Common Weakness Enumeration

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

Weakness ID: 135
Abstraction: Base
Structure: Simple
Status: Draft
Presentation Filter:
+ Description
The software does not correctly calculate the length of strings that can contain wide or multi-byte characters.
+ Relationships

The table(s) below 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)
ChildOfPillarPillar - 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.682Incorrect Calculation
+ Relevant to the view "Software Development" (CWE-699)
MemberOfCategoryCategory - a CWE entry that contains a set of other entries that share a common characteristic.133String Errors
+ Modes Of Introduction

The 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.


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
The listings below show 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 (Undetermined Prevalence)

C++ (Undetermined Prevalence)

+ Common Consequences

The table below 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: 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.

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.

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 buffers position in memory, this knowledge can be used to craft further attacks, possibly with more severe consequences.
+ Demonstrative Examples

Example 1

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

(bad code)
Example Language:
#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:

Strlen() output: 0
Wcslen() output: 53
+ 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.)
+ Memberships
This 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.
MemberOfCategoryCategory - a CWE entry that contains a set of other entries that share a common characteristic.741CERT C Secure Coding Standard (2008) Chapter 8 - Characters and Strings (STR)
MemberOfCategoryCategory - a CWE entry that contains a set of other entries that share a common characteristic.857The CERT Oracle Secure Coding Standard for Java (2011) Chapter 14 - Input Output (FIO)
MemberOfViewView - 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).884CWE Cross-section
MemberOfCategoryCategory - a CWE entry that contains a set of other entries that share a common characteristic.974SFP Secondary Cluster: Incorrect Buffer Length Computation
+ Taxonomy Mappings
Mapped Taxonomy NameNode IDFitMapped Node Name
CLASPImproper string length checking
The CERT Oracle Secure Coding Standard for Java (2011)FIO10-JEnsure the array is filled when using read() to fill an array
Software Fault PatternsSFP10Incorrect 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. 2002-12-04. <>.
[REF-18] Secure Software, Inc.. "The CLASP Application Security Process". 2005. <>.
+ Content History
+ Submissions
Submission DateSubmitterOrganization
+ Contributions
Contribution DateContributorOrganization
2010-01-11Gregory PadgettUnitrends
correction to Demonstrative_Example
+ Modifications
Modification DateModifierOrganization
2008-07-01Eric DalciCigital
updated Potential_Mitigations, Time_of_Introduction
2008-09-08CWE Content TeamMITRE
updated Applicable_Platforms, Relationships, Other_Notes, Taxonomy_Mappings
2008-11-24CWE Content TeamMITRE
updated Relationships, Taxonomy_Mappings
2009-05-27CWE Content TeamMITRE
updated Description
2010-02-16CWE Content TeamMITRE
updated Demonstrative_Examples, References
2011-06-01CWE Content TeamMITRE
updated Common_Consequences, Relationships, Taxonomy_Mappings
2011-06-27CWE Content TeamMITRE
updated Common_Consequences
2012-05-11CWE Content TeamMITRE
updated Common_Consequences, Demonstrative_Examples, Relationships, Taxonomy_Mappings
2012-10-30CWE Content TeamMITRE
updated Potential_Mitigations
2014-06-23CWE Content TeamMITRE
updated Enabling_Factors_for_Exploitation, Other_Notes
2014-07-30CWE Content TeamMITRE
updated Relationships, Taxonomy_Mappings
2017-11-08CWE Content TeamMITRE
updated Enabling_Factors_for_Exploitation, Modes_of_Introduction, References, Taxonomy_Mappings
2018-03-27CWE Content TeamMITRE
updated References
2019-01-03CWE Content TeamMITRE
updated Taxonomy_Mappings
2021-03-15CWE Content TeamMITRE
updated References
+ Previous Entry Names
Change DatePrevious Entry Name
2008-04-11Improper String Length Checking
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Page Last Updated: March 15, 2021