusted integer value is used to reference an object in an array.

public String getValue(int index) {return array[index];}

If index is outside of the range of the array, this may result in an ArrayIndexOutOfBounds Exception being raised.

The following example takes a user-supplied value to allocate an array of objects and then operates on the array.

private void buildList ( int untrustedListSize ){if ( 0 > untrustedListSize ){die("Negative value supplied for list size, die evil hacker!");}Widget[] list = new Widget [ untrustedListSize ];list[0] = new Widget();}

This example attempts to build a list from a user-specified value, and even checks to ensure a non-negative value is supplied. If, however, a 0 value is provided, the code will build an array of size 0 and then try to store a new Widget in the first location, causing an exception to be thrown.

In the following code, the method retrieves a value from an array at a specific array index location that is given as an input parameter to the method

int getValueFromArray(int *array, int len, int index) {

int value;

// check that the array index is less than the maximum

// length of the array
if (index < len) {

// get the value at the specified index of the array
value = array[index];
}
// if array index is invalid then output error message

// and return value indicating error
else {printf("Value is: %d\n", array[index]);value = -1;}
return value;
}

However, this method only verifies that the given array index is less than the maximum length of the array but does not check for the minimum value (CWE-839). This will allow a negative value to be accepted as the input array index, which will result in reading data before the beginning of the buffer (CWE-127) and may allow access to sensitive memory. The input array index should be checked to verify that is within the maximum and minimum range required for the array (CWE-129). In this example the if statement should be modified to include a minimum range check, as shown below.

...

// check that the array index is within the correct

// range of values for the array
if (index >= 0 && index < len) {
...

The following example retrieves the sizes of messages for a pop3 mail server. The message sizes are retrieved from a socket that returns in a buffer the message number and the message size, the message number (num) and size (size) are extracted from the buffer and the message size is placed into an array using the message number for the array index.

/* capture the sizes of all messages */
int getsizes(int sock, int count, int *sizes) {
...char buf[BUFFER_SIZE];int ok;int num, size;

// read values from socket and added to sizes array
while ((ok = gen_recv(sock, buf, sizeof(buf))) == 0){

// continue read from socket until buf only contains '.'
if (DOTLINE(buf))break;
else if (sscanf(buf, "%d %d", &num, &size) == 2)sizes[num - 1] = size;

}...

}

In this example the message number retrieved from the buffer could be a value that is outside the allowable range of indices for the array and could possibly be a negative number. Without proper validation of the value to be used for the array index an array overflow could occur and could potentially lead to unauthorized access to memory addresses and system crashes. The value of the array index should be validated to ensure that it is within the allowable range of indices for the array as in the following code.

/* capture the sizes of all messages */
int getsizes(int sock, int count, int *sizes) {
...char buf[BUFFER_SIZE];int ok;int num, size;

// read values from socket and added to sizes array
while ((ok = gen_recv(sock, buf, sizeof(buf))) == 0){

// continue read from socket until buf only contains '.'
if (DOTLINE(buf))break;
else if (sscanf(buf, "%d %d", &num, &size) == 2) {
if (num > 0 && num <= (unsigned)count)sizes[num - 1] = size;
else

/* warn about possible attempt to induce buffer overflow */
report(stderr, "Warning: ignoring bogus data for message sizes returned by server.\n");

}
}...

}

In the following example the method displayProductSummary is called from a Web service servlet to retrieve product summary information for display to the user. The servlet obtains the integer value of the product number from the user and passes it to the displayProductSummary method. The displayProductSummary method passes the integer value of the product number to the getProductSummary method which obtains the product summary from the array object containing the project summaries using the integer value of the product number as the array index.

// Method called from servlet to obtain product information
public String displayProductSummary(int index) {

String productSummary = new String("");
try {String productSummary = getProductSummary(index);

} catch (Exception ex) {...}
return productSummary;
}
public String getProductSummary(int index) {return products[index];}

In this example the integer value used as the array index that is provided by the user may be outside the allowable range of indices for the array which may provide unexpected results or cause the application to fail. The integer value used for the array index should be validated to ensure that it is within the allowable range of indices for the array as in the following code.

// Method called from servlet to obtain product information
public String displayProductSummary(int index) {

String productSummary = new String("");
try {String productSummary = getProductSummary(index);

} catch (Exception ex) {...}
return productSummary;
}
public String getProductSummary(int index) {
String productSummary = "";
if ((index >= 0) && (index < MAX_PRODUCTS)) {productSummary = products[index];}else {System.err.println("index is out of bounds");throw new IndexOutOfBoundsException();}
return productSummary;
}

An alternative in Java would be to use one of the collection objects such as ArrayList that will automatically generate an exception if an attempt is made to access an array index that is out of bounds.

ArrayList productArray = new ArrayList(MAX_PRODUCTS);...try {productSummary = (String) productArray.get(index);} catch (IndexOutOfBoundsException ex) {...}

The following example asks a user for an offset into an array to select an item.

int main (int argc, char **argv) {char *items[] = {"boat", "car", "truck", "train"};int index = GetUntrustedOffset();printf("You selected %s\n", items[index-1]);}

The programmer allows the user to specify which element in the list to select, however an attacker can provide an out-of-bounds offset, resulting in a buffer over-read (CWE-126).

CVE-2005-0369
large ID in packet used as array index
https://www.cve.org/CVERecord?id=CVE-2005-0369

CVE-2001-1009
negative array index as argument to POP LIST command
https://www.cve.org/CVERecord?id=CVE-2001-1009

CVE-2003-0721
Integer signedness error leads to negative array index
https://www.cve.org/CVERecord?id=CVE-2003-0721

CVE-2004-1189
product does not properly track a count and a maximum number, which can lead to resultant array index overflow.
https://www.cve.org/CVERecord?id=CVE-2004-1189

CVE-2007-5756
Chain: device driver for packet-capturing software allows access to an unintended IOCTL with resultant array index error.
https://www.cve.org/CVERecord?id=CVE-2007-5756

CVE-2005-2456
Chain: array index error (CWE-129) leads to deadlock (CWE-833)
https://www.cve.org/CVERecord?id=CVE-2005-2456

Memory Management

Memory

Unchecked array indexing

INDEX - Array index overflow

ARR00-C
Understand how arrays work

ARR30-C
Do not form or use out-of-bounds pointers or array subscripts
CWE More Specific

ARR38-C
Do not add or subtract an integer to a pointer if the resulting value does not refer to a valid array element

INT32-C
Ensure that operations on signed integers do not result in overflow

IDS32-PL
Validate any integer that is used as an array index
Imprecise

ASCSM-CWE-129

SFP8
Faulty Buffer Access

Allowed
This CWE entry is at the Variant 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.

This weakness can precede uncontrolled memory allocation (CWE-789) in languages that automatically expand an array when an index is used that is larger than the size of the array, such as JavaScript.
An improperly validated array index might lead directly to the always-incorrect behavior of "access of array using out-of-bounds index."

CLASP
2006-07-19
Draft 3
2006-07-19

Sean Eidemiller
Cigital
2008-07-01
added/updated demonstrative examples

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

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

CWE Content Team
MITRE
2009-01-12
updated Common_Consequences

CWE Content Team
MITRE
2009-10-29
updated Description, Name, Relationships

CWE Content Team
MITRE
2009-12-28
updated Applicable_Platforms, Common_Consequences, Observed_Examples, Other_Notes, Potential_Mitigations, Theoretical_Notes, Weakness_Ordinalities

CWE Content Team
MITRE
2010-02-16
updated Applicable_Platforms, Demonstrative_Examples, Detection_Factors, Likelihood_of_Exploit, Potential_Mitigations, References, Related_Attack_Patterns, Relationships

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MITRE
2010-04-05
updated Related_Attack_Patterns

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MITRE
2010-06-21
updated Common_Consequences, Potential_Mitigations, References

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MITRE
2010-09-27
updated Potential_Mitigations, Relationship_Notes, Relationships

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MITRE
2010-12-13
updated Demonstrative_Examples, Observed_Examples, Potential_Mitigations

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MITRE
2011-03-29
updated Common_Consequences, Demonstrative_Examples, Weakness_Ordinalities

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MITRE
2011-06-01
updated Common_Consequences

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MITRE
2011-06-27
updated Relationships

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MITRE
2011-09-13
updated Relationships, Taxonomy_Mappings

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MITRE
2012-05-11
updated Demonstrative_Examples, Potential_Mitigations, References, Relationships

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MITRE
2012-10-30
updated Potential_Mitigations

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2014-02-18
updated Potential_Mitigations, References

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MITRE
2014-07-30
updated Relationships, Taxonomy_Mappings

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2015-12-07
updated Relationships

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2017-11-08
updated Causal_Nature, References, Relationships, Taxonomy_Mappings

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2018-03-27
updated References

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2019-01-03
updated References, Relationships, Taxonomy_Mappings

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2019-09-19
updated Potential_Mitigations

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2020-02-24
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2020-06-25
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2020-08-20
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2020-12-10
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2021-03-15
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2022-10-13
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2023-04-27
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2023-06-29
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CWE Content Team
MITRE
2025-09-09
4.18
2025-09-09
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2025-12-11
4.19
2025-12-11
updated Detection_Factors, References, Relationships

Unchecked Array Indexing