CWE - CWE-135: Incorrect Calculation of Multi-Byte String Length (4.15)

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CWE Glossary Definition

CWE-135: Incorrect Calculation of Multi-Byte String Length

Weakness ID: 135

Vulnerability Mapping: ALLOWEDThis CWE ID may be used to map to real-world vulnerabilities
Abstraction: BaseBase - 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.

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Description

The product does not correctly calculate the length of strings that can contain wide or multi-byte characters.

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.

Scope	Impact	Likelihood
Integrity Confidentiality Availability	Technical Impact: Execute Unauthorized Code or Commands This weakness may lead to a buffer overflow. 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 Confidentiality	Technical Impact: Read Memory; DoS: Crash, Exit, or Restart; DoS: Resource Consumption (CPU); DoS: Resource Consumption (Memory) 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	Technical Impact: Read Memory 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 buffer's position in memory, this knowledge can be used to craft further attacks, possibly with more severe consequences.

Potential Mitigations

Phase: Implementation

Strategy: Input Validation

Always verify the length of the string unit character.

Phase: Implementation

Strategy: Libraries or Frameworks

Use length computing functions (e.g. strlen, wcslen, etc.) appropriately with their equivalent type (e.g.: byte, wchar_t, etc.)

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)

Nature	Type	ID	Name
ChildOf	PillarPillar - a weakness that is the most abstract type of weakness and represents a theme for all class/base/variant weaknesses related to it. A Pillar is different from a Category as a Pillar is still technically a type of weakness that describes a mistake, while a Category represents a common characteristic used to group related things.	682	Incorrect Calculation

Relevant to the view "Software Development" (CWE-699)

Nature	Type	ID	Name
MemberOf	CategoryCategory - a CWE entry that contains a set of other entries that share a common characteristic.	133	String Errors

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.

Phase	Note
Implementation	There are several ways in which improper string length checking may result in an exploitable condition. All of these, however, involve the introduction of buffer overflow conditions in order to reach an exploitable state. The first of these issues takes place when the output of a wide or multi-byte character string, string-length function is used as a size for the allocation of memory. While this will result in an output of the number of characters in the string, note that the characters are most likely not a single byte, as they are with standard character strings. So, using the size returned as the size sent to new or malloc and copying the string to this newly allocated memory will result in a buffer overflow. Another common way these strings are misused involves the mixing of standard string and wide or multi-byte string functions on a single string. Invariably, this mismatched information will result in the creation of a possibly exploitable buffer overflow condition.

Phase

Note

Implementation

There are several ways in which improper string length checking may result in an exploitable condition. All of these, however, involve the introduction of buffer overflow conditions in order to reach an exploitable state.

The first of these issues takes place when the output of a wide or multi-byte character string, string-length function is used as a size for the allocation of memory. While this will result in an output of the number of characters in the string, note that the characters are most likely not a single byte, as they are with standard character strings. So, using the size returned as the size sent to new or malloc and copying the string to this newly allocated memory will result in a buffer overflow.

Another common way these strings are misused involves the mixing of standard string and wide or multi-byte string functions on a single string. Invariably, this mismatched information will result in the creation of a possibly exploitable buffer overflow condition.

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.

Languages

C (Undetermined Prevalence)

C++ (Undetermined Prevalence)

Demonstrative Examples

Example 1

The following example would be exploitable if any of the commented incorrect malloc calls were used.

(bad code)

Example Language: C

#include <stdio.h>
#include <strings.h>
#include <wchar.h>

int main() {

wchar_t wideString[] = L"The spazzy orange tiger jumped " \
"over the tawny jaguar.";
wchar_t *newString;

printf("Strlen() output: %d\nWcslen() output: %d\n",
strlen(wideString), wcslen(wideString));

/* Wrong because the number of chars in a string isn't related to its length in bytes //
newString = (wchar_t *) malloc(strlen(wideString));
*/

/* Wrong because wide characters aren't 1 byte long! //
newString = (wchar_t *) malloc(wcslen(wideString));
*/

/* Wrong because wcslen does not include the terminating null */
newString = (wchar_t *) malloc(wcslen(wideString) * sizeof(wchar_t));

/* correct! */
newString = (wchar_t *) malloc((wcslen(wideString) + 1) * sizeof(wchar_t));

/* ... */

}

The output from the printf() statement would be:

(result)

Strlen() output: 0
Wcslen() output: 53

Detection Methods

Automated Static Analysis

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 searching for potentially-vulnerable patterns that connect "sources" (origins of input) with "sinks" (destinations where the data interacts with external components, a lower layer such as the OS, etc.)

Effectiveness: High

Memberships

Section HelpThis MemberOf Relationships table shows additional CWE Categories and Views that reference this weakness as a member. This information is often useful in understanding where a weakness fits within the context of external information sources.

Nature	Type	ID	Name
MemberOf	CategoryCategory - a CWE entry that contains a set of other entries that share a common characteristic.	741	CERT C Secure Coding Standard (2008) Chapter 8 - Characters and Strings (STR)
MemberOf	CategoryCategory - a CWE entry that contains a set of other entries that share a common characteristic.	857	The CERT Oracle Secure Coding Standard for Java (2011) Chapter 14 - Input Output (FIO)
MemberOf	ViewView - a subset of CWE entries that provides a way of examining CWE content. The two main view structures are Slices (flat lists) and Graphs (containing relationships between entries).	884	CWE Cross-section
MemberOf	CategoryCategory - a CWE entry that contains a set of other entries that share a common characteristic.	974	SFP Secondary Cluster: Incorrect Buffer Length Computation
MemberOf	CategoryCategory - a CWE entry that contains a set of other entries that share a common characteristic.	1408	Comprehensive Categorization: Incorrect Calculation

Vulnerability Mapping Notes

Usage: ALLOWED

(this CWE ID could be used to map to real-world vulnerabilities)

Reason: Acceptable-Use

Rationale:

This CWE entry is at the Base level of abstraction, which is a preferred level of abstraction for mapping to the root causes of vulnerabilities.

Comments:

Carefully read both the name and description to ensure that this mapping is an appropriate fit. Do not try to 'force' a mapping to a lower-level Base/Variant simply to comply with this preferred level of abstraction.

Taxonomy Mappings

Mapped Taxonomy Name	Node ID	Fit
CLASP	Improper string length checking
The CERT Oracle Secure Coding Standard for Java (2011)	FIO10-J	Ensure the array is filled when using read() to fill an array
Software Fault Patterns	SFP10	Incorrect Buffer Length Computation

References

[REF-7] Michael Howard and David LeBlanc. "Writing Secure Code". Chapter 5, "Unicode and ANSI Buffer Size Mismatches" Page 153. 2nd Edition. Microsoft Press. 2002年12月04日. <https://www.microsoftpressstore.com/store/writing-secure-code-9780735617223>.

[REF-18] Secure Software, Inc.. "The CLASP Application Security Process". 2005. <https://cwe.mitre.org/documents/sources/TheCLASPApplicationSecurityProcess.pdf>.

Content History

Submissions
Submission Date	Submitter	Organization
2006年07月19日 (CWE Draft 3, 2006年07月19日)	CLASP
2006年07月19日 (CWE Draft 3, 2006年07月19日)
Contributions
Contribution Date	Contributor	Organization
2010年01月11日	Gregory Padgett	Unitrends
2010年01月11日	correction to Demonstrative_Example
Modifications
Modification Date	Modifier	Organization
2008年07月01日	Eric Dalci	Cigital
2008年07月01日	updated Potential_Mitigations, Time_of_Introduction
2008年09月08日	CWE Content Team	MITRE
2008年09月08日	updated Applicable_Platforms, Relationships, Other_Notes, Taxonomy_Mappings
2008年11月24日	CWE Content Team	MITRE
2008年11月24日	updated Relationships, Taxonomy_Mappings
2009年05月27日	CWE Content Team	MITRE
2009年05月27日	updated Description
2010年02月16日	CWE Content Team	MITRE
2010年02月16日	updated Demonstrative_Examples, References
2011年06月01日	CWE Content Team	MITRE
2011年06月01日	updated Common_Consequences, Relationships, Taxonomy_Mappings
2011年06月27日	CWE Content Team	MITRE
2011年06月27日	updated Common_Consequences
2012年05月11日	CWE Content Team	MITRE
2012年05月11日	updated Common_Consequences, Demonstrative_Examples, Relationships, Taxonomy_Mappings
2012年10月30日	CWE Content Team	MITRE
2012年10月30日	updated Potential_Mitigations
2014年06月23日	CWE Content Team	MITRE
2014年06月23日	updated Enabling_Factors_for_Exploitation, Other_Notes
2014年07月30日	CWE Content Team	MITRE
2014年07月30日	updated Relationships, Taxonomy_Mappings
2017年11月08日	CWE Content Team	MITRE
2017年11月08日	updated Enabling_Factors_for_Exploitation, Modes_of_Introduction, References, Taxonomy_Mappings
2018年03月27日	CWE Content Team	MITRE
2018年03月27日	updated References
2019年01月03日	CWE Content Team	MITRE
2019年01月03日	updated Taxonomy_Mappings
2021年03月15日	CWE Content Team	MITRE
2021年03月15日	updated References
2023年01月31日	CWE Content Team	MITRE
2023年01月31日	updated Description
2023年04月27日	CWE Content Team	MITRE
2023年04月27日	updated Detection_Factors, Relationships
2023年06月29日	CWE Content Team	MITRE
2023年06月29日	updated Mapping_Notes
2024年07月16日 (CWE 4.15, 2024年07月16日)	CWE Content Team	MITRE
2024年07月16日 (CWE 4.15, 2024年07月16日)	updated Common_Consequences
Previous Entry Names
Change Date	Previous Entry Name
2008年04月11日	Improper String Length Checking

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Page Last Updated: July 16, 2024

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