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CWE

Buffer Copy without Checking Size of Input ('Classic Buffer Overflow')

ID: 120		Date: (C)2012-05-14 (M)2022-10-10
Type: weakness		Status: INCOMPLETE
Abstraction Type: Base

Description

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 restricting how much is copied. 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.

Likelihood of Exploit: High to Very High

Applicable Platforms
Language: C
Language: C++
Language: Assembly

Time Of Introduction

Implementation

Related Attack Patterns

Common Consequences

Scope	Technical Impact	Notes
Integrity Confidentiality Availability	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.
Availability	DoS: crash / exit / 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.

Detection Methods

Name	Description	Effectiveness
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.	High
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.

Potential Mitigations

Phase	Strategy	Description	Effectiveness	Notes
Requirements	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.
Architecture and Design	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 [R.120.4], and the Strsafe.h library from Microsoft [R.120.3]. 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.
Build and Compilation	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.	Defense in Depth	This is not necessarily a complete solution, since these mechanisms can only detect certain types of overflows. In addition, an attack could still cause a denial of service, since the typical response is to exit the application.
Implementation		Consider adhering to the following rules when allocating and managing an application's memory:
Implementation	Input Validation	Assume all input is malicious. Use an "accept known good" input validation strategy, i.e., use a whitelist of acceptable inputs that strictly conform to specifications. Reject any input that does not strictly conform to specifications, or transform it into something that does. When performing input validation, consider all potentially relevant properties, including length, type of input, the full range of acceptable values, missing or extra inputs, syntax, consistency across related fields, and conformance to business rules. As an example of business rule logic, "boat" may be syntactically valid because it only contains alphanumeric characters, but it is not valid if the input is only expected to contain colors such as "red" or "blue." Do not rely exclusively on looking for malicious or malformed inputs (i.e., do not rely on a blacklist). A blacklist is likely to miss at least one undesirable input, especially if the code's environment changes. This can give attackers enough room to bypass the intended validation. However, blacklists can be useful for detecting potential attacks or determining which inputs are so malformed that they should be rejected outright.
Architecture and Design		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.
Operation	Environment Hardening	Use a feature like Address Space Layout Randomization (ASLR) [R.120.5] [R.120.7].	Defense in Depth	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.
Operation	Environment Hardening	Use a CPU and operating system that offers Data Execution Protection (NX) or its equivalent [R.120.7] [R.120.9].	Defense in Depth	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.
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.
Implementation		Replace unbounded copy functions with analogous functions that support length arguments, such as strcpy with strncpy. Create these if they are not available.	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).
Architecture and Design	Enforcement by Conversion	When the set of acceptable objects, such as filenames or URLs, is limited or known, create a mapping from a set of fixed input values (such as numeric IDs) to the actual filenames or URLs, and reject all other inputs.
Architecture and Design Operation	Environment Hardening	Run your code using the lowest privileges that are required to accomplish the necessary tasks [R.120.10]. 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.
Architecture and Design Operation	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.	Limited	The effectiveness of this mitigation depends on the prevention capabilities of the specific sandbox or jail being used and might only help to reduce the scope of an attack, such as restricting the attacker to certain system calls or limiting the portion of the file system that can be accessed.

Relationships
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.

Related CWE	Type	View	Chain
CWE-120 ChildOf CWE-890	Category	CWE-888

Demonstrative Examples (Details)

Observed Examples

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

For more examples, refer to CVE relations in the bottom box.

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

Black Box Definitions
None

Taxynomy Mappings

Taxynomy	Id	Name	Fit
PLOVER		Unbounded Transfer ('classic overflow')
7 Pernicious Kingdoms		Buffer Overflow
CLASP		Buffer overflow
OWASP Top Ten 2004	A1	Unvalidated Input	CWE_More_Specific
OWASP Top Ten 2004	A5	Buffer Overflows	CWE_More_Specific
CERT C Secure Coding	STR35-C	Do not copy data from an unbounded source to a fixed-length array
WASC	7	Buffer Overflow
CERT C++ Secure Coding	STR35-CPP	Do not copy data from an unbounded source to a fixed-length array

References:

M. Howard D. LeBlanc .Writing Secure Code 2nd Edition. Microsoft. Section:'Chapter 5, "Public Enemy #1: The Buffer Overrun" Page 127'. Published on 2002.
Michael Howard David LeBlanc John Viega .24 Deadly Sins of Software Security. McGraw-Hill. Section:'"Sin 5: Buffer Overruns." Page 89'. Published on 2010.
Microsoft .Using the Strsafe.h Functions.
Matt Messier John Viega .Safe C String Library v1.0.3.
Michael Howard .Address Space Layout Randomization in Windows Vista.
Arjan van de Ven .Limiting buffer overflows with ExecShield.
.PaX.
Jason Lam .Top 25 Series - Rank 3 - Classic Buffer Overflow. SANS Software Security Institute. 2010-03-02.
Microsoft .Understanding DEP as a mitigation technology part 1.
Sean Barnum Michael Gegick .Least Privilege. Published on 2005-09-14.
Mark Dowd John McDonald Justin Schuh .The Art of Software Security Assessment 1st Edition. Addison Wesley. Section:'Chapter 3, "Nonexecutable Stack", Page 76.'. Published on 2006.
Mark Dowd John McDonald Justin Schuh .The Art of Software Security Assessment 1st Edition. Addison Wesley. Section:'Chapter 5, "Protection Mechanisms", Page 189.'. Published on 2006.
Mark Dowd John McDonald Justin Schuh .The Art of Software Security Assessment 1st Edition. Addison Wesley. Section:'Chapter 8, "C String Handling", Page 388.'. Published on 2006.


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