Incorrect Calculation of Buffer Size
Weakness ID: 131 (Weakness Base)Status: Draft
+ Description

Description Summary

The software does not correctly calculate the size to be used when allocating a buffer, which could lead to a buffer overflow.
+ Time of Introduction
  • Implementation
+ Applicable Platforms




+ Common Consequences

If the incorrect calculation is used in the context of memory allocation, then the software may create a buffer that is smaller or larger than expected. If the allocated buffer is smaller than expected, this could lead to an out-of-bounds read or write (CWE-119), possibly causing a crash, allowing arbitrary code execution, or exposing sensitive data.

+ Likelihood of Exploit

High to Very High

+ Detection Methods

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 potential errors in buffer calculations. This can make it difficult for users to determine which warnings should be investigated first. For example, an analysis tool might report buffer overflows that originate from command line arguments in a program that is not expected to run with setuid or other special privileges.

Effectiveness: High

Detection techniques for buffer-related errors are more mature than for most other weakness types.

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.

Manual Analysis

Manual analysis can be useful for finding this weakness, but it might not achieve desired code coverage within limited time constraints. This becomes difficult for weaknesses that must be considered for all inputs, since the attack surface can be too large.

Black Box

Evidence of 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.

Effectiveness: Moderate

Without visibility into the code, black box methods may not be able to sufficiently distinguish this weakness from others, requiring follow-up manual methods to diagnose the underlying problem.

+ Demonstrative Examples

Example 1

The following code allocates memory for a maximum number of widgets. It then gets a user-specified number of widgets, making sure that the user does not request too many. It then initializes the elements of the array using InitializeWidget(). Because the number of widgets can vary for each request, the code inserts a NULL pointer to signify the location of the last widget.

(Bad Code)
Example Language:
int i;
unsigned int numWidgets;
Widget **WidgetList;

numWidgets = GetUntrustedSizeValue();
if ((numWidgets == 0) || (numWidgets > MAX_NUM_WIDGETS)) {
ExitError("Incorrect number of widgets requested!");
WidgetList = (Widget **)malloc(numWidgets * sizeof(Widget *));
printf("WidgetList ptr=%p\n", WidgetList);
for(i=0; i<numWidgets; i++) {
WidgetList[i] = InitializeWidget();
WidgetList[numWidgets] = NULL;

However, this code contains an off-by-one calculation error. It allocates exactly enough space to contain the specified number of widgets, but it does not include the space for the NULL pointer. As a result, the allocated buffer is smaller than it is supposed to be. So if the user ever requests MAX_NUM_WIDGETS, there is an off-by-one buffer overflow (CWE-193) when the NULL is assigned. Depending on the environment and compilation settings, this could cause memory corruption.

Example 2

This example applies an encoding procedure to an input string and stores it into a buffer.

(Bad Code)
Example Language:
char * copy_input(char *user_supplied_string){
int i, dst_index;
char *dst_buf = (char*)malloc(4*sizeof(char) * MAX_SIZE);
if ( MAX_SIZE <= strlen(user_supplied_string) ){
die("user string too long, die evil hacker!");
dst_index = 0;
for ( i = 0; i < strlen; i++ ){
if( '&' == user_supplied_string[i] ){
dst_buf[dst_index++] = '&';
dst_buf[dst_index++] = 'a';
dst_buf[dst_index++] = 'm';
dst_buf[dst_index++] = 'p';
dst_buf[dst_index++] = ';';
else if ('<' == user_supplied_string[i] ){
/* encode to &lt; */
else dst_buf[dst_index++] = user_supplied_string[i];
return dst_buf;

The programmer attempts to encode the ampersand character in the user-controlled string, however the length of the string is validated before the encoding procedure is applied. Furthermore, the programmer assumes encoding expansion will only expand a given character by a factor of 4, while the encoding of the ampersand expands by 5. As a result, when the encoding procedure expands the string it is possible to overflow the destination buffer if the attacker provides a string of many ampersands.

Example 3

The following code is intended to read an incoming packet from a socket and extract one or more headers.

(Bad Code)
Example Language:
DataPacket *packet;
int numHeaders;
PacketHeader *headers;

ReadPacket(packet, sock);
numHeaders =packet->headers;

if (numHeaders > 100) {
ExitError("too many headers!");
headers = malloc(numHeaders * sizeof(PacketHeader);
ParsePacketHeaders(packet, headers);

The code performs a check to make sure that the packet does not contain too many headers. However, numHeaders is defined as a signed int, so it could be negative. If the incoming packet specifies a value such as -3, then the malloc calculation will generate a negative number (say, -300 if each header can be a maximum of 100 bytes). When this result is provided to malloc(), it is first converted to a size_t type. This conversion then produces a large value such as 4294966996, which may cause malloc() to fail or to allocate an extremely large amount of memory (CWE-195). With the appropriate negative numbers, an attacker could trick malloc() into using a very small positive number, which then allocates a buffer that is much smaller than expected, potentially leading to a buffer overflow.

Example 4

The following code attempts to save three different identification numbers into an array. The array is allocated from memory using a call to malloc().

(Bad Code)
Example Language:
int *id_sequence;

/* Allocate space for an array of three ids. */

id_sequence = (int*) malloc(3);
if (id_sequence == NULL) exit(1);

/* Populate the id array. */

id_sequence[0] = 13579;
id_sequence[1] = 24680;
id_sequence[2] = 97531;

The problem with the code above is the value of the size parameter used during the malloc() call. It uses a value of '3' which by definition results in a buffer of three bytes to be created. However the intention was to create a buffer that holds three ints, and in C, each int requires 4 bytes worth of memory, so an array of 12 bytes is needed, 4 bytes for each int. Executing the above code could result in a buffer overflow as 12 bytes of data is being saved into 3 bytes worth of allocated space. The overflow would occur during the assignment of id_sequence[0] and would continue with the assignment of id_sequence[1] and id_sequence[2].

The malloc() call could have used '3*sizeof(int)' as the value for the size parameter in order to allocate the correct amount of space required to store the three ints.

+ Observed Examples
CVE-2004-1363substitution overflow: buffer overflow using environment variables that are expanded after the length check is performed
CVE-2004-0747substitution overflow: buffer overflow using expansion of environment variables
CVE-2005-2103substitution overflow: buffer overflow using a large number of substitution strings
CVE-2005-3120transformation overflow: product adds extra escape characters to incoming data, but does not account for them in the buffer length
CVE-2003-0899transformation overflow: buffer overflow when expanding ">" to "&gt;", etc.
CVE-2001-0334expansion overflow: buffer overflow using wildcards
CVE-2001-0248expansion overflow: long pathname + glob = overflow
CVE-2001-0249expansion overflow: long pathname + glob = overflow
CVE-2002-0184special characters in argument are not properly expanded
CVE-2004-0434small length value leads to heap overflow
CVE-2002-1347multiple variants
CVE-2005-0490needs closer investigation, but probably expansion-based
CVE-2004-0940needs closer investigation, but probably expansion-based
CVE-2008-0599Chain: Language interpreter calculates wrong buffer size (CWE-131) by using "size = ptr ? X : Y" instead of "size = (ptr ? X : Y)" expression.
+ Potential Mitigations

Phase: Implementation

If you allocate a buffer for the purpose of transforming, converting, or encoding an input, make sure that you allocate enough memory to handle the largest possible encoding. For example, in a routine that converts "&" characters to "&amp;" for HTML entity encoding, you will need an output buffer that is at least 5 times as large as the input buffer.

Phase: Implementation

Understand your programming language's underlying representation and how it interacts with numeric calculation (CWE-681). Pay close attention to byte size discrepancies, precision, signed/unsigned distinctions, truncation, conversion and casting between types, "not-a-number" calculations, and how your language handles numbers that are too large or too small for its underlying representation.

Phase: Implementation

Strategy: Input Validation

Perform input validation on any numeric input by ensuring that it is within the expected range. Enforce that the input meets both the minimum and maximum requirements for the expected range.

Phase: Implementation

When processing structured incoming data containing a size field followed by raw data, ensure that you identify and resolve any inconsistencies between the size field and the actual size of the data (CWE-130).

Phase: Implementation

When allocating memory that uses sentinels to mark the end of a data structure - such as NUL bytes in strings - make sure you also include the sentinel in your calculation of the total amount of memory that must be allocated.

Phase: Implementation

Use sizeof() on the appropriate data type to avoid CWE-467.

Phase: Implementation

Use the appropriate type for the desired action. For example, in C/C++, only use unsigned types for values that could never be negative, such as height, width, or other numbers related to quantity. This will simplify your sanity checks and will reduce surprises related to unexpected casting.

Phase: Architecture and Design

Strategy: Libraries or Frameworks

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

Use libraries or frameworks that make it easier to handle numbers without unexpected consequences, or buffer allocation routines that automatically track buffer size.

Examples include safe integer handling packages such as SafeInt (C++) or IntegerLib (C or C++).

+ Relationships
NatureTypeIDNameView(s) this relationship pertains toView(s)
ChildOfWeakness ClassWeakness Class682Incorrect Calculation
Development Concepts (primary)699
Research Concepts (primary)1000
ChildOfCategoryCategory742CERT C Secure Coding Section 08 - Memory Management (MEM)
Weaknesses Addressed by the CERT C Secure Coding Standard (primary)734
ChildOfCategoryCategory8022010 Top 25 - Risky Resource Management
Weaknesses in the 2010 CWE/SANS Top 25 Most Dangerous Programming Errors (primary)800
CanPrecedeWeakness ClassWeakness Class119Failure to Constrain Operations within the Bounds of a Memory Buffer
Development Concepts699
Research Concepts1000
CanFollowWeakness VariantWeakness Variant467Use of sizeof() on a Pointer Type
Research Concepts1000
+ Taxonomy Mappings
Mapped Taxonomy NameNode IDFitMapped Node Name
PLOVEROther length calculation error
CERT C Secure CodingMEM35-CAllocate sufficient memory for an object
+ Related Attack Patterns
CAPEC-IDAttack Pattern Name
(CAPEC Version: 1.4)
100Overflow Buffers
47Buffer Overflow via Parameter Expansion
123Buffer Attacks
+ Maintenance Notes

This is a broad category. Some examples include: (1) simple math errors, (2) incorrectly updating parallel counters, (3) not accounting for size differences when "transforming" one input to another format (e.g. URL canonicalization or other transformation that can generate a result that's larger than the original input, i.e. "expansion").

This level of detail is rarely available in public reports, so it is difficult to find good examples.

This weakness may be a composite or a chain. It also may contain layering or perspective differences.

This issue may be associated with many different types of incorrect calculations (CWE-682), although the integer overflow (CWE-190) is probably the most prevalent. This can be primary to resource consumption problems (CWE-400), including uncontrolled memory allocation (CWE-789). However, its relationship with out-of-bounds buffer access (CWE-119) must also be considered.

+ Content History
Submission DateSubmitterOrganizationSource
PLOVERExternally Mined
Modification DateModifierOrganizationSource
2008-07-01Eric DalciCigitalExternal
updated Potential Mitigations, Time of Introduction
2008-09-08CWE Content TeamMITREInternal
updated Applicable Platforms, Maintenance Notes, Relationships, Taxonomy Mappings, Type
2008-10-14CWE Content TeamMITREInternal
updated Relationships
2008-11-24CWE Content TeamMITREInternal
updated Relationships, Taxonomy Mappings
2009-12-28CWE Content TeamMITREInternal
updated Demonstrative Examples, Likelihood of Exploit, Observed Examples, Potential Mitigations
Previous Entry Names
Change DatePrevious Entry Name
2008-01-30Other Length Calculation Error