CWE-805 使用不正确的长度值访问缓冲区

Buffer Access with Incorrect Length Value

结构: Simple

Abstraction: Base

状态: Incomplete

被利用可能性: High

基本描述

The software uses a sequential operation to read or write a buffer, but it uses an incorrect length value that causes it to access memory that is outside of the bounds of the buffer.

扩展描述

When the length value exceeds the size of the destination, a buffer overflow could occur.

相关缺陷

• cwe_Nature: ChildOf cwe_CWE_ID: 119 cwe_View_ID: 1000 cwe_Ordinal: Primary

• cwe_Nature: ChildOf cwe_CWE_ID: 119 cwe_View_ID: 699 cwe_Ordinal: Primary

适用平台

Language: [{'cwe_Name': 'C', 'cwe_Prevalence': 'Often'}, {'cwe_Name': 'C++', 'cwe_Prevalence': 'Often'}, {'cwe_Class': 'Assembly', '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 manual methods to diagnose the underlying problem.

DM-9 Manual Analysis

可能的缓解方案

MIT-3 Requirements

策略: 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, many languages that perform their own memory management, such as Java and Perl, are not subject to buffer overflows. Other languages, such as Ada and C#, typically provide overflow protection, but the protection can be disabled by the programmer. Be wary that a language's interface to native code may still be subject to overflows, even if the language itself is theoretically safe.

MIT-4.1 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. Examples include the Safe C String Library (SafeStr) by Messier and Viega [REF-57], and the Strsafe.h library from Microsoft [REF-56]. These libraries provide safer versions of overflow-prone string-handling functions.

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-9 Implementation

策略: Consider adhering to the following rules when allocating and managing an application's memory:

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.

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-59] [REF-57].

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

This example takes an IP address from a user, verifies that it is well formed and then looks up the hostname and copies it into a buffer.

bad C

This function allocates a buffer of 64 bytes to store the hostname under the assumption that the maximum length value of hostname is 64 bytes, however there is no guarantee that the hostname will not be larger than 64 bytes. If an attacker specifies an address which resolves to a very large hostname, then we may overwrite sensitive data or even relinquish control flow to the attacker.

Note that this example also contains an unchecked return value (CWE-252) that can lead to a NULL pointer dereference (CWE-476).

例

In the following example, it is possible to request that memcpy move a much larger segment of memory than assumed:

bad C

If returnChunkSize() happens to encounter an error it will return -1. Notice that the return value is not checked before the memcpy operation (CWE-252), so -1 can be passed as the size argument to memcpy() (CWE-805). Because memcpy() assumes that the value is unsigned, it will be interpreted as MAXINT-1 (CWE-195), and therefore will copy far more memory than is likely available to the destination buffer (CWE-787, CWE-788).

例

In the following example, the source character string is copied to the dest character string using the method strncpy.

bad C

However, in the call to strncpy the source character string is used within the sizeof call to determine the number of characters to copy. This will create a buffer overflow as the size of the source character string is greater than the dest character string. The dest character string should be used within the sizeof call to ensure that the correct number of characters are copied, as shown below.

good C

例

In this example, the method outputFilenameToLog outputs a filename to a log file. The method arguments include a pointer to a character string containing the file name and an integer for the number of characters in the string. The filename is copied to a buffer where the buffer size is set to a maximum size for inputs to the log file. The method then calls another method to save the contents of the buffer to the log file.

bad C

However, in this case the string copy method, strncpy, mistakenly uses the length method argument to determine the number of characters to copy rather than using the size of the local character string, buf. This can lead to a buffer overflow if the number of characters contained in character string pointed to by filename is larger then the number of characters allowed for the local character string. The string copy method should use the buf character string within a sizeof call to ensure that only characters up to the size of the buf array are copied to avoid a buffer overflow, as shown below.

good C

分析过的案例

分类映射

相关攻击模式

• CAPEC-100
• CAPEC-256

引用

• REF-7 Writing Secure Code

• REF-58 Address Space Layout Randomization in Windows Vista

• REF-59 Limiting buffer overflows with ExecShield

• REF-60 PaX

• REF-741 Top 25 Series - Rank 12 - Buffer Access with Incorrect Length Value

• REF-57 Safe C String Library v1.0.3

• REF-56 Using the Strsafe.h Functions

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

• REF-76 Least Privilege

• REF-64 Position Independent Executables (PIE)