CWE-131 缓冲区大小计算不正确

Incorrect Calculation of Buffer Size

结构: Simple

Abstraction: Base

状态: Draft

被利用可能性: High

基本描述

The software does not correctly calculate the size to be used when allocating a buffer, which could lead to a buffer overflow.

相关缺陷

• cwe_Nature: ChildOf cwe_CWE_ID: 682 cwe_View_ID: 1000 cwe_Ordinal: Primary

• cwe_Nature: ChildOf cwe_CWE_ID: 682 cwe_View_ID: 1003 cwe_Ordinal: Primary

• cwe_Nature: ChildOf cwe_CWE_ID: 682 cwe_View_ID: 699 cwe_Ordinal: Primary

• cwe_Nature: CanPrecede cwe_CWE_ID: 119 cwe_View_ID: 1000

• cwe_Nature: CanPrecede cwe_CWE_ID: 119 cwe_View_ID: 699

适用平台

Language: [{'cwe_Name': 'C', 'cwe_Prevalence': 'Undetermined'}, {'cwe_Name': 'C++', 'cwe_Prevalence': 'Undetermined'}]

常见的影响

检测方法

DM-1 Automated Static Analysis

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

DM-2 Automated Dynamic Analysis

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.

DM-9 Manual Analysis

DM-7 Manual Analysis

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

Automated Static Analysis - Binary or Bytecode

Manual Static Analysis - Binary or Bytecode

Manual Static Analysis - Source Code

Automated Static Analysis - Source Code

Architecture or Design Review

可能的缓解方案

Implementation

策略:

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.

MIT-36 Implementation

策略:

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. [REF-7] Also be careful to account for 32-bit, 64-bit, and other potential differences that may affect the numeric representation.

MIT-8 Implementation

策略: 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.

MIT-15 Architecture and Design

策略:

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.

Implementation

策略:

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).

Implementation

策略:

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.

MIT-13 Implementation

策略:

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

Implementation

策略:

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

Implementation

策略:

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.

MIT-4 Architecture and Design

策略: 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++). [REF-106]

MIT-10 Build and Compilation

策略: 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.

MIT-11 Operation

策略: 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].

MIT-12 Operation

策略: Environment Hardening

Use a CPU and operating system that offers Data Execution Protection (NX) or its equivalent [REF-61] [REF-60].

MIT-26 Implementation

策略: 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.

MIT-17 ['Architecture and Design', 'Operation']

策略: 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.

MIT-22 ['Architecture and Design', 'Operation']

策略: 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.

示例代码

例

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 C

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.

例

The following image processing code allocates a table for images.

bad C

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).

例

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

bad C

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.

例

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

bad C

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.

例

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 C

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.

分析过的案例

Notes

Maintenance

Maintenance

分类映射

相关攻击模式

• CAPEC-100
• CAPEC-47

引用

• REF-106 SafeInt

• REF-107 Top 25 Series - Rank 18 - Incorrect Calculation of Buffer Size

• REF-58 Address Space Layout Randomization in Windows Vista

• REF-61 Understanding DEP as a mitigation technology part 1

• REF-60 PaX

• REF-76 Least Privilege

• REF-7 Writing Secure Code

• REF-44 24 Deadly Sins of Software Security

• REF-62 The Art of Software Security Assessment

• REF-64 Position Independent Executables (PIE)