Security Database IT Watching

Failure to Constrain Operations within the Bounds of a Memory Buffer

Weakness ID: 119 (Weakness Class)

Status: Usable

Description

Description Summary

The software performs operations on a memory buffer, but it can read from or write to a memory location that is outside of the intended boundary of the buffer.

Extended Description

Certain languages allow direct addressing of memory locations and do not automatically ensure that these locations are valid for the memory buffer that is being referenced. This can cause read or write operations to be performed on memory locations that may be associated with other variables, data structures, or internal program data.

As a result, an attacker may be able to execute arbitrary code, alter the intended control flow, read sensitive information, or cause the system to crash.

Alternate Terms

Memory Corruption:	The generic term "memory corruption" is often used to describe the consequences of writing to memory outside the bounds of a buffer, when the root cause is something other than a sequential copies of excessive data from a fixed starting location (i.e., classic buffer overflows or CWE-120). This may include issues such as incorrect pointer arithmetic, accessing invalid pointers due to incomplete initialization or memory release, etc.

Memory Corruption:

The generic term "memory corruption" is often used to describe the consequences of writing to memory outside the bounds of a buffer, when the root cause is something other than a sequential copies of excessive data from a fixed starting location (i.e., classic buffer overflows or CWE-120). This may include issues such as incorrect pointer arithmetic, accessing invalid pointers due to incomplete initialization or memory release, etc.

Time of Introduction

Architecture and Design
Implementation
Operation

Applicable Platforms

Languages

C: (Often)

C++: (Often)

Assembly

Languages without memory management support

Platform Notes

It is possible in many programming languages to attempt an operation outside of the bounds of a memory buffer, but the consequences will vary widely depending on the language, platform, and chip architecture.

Common Consequences

Scope	Effect
Integrity	Technical Impact: Execute unauthorized code or commands; Memory corruption If the memory accessible by the attacker can be effectively controlled, it may be possible to execute arbitrary code, as with a standard buffer overflow. If the attacker can overwrite a pointer's worth of memory (usually 32 or 64 bits), he can redirect a function pointer to his own malicious code. Even when the attacker can only modify a single byte arbitrary code execution can be possible. Sometimes this is because the same problem can be exploited repeatedly to the same effect. Other times it is because the attacker can overwrite security-critical application-specific data -- such as a flag indicating whether the user is an administrator.
Availability	Out of bounds memory access will very likely result in the corruption of relevant memory, and perhaps instructions, possibly leading to a crash. Other attacks leading to lack of availability are possible, including putting the program into an infinite loop.
Confidentiality	In the case of an out-of-bounds read, the attacker may have access to sensitive information. If the sensitive information contains system details, such as the current buffers position in memory, this knowledge can be used to craft further attacks, possibly with more severe consequences.

Likelihood of Exploit

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.

Demonstrative Examples

Example 1

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 Code)

Example Language: C

void host_lookup(char *user_supplied_addr){

struct hostent *hp;

in_addr_t *addr;

char hostname[64];

in_addr_t inet_addr(const char *cp);

/*routine that ensures user_supplied_addr is in the right format for conversion */

validate_addr_form(user_supplied_addr);

addr = inet_addr(user_supplied_addr);

hp = gethostbyaddr( addr, sizeof(struct in_addr), AF_INET);

strcpy(&hostname, hp->h_name);

}

This function allocates a buffer of 64 bytes to store the hostname, 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).

Example 2

This example applies an encoding procedure to an input string and stores it into a buffer.

(Bad Code)

Example Language: C

char * copy_input(char *user_supplied_string){

int i, dst_index;

char *dst_buf = (char*)malloc(4*sizeof(char) * MAX_SIZE);

if ( MAX_SIZE <= strlen(user_supplied_string) ){

die("user string too long, die evil hacker!");

}

dst_index = 0;

for ( i = 0; i < strlen; i++ ){

if( '&' == user_supplied_string[i] ){

dst_buf[dst_index++] = '&';

dst_buf[dst_index++] = 'a';

dst_buf[dst_index++] = 'm';

dst_buf[dst_index++] = 'p';

dst_buf[dst_index++] = ';';

}

else if ('<' == user_supplied_string[i] ){

/* encode to < */

}

else dst_buf[dst_index++] = user_supplied_string[i];

}

return dst_buf;

}

The programmer attempts to encode the ampersand character in the user-controlled string, however the length of the string is validated before the encoding procedure is applied. Furthermore, the programmer assumes encoding expansion will only expand a given character by a factor of 4, while the encoding of the ampersand expands by 5. As a result, when the encoding procedure expands the string it is possible to overflow the destination buffer if the attacker provides a string of many ampersands.

Example 3

The following example asks a user for an offset into an array to select an item.

(Bad Code)

Example Language: C

int main (int argc, char **argv) {

char *items[] = {"boat", "car", "truck", "train"};

int index = GetUntrustedOffset();

printf("You selected %s\n", items[index-1]);

}

The programmer allows the user to specify which element in the list to select, however an attacker can provide an out-of-bounds offset, resulting in a buffer over-read (CWE-126).

Observed Examples

Reference	Description
CVE-2009-2550	Classic stack-based buffer overflow in media player using a long entry in a playlist
CVE-2009-2403	Heap-based buffer overflow in media player using a long entry in a playlist
CVE-2009-0689	large precision value in a format string triggers overflow
CVE-2009-0690	negative offset value leads to out-of-bounds read
CVE-2009-1532	malformed inputs cause accesses of uninitialized or previously-deleted objects, leading to memory corruption
CVE-2009-1528	chain: lack of synchronization leads to memory corruption
CVE-2009-0558	attacker-controlled array index leads to code execution
CVE-2009-0269	chain: -1 value from a function call was intended to indicate an error, but is used as an array index instead.
CVE-2009-0566	chain: incorrect calculations lead to incorrect pointer dereference and memory corruption
CVE-2009-1350	product accepts crafted messages that lead to a dereference of an arbitrary pointer
CVE-2009-0191	chain: malformed input causes dereference of uninitialized memory
CVE-2008-4113	OS kernel trusts userland-supplied length value, allowing reading of sensitive information

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.

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).

Relationships

Nature	Type	ID	Name	View(s) this relationship pertains to	Named Chain(s) this relationship pertains to
ChildOf	Weakness Class	20	Improper Input Validation	Development Concepts699 Seven Pernicious Kingdoms (primary)700
ChildOf	Weakness Class	118	Improper Access of Indexable Resource ('Range Error')	Development Concepts (primary)699 Research Concepts (primary)1000
ChildOf	Category	633	Weaknesses that Affect Memory	Resource-specific Weaknesses (primary)631
ChildOf	Category	726	OWASP Top Ten 2004 Category A5 - Buffer Overflows	Weaknesses in OWASP Top Ten (2004) (primary)711
ChildOf	Category	740	CERT C Secure Coding Section 06 - Arrays (ARR)	Weaknesses Addressed by the CERT C Secure Coding Standard (primary)734
ChildOf	Category	741	CERT C Secure Coding Section 07 - Characters and Strings (STR)	Weaknesses Addressed by the CERT C Secure Coding Standard734
ChildOf	Category	742	CERT C Secure Coding Section 08 - Memory Management (MEM)	Weaknesses Addressed by the CERT C Secure Coding Standard734
ChildOf	Category	743	CERT C Secure Coding Section 09 - Input Output (FIO)	Weaknesses Addressed by the CERT C Secure Coding Standard734
ChildOf	Category	744	CERT C Secure Coding Section 10 - Environment (ENV)	Weaknesses Addressed by the CERT C Secure Coding Standard734
ChildOf	Category	752	2009 Top 25 - Risky Resource Management	Weaknesses in the 2009 CWE/SANS Top 25 Most Dangerous Programming Errors (primary)750
ParentOf	Weakness Base	120	Buffer Copy without Checking Size of Input ('Classic Buffer Overflow')	Development Concepts (primary)699 Research Concepts (primary)1000
ParentOf	Weakness Base	123	Write-what-where Condition	Development Concepts (primary)699 Research Concepts (primary)1000
ParentOf	Weakness Base	125	Out-of-bounds Read	Development Concepts (primary)699 Research Concepts (primary)1000
ParentOf	Weakness Base	130	Improper Handling of Length Parameter Inconsistency	Development Concepts (primary)699
ParentOf	Weakness Base	466	Return of Pointer Value Outside of Expected Range	Research Concepts (primary)1000
ParentOf	Weakness Base	786	Access of Memory Location Before Start of Buffer	Development Concepts (primary)699 Research Concepts (primary)1000
ParentOf	Weakness Base	787	Out-of-bounds Write	Development Concepts (primary)699 Research Concepts (primary)1000
ParentOf	Weakness Base	788	Access of Memory Location After End of Buffer	Development Concepts (primary)699 Research Concepts (primary)1000
ParentOf	Weakness Base	805	Buffer Access with Incorrect Length Value	Development Concepts (primary)699 Research Concepts (primary)1000
MemberOf	View	635	Weaknesses Used by NVD	Weaknesses Used by NVD (primary)635
CanFollow	Weakness Base	128	Wrap-around Error	Research Concepts1000
CanFollow	Weakness Base	129	Improper Validation of Array Index	Research Concepts1000
CanFollow	Weakness Base	131	Incorrect Calculation of Buffer Size	Development Concepts699 Research Concepts1000
CanFollow	Weakness Base	190	Integer Overflow or Wraparound	Research Concepts1000	Integer Overflow to Buffer Overflow680
CanFollow	Weakness Base	193	Off-by-one Error	Research Concepts1000
CanFollow	Weakness Variant	195	Signed to Unsigned Conversion Error	Research Concepts1000

Affected Resources

Memory

Taxonomy Mappings

Mapped Taxonomy Name	Node ID	Fit	Mapped Node Name
OWASP Top Ten 2004	A5	Exact	Buffer Overflows
CERT C Secure Coding	ARR00-C		Understand how arrays work
CERT C Secure Coding	ARR33-C		Guarantee that copies are made into storage of sufficient size
CERT C Secure Coding	ARR34-C		Ensure that array types in expressions are compatible
CERT C Secure Coding	ARR35-C		Do not allow loops to iterate beyond the end of an array
CERT C Secure Coding	ENV01-C		Do not make assumptions about the size of an environment variable
CERT C Secure Coding	FIO37-C		Do not assume character data has been read
CERT C Secure Coding	MEM09-C		Do not assume memory allocation routines initialize memory
CERT C Secure Coding	STR31-C		Guarantee that storage for strings has sufficient space for character data and the null terminator
CERT C Secure Coding	STR32-C		Null-terminate byte strings as required
CERT C Secure Coding	STR33-C		Size wide character strings correctly
WASC	7		Buffer Overflow

Related Attack Patterns

CAPEC-ID	Attack Pattern Name	(CAPEC Version: 1.4)
8	Buffer Overflow in an API Call
9	Buffer Overflow in Local Command-Line Utilities
10	Buffer Overflow via Environment Variables
14	Client-side Injection-induced Buffer Overflow
24	Filter Failure through Buffer Overflow
42	MIME Conversion
44	Overflow Binary Resource File
45	Buffer Overflow via Symbolic Links
100	Overflow Buffers
46	Overflow Variables and Tags
47	Buffer Overflow via Parameter Expansion

References

[REF-11] M. Howard and D. LeBlanc. "Writing Secure Code". Chapter 5, "Public Enemy #1: The Buffer Overrun" Page 127; Chapter 14, "Prevent I18N Buffer Overruns" Page 441. 2nd Edition. Microsoft. 2002.

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

Submissions
Submission Date	Submitter	Organization	Source
	PLOVER		Externally Mined

Modifications
Modification Date	Modifier	Organization	Source
2008-07-01	Eric Dalci	Cigital	External
2008-07-01	updated Time of Introduction
2008-08-15		Veracode	External
2008-08-15	Suggested OWASP Top Ten 2004 mapping
2008-09-08	CWE Content Team	MITRE	Internal
2008-09-08	updated Description, Relationships, Taxonomy Mappings
2008-10-14	CWE Content Team	MITRE	Internal
2008-10-14	updated Relationships
2008-11-24	CWE Content Team	MITRE	Internal
2008-11-24	updated Relationships, Taxonomy Mappings
2009-01-12	CWE Content Team	MITRE	Internal
2009-01-12	updated Applicable Platforms, Common Consequences, Demonstrative Examples, Likelihood of Exploit, Name, Potential Mitigations, References, Relationships
2009-03-10	CWE Content Team	MITRE	Internal
2009-03-10	updated Potential Mitigations
2009-05-27	CWE Content Team	MITRE	Internal
2009-05-27	updated Demonstrative Examples
2009-07-27	CWE Content Team	MITRE	Internal
2009-07-27	updated Observed Examples
2009-10-29	CWE Content Team	MITRE	Internal
2009-10-29	updated Applicable Platforms, Common Consequences, Demonstrative Examples, Description, Relationships, Time of Introduction
2009-12-28	CWE Content Team	MITRE	Internal
2009-12-28	updated Common Consequences, Demonstrative Examples, Detection Factors, Observed Examples
Previous Entry Names
Change Date	Previous Entry Name
2008-04-11	Buffer Errors

2009-01-12	Failure to Constrain Operations within the Bounds of an Allocated Memory Buffer