CWE-806: Buffer Access Using Size of Source Buffer

Description

The product uses the size of a source buffer when reading from or writing to a destination buffer, which may cause it to access memory that is outside of the bounds of the buffer.

When the size of the destination is smaller than the size of the source, a buffer overflow could occur.

Potential Impact

Availability

Modify Memory, DoS: Crash, Exit, or Restart, DoS: Resource Consumption (CPU)

Integrity, Confidentiality, Availability

Read Memory, Modify Memory, Execute Unauthorized Code or Commands

Access Control

Bypass Protection Mechanism

Demonstrative Examples

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

Bad

...char source[21] = "the character string";char dest[12];strncpy(dest, source, sizeof(source)-1);...

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

...char source[21] = "the character string";char dest[12];strncpy(dest, source, sizeof(dest)-1);...

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

#define LOG_INPUT_SIZE 40
                     
                     // saves the file name to a log file
                     int outputFilenameToLog(char *filename, int length) {
                        int success;
                           
                           // buffer with size set to maximum size for input to log file
                           char buf[LOG_INPUT_SIZE];
                           
                           // copy filename to buffer
                           strncpy(buf, filename, length);
                           
                           // save to log file
                           success = saveToLogFile(buf);
                           return success;
                     }

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

...
                     // copy filename to buffer
                     strncpy(buf, filename, sizeof(buf)-1);...

Mitigations & Prevention

Architecture and Design

Use an abstraction library to abstract away risky APIs. Examples include the Safe C String Library (SafeStr) by Viega, and the Strsafe.h library from Microsoft. This is not a complete solution, since many buffer overflows are not related to strings.

OperationBuild and Compilation Defense in Depth

Use automatic buffer overflow detection mechanisms that are offered by certain compilers or compiler extensions. Examples include: the Microsoft Visual Studio /GS flag, Fedora/Red Hat FORTIFY_SOURCE GCC flag, StackGuard, and ProPolice, which provide various mechanisms including canary-based detection and range/index checking. D3-SFCV (Stack Frame Canary Validation) from D3FEND [REF-1334] discusses canary-based detection in detail.

Implementation

Programmers should adhere to the following rules when allocating and managing their applications memory: Double check that your buffer is as large as you specify. When using functions that accept a number of bytes to copy, such as strncpy(), be aware that if the destination buffer size is equal to the source buffer size, it may not NULL-terminate the string. Check buffer boundaries if calling this function in a loop and make sure there is no danger of writing past the allocated space. Truncate a

OperationBuild and Compilation Defense in Depth

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]. Imported modules may be similarly realigned if their default memory addresses conflict with other mo

Operation Defense in Depth

Use a CPU and operating system that offers Data Execution Protection (using hardware NX or XD bits) or the equivalent techniques that simulate this feature in software, such as PaX [REF-60] [REF-61]. These techniques ensure that any instruction executed is exclusively at a memory address that is part of the code segment. For more information on these techniques see D3-PSEP (Process Segment Execution Prevention) from D3FEND [REF-1336].

Build and CompilationOperation

Most mitigating technologies at the compiler or OS level to date address only a subset of buffer overflow problems and rarely provide complete protection against even that subset. It is good practice to implement strategies to increase the workload of an attacker, such as leaving the attacker to guess an unknown value that changes every program execution.

Detection Methods

Automated Static Analysis High — Automated static analysis, commonly referred to as Static Application Security Testing (SAST), can find some instances of this weakness by analyzing source code (or binary/compiled code) without having to execute it. Typically, this is done by building a model of data flow and control flow, then sea
Automated Dynamic Analysis Moderate — Use tools that are integrated during compilation to insert runtime error-checking mechanisms related to memory safety errors, such as AddressSanitizer (ASan) for C/C++ [REF-1518].

Frequently Asked Questions

What is CWE-806?

CWE-806 (Buffer Access Using Size of Source Buffer) is a software weakness identified by MITRE's Common Weakness Enumeration. It is classified as a Variant-level weakness. The product uses the size of a source buffer when reading from or writing to a destination buffer, which may cause it to access memory that is outside of the bounds of the buffer.

How can CWE-806 be exploited?

Attackers can exploit CWE-806 (Buffer Access Using Size of Source Buffer) to modify memory, dos: crash, exit, or restart, dos: resource consumption (cpu). This weakness is typically introduced during the Implementation phase of software development.

How do I prevent CWE-806?

Key mitigations include: Use an abstraction library to abstract away risky APIs. Examples include the Safe C String Library (SafeStr) by Viega, and the Strsafe.h library from Microsoft. This is not a complete solution, since

What is the severity of CWE-806?

CWE-806 is classified as a Variant-level weakness (Low-Medium abstraction). Its actual severity depends on the specific context and how the weakness manifests in your application.