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: C
int i;
unsigned int numWidgets;
Widget **WidgetList;
numWidgets = GetUntrustedSizeValue();
if ((numWidgets == 0) || (numWidgets > MAX_NUM_WIDGETS))
{
ExitError("Incorrect number of widgets requested!");
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.
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.
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.
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: C
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.
Chain: 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 "&" 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
Use a feature like Address Space Layout Randomization (ASLR) [R.131.3] [R.131.5].
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.
[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.
Maintenance Notes
This is a broad category. Some examples include:
simple math errors,
incorrectly updating parallel counters,
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.
Description
