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CWE-806: Buffer Access Using Size of Source Buffer

Weakness ID: 806
Abstraction: Variant
Structure: Simple
View customized information:
+ Description
The software 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.
+ Extended Description
When the size of the destination is smaller than the size of the source, a buffer overflow could occur.
+ Relationships
Section HelpThis table shows the weaknesses and high level categories that are related to this weakness. These relationships are defined as ChildOf, ParentOf, MemberOf and give insight to similar items that may exist at higher and lower levels of abstraction. In addition, relationships such as PeerOf and CanAlsoBe are defined to show similar weaknesses that the user may want to explore.
+ Relevant to the view "Research Concepts" (CWE-1000)
ChildOfBaseBase - a weakness that is still mostly independent of a resource or technology, but with sufficient details to provide specific methods for detection and prevention. Base level weaknesses typically describe issues in terms of 2 or 3 of the following dimensions: behavior, property, technology, language, and resource.805Buffer Access with Incorrect Length Value
+ Modes Of Introduction
Section HelpThe different Modes of Introduction provide information about how and when this weakness may be introduced. The Phase identifies a point in the life cycle at which introduction may occur, while the Note provides a typical scenario related to introduction during the given phase.
+ Applicable Platforms
Section HelpThis listing shows possible areas for which the given weakness could appear. These may be for specific named Languages, Operating Systems, Architectures, Paradigms, Technologies, or a class of such platforms. The platform is listed along with how frequently the given weakness appears for that instance.


C (Sometimes Prevalent)

C++ (Sometimes Prevalent)

+ Common Consequences
Section HelpThis table specifies different individual consequences associated with the weakness. The Scope identifies the application security area that is violated, while the Impact describes the negative technical impact that arises if an adversary succeeds in exploiting this weakness. The Likelihood provides information about how likely the specific consequence is expected to be seen relative to the other consequences in the list. For example, there may be high likelihood that a weakness will be exploited to achieve a certain impact, but a low likelihood that it will be exploited to achieve a different impact.

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

Buffer overflows generally lead to crashes. Other attacks leading to lack of availability are possible, including putting the program into an infinite loop.

Technical Impact: Read Memory; Modify Memory; Execute Unauthorized Code or Commands

Buffer overflows often can be used to execute arbitrary code, which is usually outside the scope of a program's implicit security policy.
Access Control

Technical Impact: Bypass Protection Mechanism

When the consequence is arbitrary code execution, this can often be used to subvert any other security service.
+ Demonstrative Examples

Example 1

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

(bad code)
Example Language:
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 code)
Example Language:
char source[21] = "the character string";
char dest[12];
strncpy(dest, source, sizeof(dest)-1);

Example 2

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 code)
Example Language:
#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 code)
Example Language:
// copy filename to buffer
strncpy(buf, filename, sizeof(buf)-1);
+ Potential Mitigations

Phase: 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.

Phase: Build and Compilation

Use automatic buffer overflow detection mechanisms that are offered by certain compilers or compiler extensions. Examples include StackGuard, ProPolice and the Microsoft Visual Studio /GS flag. This is not necessarily a complete solution, since these canary-based mechanisms only detect certain types of overflows. In addition, the result is still a denial of service, since the typical response is to exit the application.

Phase: 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 all input strings to a reasonable length before passing them to the copy and concatenation functions

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) [REF-58] [REF-60] and Position-Independent Executables (PIE) [REF-64].

Effectiveness: Defense in Depth

Note: 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 [REF-60] [REF-61].

Effectiveness: Defense in Depth

Note: 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.

Phases: Build and Compilation; Operation

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.
+ Weakness Ordinalities
(where the weakness is typically related to the presence of some other weaknesses)
(where the weakness exists independent of other weaknesses)
+ Affected Resources
  • Memory
+ References
[REF-56] Microsoft. "Using the Strsafe.h Functions". <>.
[REF-57] Matt Messier and John Viega. "Safe C String Library v1.0.3". <>.
[REF-58] Michael Howard. "Address Space Layout Randomization in Windows Vista". <>.
[REF-59] Arjan van de Ven. "Limiting buffer overflows with ExecShield". <>.
[REF-61] Microsoft. "Understanding DEP as a mitigation technology part 1". <>.
[REF-64] Grant Murphy. "Position Independent Executables (PIE)". Red Hat. 2012-11-28. <>.
+ Content History
+ Submissions
Submission DateSubmitterOrganization
2010-01-15CWE Content TeamMITRE
+ Modifications
Modification DateModifierOrganization
2011-03-29CWE Content TeamMITRE
updated Demonstrative_Examples
2011-06-01CWE Content TeamMITRE
updated Common_Consequences
2012-05-11CWE Content TeamMITRE
updated Potential_Mitigations, References
2014-02-18CWE Content TeamMITRE
updated Potential_Mitigations, References
2017-11-08CWE Content TeamMITRE
updated Causal_Nature, Demonstrative_Examples, Likelihood_of_Exploit, References
2020-02-24CWE Content TeamMITRE
updated Relationships
2020-06-25CWE Content TeamMITRE
updated Common_Consequences
2021-07-20CWE Content TeamMITRE
updated Potential_Mitigations
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Page Last Updated: October 13, 2022