Buffer Copy without Checking Size of Input ('Classic Buffer Overflow')
Weakness ID: 120 (Weakness Base)Status: Incomplete
+ Description

Description Summary

The program copies an input buffer to an output buffer without verifying that the size of the input buffer is less than the size of the output buffer, leading to a buffer overflow.

Extended Description

A buffer overflow condition exists when a program attempts to put more data in a buffer than it can hold, or when a program attempts to put data in a memory area outside of the boundaries of a buffer. The simplest type of error, and the most common cause of buffer overflows, is the "classic" case in which the program copies the buffer without checking its length at all. Other variants exist, but the existence of a classic overflow strongly suggests that the programmer is not considering even the most basic of security protections.

+ Alternate Terms
buffer overrun:

Some prominent vendors and researchers use the term "buffer overrun," but most people use "buffer overflow."

Unbounded Transfer
+ Terminology Notes

Many issues that are now called "buffer overflows" are substantively different than the "classic" overflow, including entirely different bug types that rely on overflow exploit techniques, such as integer signedness errors, integer overflows, and format string bugs. This imprecise terminology can make it difficult to determine which variant is being reported.

+ Time of Introduction
  • Implementation
+ Applicable Platforms





+ Common Consequences

Technical Impact: 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. This can often be used to subvert any other security service.


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

+ 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 out-of-bounds memory operations. 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.

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.

+ Demonstrative Examples

Example 1

The following code asks the user to enter their last name and then attempts to store the value entered in the last_name array.

(Bad Code)
Example Language:
char last_name[20];
printf ("Enter your last name: ");
scanf ("%s", last_name);

The problem with the code above is that it does not check the size of the name entered by the user. If the user enters "Very_very_long_last_name" which is 24 characters long, then a buffer overflow will occur since the array can only hold 20 characters total.

Example 2

The following code attempts to create a local copy of a buffer to perform some manipulations to the data.

(Bad Code)
Example Language:
void manipulate_string(char* string){
char buf[24];
strcpy(buf, string);

However, the programmer does not ensure that the size of the data pointed to by string will fit in the local buffer and blindly copies the data with the potentially dangerous strcpy() function. This may result in a buffer overflow condition if an attacker can influence the contents of the string parameter.

Example 3

The excerpt below calls the gets() function in C, which is inherently unsafe.

(Bad Code)
Example Language:
char buf[24];
printf("Please enter your name and press <Enter>\n");

However, the programmer uses the function gets() which is inherently unsafe because it blindly copies all input from STDIN to the buffer without checking size. This allows the user to provide a string that is larger than the buffer size, resulting in an overflow condition.

+ Observed Examples
CVE-2000-1094buffer overflow using command with long argument
CVE-1999-0046buffer overflow in local program using long environment variable
CVE-2002-1337buffer overflow in comment characters, when product increments a counter for a ">" but does not decrement for "<"
CVE-2003-0595By replacing a valid cookie value with an extremely long string of characters, an attacker may overflow the application's buffers.
CVE-2001-0191By replacing a valid cookie value with an extremely long string of characters, an attacker may overflow the application's buffers.
+ Potential Mitigations

Phase: Requirements

Strategy: Language Selection

Use a language with features that can automatically mitigate or eliminate buffer overflows.

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.

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.

Examples include the Safe C String Library (SafeStr) by Messier and Viega, and the Strsafe.h library from Microsoft. These libraries provide safer versions of overflow-prone string-handling functions. This is not a complete solution, since many buffer overflows are not related to strings.

Phase: Build and Compilation

Run or compile your 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.

This is not necessarily a complete solution, since these mechanisms can only detect certain types of overflows. In addition, a buffer overflow attack can still cause 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 you are not in danger of writing past the allocated space.

  • If necessary, truncate all input strings to a reasonable length before passing them to the copy and concatenation functions.

Phase: Operation

Use a feature like Address Space Layout Randomization (ASLR). This is not a complete solution. However, it forces the attacker to guess an unknown value that changes every program execution.

Phase: Operation

Use a CPU and operating system that offers Data Execution Protection (NX) or its equivalent. 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.

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.

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).

+ Weakness Ordinalities
(where the weakness is typically related to the presence of some other weaknesses)
(where the weakness exists independent of other weaknesses)
+ Relationships
NatureTypeIDNameView(s) this relationship pertains toView(s)
ChildOfWeakness ClassWeakness Class20Improper Input Validation
Seven Pernicious Kingdoms (primary)700
ChildOfWeakness ClassWeakness Class119Failure to Constrain Operations within the Bounds of a Memory Buffer
Development Concepts (primary)699
Research Concepts (primary)1000
ChildOfCategoryCategory633Weaknesses that Affect Memory
Resource-specific Weaknesses (primary)631
ChildOfCategoryCategory722OWASP Top Ten 2004 Category A1 - Unvalidated Input
Weaknesses in OWASP Top Ten (2004)711
ChildOfCategoryCategory726OWASP Top Ten 2004 Category A5 - Buffer Overflows
Weaknesses in OWASP Top Ten (2004) (primary)711
ChildOfCategoryCategory741CERT C Secure Coding Section 07 - Characters and Strings (STR)
Weaknesses Addressed by the CERT C Secure Coding Standard (primary)734
ChildOfCategoryCategory8022010 Top 25 - Risky Resource Management
Weaknesses in the 2010 CWE/SANS Top 25 Most Dangerous Programming Errors (primary)800
CanPrecedeWeakness BaseWeakness Base123Write-what-where Condition
Research Concepts1000
ParentOfWeakness VariantWeakness Variant785Use of Path Manipulation Function without Maximum-sized Buffer
Development Concepts (primary)699
Research Concepts1000
CanFollowWeakness BaseWeakness Base170Improper Null Termination
Research Concepts1000
CanFollowWeakness BaseWeakness Base231Improper Handling of Extra Values
Research Concepts1000
CanFollowWeakness BaseWeakness Base242Use of Inherently Dangerous Function
Research Concepts1000
CanFollowWeakness BaseWeakness Base242Use of Inherently Dangerous Function
Research Concepts1000
CanFollowWeakness BaseWeakness Base416Use After Free
Research Concepts1000
CanFollowWeakness BaseWeakness Base456Missing Initialization
Research Concepts1000
PeerOfWeakness BaseWeakness Base124Buffer Underwrite ('Buffer Underflow')
Research Concepts1000
CanAlsoBeWeakness VariantWeakness Variant196Unsigned to Signed Conversion Error
Research Concepts1000
+ Relationship Notes

At the code level, stack-based and heap-based overflows do not differ significantly, so there usually is not a need to distinguish them. From the attacker perspective, they can be quite different, since different techniques are required to exploit them.

+ Affected Resources
  • Memory
+ Functional Areas
  • Memory Management
+ Causal Nature


+ Taxonomy Mappings
Mapped Taxonomy NameNode IDFitMapped Node Name
PLOVERUnbounded Transfer ('classic overflow')
7 Pernicious KingdomsBuffer Overflow
CLASPBuffer overflow
OWASP Top Ten 2004A1CWE More SpecificUnvalidated Input
OWASP Top Ten 2004A5CWE More SpecificBuffer Overflows
CERT C Secure CodingSTR35-CDo not copy data from an unbounded source to a fixed-length array
WASC7Buffer Overflow
+ Related Attack Patterns
CAPEC-IDAttack Pattern Name
(CAPEC Version: 1.4)
8Buffer Overflow in an API Call
9Buffer Overflow in Local Command-Line Utilities
10Buffer Overflow via Environment Variables
14Client-side Injection-induced Buffer Overflow
24Filter Failure through Buffer Overflow
92Forced Integer Overflow
42MIME Conversion
45Buffer Overflow via Symbolic Links
100Overflow Buffers
46Overflow Variables and Tags
47Buffer Overflow via Parameter Expansion
67String Format Overflow in syslog()
123Buffer Attacks
+ White Box Definitions

A weakness where the code path includes a Buffer Write Operation such that:

1. the expected size of the buffer is greater than the actual size of the buffer where expected size is equal to the sum of the size of the data item and the position in the buffer

Where Buffer Write Operation is a statement that writes a data item of a certain size into a buffer at a certain position and at a certain index

+ References
[REF-11] M. Howard and D. LeBlanc. "Writing Secure Code". Chapter 5, "Public Enemy #1: The Buffer Overrun" Page 127. 2nd Edition. Microsoft. 2002.
[REF-17] Michael Howard, David LeBlanc and John Viega. "24 Deadly Sins of Software Security". "Sin 5: Buffer Overruns." Page 89. McGraw-Hill. 2010.
Microsoft. "Using the Strsafe.h Functions". <http://msdn.microsoft.com/en-us/library/ms647466.aspx>.
Matt Messier and John Viega. "Safe C String Library v1.0.3". <http://www.zork.org/safestr/>.
Michael Howard. "Address Space Layout Randomization in Windows Vista". <http://blogs.msdn.com/michael_howard/archive/2006/05/26/address-space-layout-randomization-in-windows-vista.aspx>.
Arjan van de Ven. "Limiting buffer overflows with ExecShield". <http://www.redhat.com/magazine/009jul05/features/execshield/>.
"PaX". <http://en.wikipedia.org/wiki/PaX>.
+ Content History
Submission DateSubmitterOrganizationSource
PLOVERExternally Mined
Modification DateModifierOrganizationSource
2008-07-01Eric DalciCigitalExternal
updated Time of Introduction
2008-08-01KDM AnalyticsExternal
added/updated white box definitions
Suggested OWASP Top Ten 2004 mapping
2008-09-08CWE Content TeamMITREInternal
updated Alternate Terms, Applicable Platforms, Common Consequences, Relationships, Observed Example, Other Notes, Taxonomy Mappings, Weakness Ordinalities
2008-10-10CWE Content TeamMITREInternal
Changed name and description to more clearly emphasize the "classic" nature of the overflow.
2008-10-14CWE Content TeamMITREInternal
updated Alternate Terms, Description, Name, Other Notes, Terminology Notes
2008-11-24CWE Content TeamMITREInternal
updated Other Notes, Relationships, Taxonomy Mappings
2009-01-12CWE Content TeamMITREInternal
updated Common Consequences, Other Notes, Potential Mitigations, References, Relationship Notes, Relationships
2009-07-27CWE Content TeamMITREInternal
updated Other Notes, Potential Mitigations, Relationships
2009-10-29CWE Content TeamMITREInternal
updated Common Consequences, Relationships
Previous Entry Names
Change DatePrevious Entry Name
2008-10-14Unbounded Transfer ('Classic Buffer Overflow')