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CWE-131: Incorrect Calculation of Buffer Size

 
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

Languages

C

C++

+ Common Consequences
ScopeEffect
Integrity
Availability
Confidentiality

Technical Impact: DoS: crash / exit / restart; Execute unauthorized code or commands; Read memory; Modify memory

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.

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.

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.

Manual Analysis

This weakness can be detected using tools and techniques that require manual (human) analysis, such as penetration testing, threat modeling, and interactive tools that allow the tester to record and modify an active session.

Specifically, manual static analysis is useful for evaluating the correctness of allocation calculations. This can be useful for detecting overflow conditions (CWE-190) or similar weaknesses that might have serious security impacts on the program.

Effectiveness: High

These may be more effective than strictly automated techniques. This is especially the case with weaknesses that are related to design and business rules.

+ 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;
showWidgets(WidgetList);

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

The following image processing code allocates a table for images.

(Bad Code)
Example Language:
img_t table_ptr; /*struct containing img data, 10kB each*/
int num_imgs;
...
num_imgs = get_num_imgs();
table_ptr = (img_t*)malloc(sizeof(img_t)*num_imgs);
...

This code intends to allocate a table of size num_imgs, however as num_imgs grows large, the calculation determining the size of the list will eventually overflow (CWE-190). This will result in a very small list to be allocated instead. If the subsequent code operates on the list as if it were num_imgs long, it may result in many types of out-of-bounds problems (CWE-119).

Example 3

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(user_supplied_string); 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 4

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;

sock=AcceptSocketConnection();
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 5

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
ReferenceDescription
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

When allocating a buffer for the purpose of transforming, converting, or encoding an input, allocate enough memory to handle the largest possible encoding. For example, in a routine that converts "&" characters to "&amp;" for HTML entity encoding, the output buffer needs to be at least 5 times as large as the input buffer.

Phase: Implementation

Understand the 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 the language handles numbers that are too large or too small for its underlying representation. [R.131.7]

Also be careful to account for 32-bit, 64-bit, and other potential differences that may affect the numeric 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: 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.

Phase: Implementation

When processing structured incoming data containing a size field followed by raw data, 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

Replace unbounded copy functions with analogous functions that support length arguments, such as strcpy with strncpy. Create these if they are not available.

Effectiveness: Moderate

This approach is still susceptible to calculation errors, including issues such as off-by-one errors (CWE-193) and incorrectly calculating buffer lengths (CWE-131).

Additionally, this only addresses potential overflow issues. Resource consumption / exhaustion issues are still possible.

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 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++). [R.131.1]

Phase: Build and Compilation

Strategy: Compilation or Build Hardening

Run or compile the software using features or extensions that automatically provide a protection mechanism that mitigates or eliminates buffer overflows.

For example, certain compilers and extensions provide automatic buffer overflow detection mechanisms that are built into the compiled code. Examples include the Microsoft Visual Studio /GS flag, Fedora/Red Hat FORTIFY_SOURCE GCC flag, StackGuard, and ProPolice.

Effectiveness: Defense in Depth

This is not necessarily a complete solution, since these mechanisms can only detect certain types of overflows. In addition, an attack could still cause a denial of service, since the typical response is to exit the application.

Phase: Operation

Strategy: 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) [R.131.3] [R.131.5] and Position-Independent Executables (PIE) [R.131.10].

Effectiveness: Defense in Depth

This is not a complete solution. However, it forces the attacker to guess an unknown value that changes every program execution. In addition, an attack could still cause a denial of service, since the typical response is to exit the application.

Phase: Operation

Strategy: Environment Hardening

Use a CPU and operating system that offers Data Execution Protection (NX) or its equivalent [R.131.4] [R.131.5].

Effectiveness: 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.

Phase: Implementation

Strategy: Compilation or Build Hardening

Examine compiler warnings closely and eliminate problems with potential security implications, such as signed / unsigned mismatch in memory operations, or use of uninitialized variables. Even if the weakness is rarely exploitable, a single failure may lead to the compromise of the entire system.

Phases: Architecture and Design; Operation

Strategy: Environment Hardening

Run your code using the lowest privileges that are required to accomplish the necessary tasks [R.131.6]. 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.

Phases: Architecture and Design; Operation

Strategy: 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.

Effectiveness: 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.

+ 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
ChildOfCategoryCategory8652011 Top 25 - Risky Resource Management
Weaknesses in the 2011 CWE/SANS Top 25 Most Dangerous Software Errors (primary)900
ChildOfCategoryCategory876CERT C++ Secure Coding Section 08 - Memory Management (MEM)
Weaknesses Addressed by the CERT C++ Secure Coding Standard (primary)868
ChildOfCategoryCategory890SFP Cluster: Memory Access
Software Fault Pattern (SFP) Clusters (primary)888
CanPrecedeWeakness ClassWeakness Class119Improper Restriction of Operations within the Bounds of a Memory Buffer
Development Concepts699
Research Concepts1000
MemberOfViewView884CWE Cross-section
CWE Cross-section (primary)884
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
CERT C++ Secure CodingMEM35-CPPAllocate sufficient memory for an object
+ References
[R.131.1] [REF-18] David LeBlanc and Niels Dekker. "SafeInt". <http://safeint.codeplex.com/>.
[R.131.2] Jason Lam. "Top 25 Series - Rank 18 - Incorrect Calculation of Buffer Size". SANS Software Security Institute. 2010-03-19. <http://software-security.sans.org/blog/2010/03/19/top-25-series-rank-18-incorrect-calculation-of-buffer-size>.
[R.131.3] [REF-22] Michael Howard. "Address Space Layout Randomization in Windows Vista". <http://blogs.msdn.com/michael_howard/archive/2006/05/26/address-space-layout-randomization-in-windows-vista.aspx>.
[R.131.4] [REF-25] Microsoft. "Understanding DEP as a mitigation technology part 1". <http://blogs.technet.com/b/srd/archive/2009/06/12/understanding-dep-as-a-mitigation-technology-part-1.aspx>.
[R.131.5] [REF-29] "PaX". <http://en.wikipedia.org/wiki/PaX>.
[R.131.6] [REF-31] Sean Barnum and Michael Gegick. "Least Privilege". 2005-09-14. <https://buildsecurityin.us-cert.gov/daisy/bsi/articles/knowledge/principles/351.html>.
[R.131.7] [REF-11] M. Howard and D. LeBlanc. "Writing Secure Code". Chapter 20, "Integer Overflows" Page 620. 2nd Edition. Microsoft. 2002.
[R.131.8] [REF-17] Michael Howard, David LeBlanc and John Viega. "24 Deadly Sins of Software Security". "Sin 5: Buffer Overruns." Page 89. McGraw-Hill. 2010.
[R.131.9] [REF-7] Mark Dowd, John McDonald and Justin Schuh. "The Art of Software Security Assessment". Chapter 8, "Incrementing Pointers Incorrectly", Page 401.. 1st Edition. Addison Wesley. 2006.
[R.131.10] [REF-37] Grant Murphy. "Position Independent Executables (PIE)". Red Hat. 2012-11-28. <https://securityblog.redhat.com/2012/11/28/position-independent-executables-pie/>.
+ 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
Submissions
Submission DateSubmitterOrganizationSource
PLOVERExternally Mined
Modifications
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
2010-02-16CWE Content TeamMITREInternal
updated Common_Consequences, Demonstrative_Examples, Detection_Factors, Maintenance_Notes, Potential_Mitigations, Related_Attack_Patterns, Relationships
2010-04-05CWE Content TeamMITREInternal
updated Detection_Factors, Potential_Mitigations, References, Related_Attack_Patterns
2010-06-21CWE Content TeamMITREInternal
updated Common_Consequences, Detection_Factors, Potential_Mitigations, References
2010-09-27CWE Content TeamMITREInternal
updated Potential_Mitigations
2010-12-13CWE Content TeamMITREInternal
updated Potential_Mitigations
2011-03-29CWE Content TeamMITREInternal
updated Maintenance_Notes
2011-06-01CWE Content TeamMITREInternal
updated Common_Consequences
2011-06-27CWE Content TeamMITREInternal
updated Relationships
2011-09-13CWE Content TeamMITREInternal
updated Potential_Mitigations, References, Relationships, Taxonomy_Mappings
2012-05-11CWE Content TeamMITREInternal
updated Demonstrative_Examples, Potential_Mitigations, References, Relationships
2012-10-30CWE Content TeamMITREInternal
updated Potential_Mitigations
2013-02-21CWE Content TeamMITREInternal
updated Demonstrative_Examples
2013-07-17CWE Content TeamMITREInternal
updated References
2014-02-18CWE Content TeamMITREInternal
updated Potential_Mitigations, References
Previous Entry Names
Change DatePrevious Entry Name
2008-01-30Other Length Calculation Error
Page Last Updated: February 18, 2014