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

Code Examples & CVEs

Demonstrative Examples

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

The output from the printf() statement would be:

BadC

#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));                            /* ... */                      }

Source-backed guidance

Additional facts reviewed against primary or authoritative security sources.

Verify controls for CWE-135 with SSDF evidence

Use NIST SSDF verification and vulnerability-response practices to detect CWE-135, Incorrect Calculation of Multi-Byte String Length, throughout the product lifecycle. Derive review questions, static or dynamic checks, and negative tests from the CWE's causal behavior; define the components and lifecycle stages each check covers; and retain findings with enough evidence to distinguish the root cause from symptoms and impacts. Track escapes and false negatives, then improve the verification plan after every confirmed occurrence.

NIST SP 800-218 Secure Software Development Framework — National Institute of Standards and Technology

Apply Input Validation controls for Incorrect Calculation of Multi-Byte String Length

MITRE associates mitigation with Implementation; the listed strategies include Input Validation, and Libraries or Frameworks; documented detection approaches include Automated Static Analysis. Use these source-defined anchors to turn CWE-135 into implementation, review, and verification checks for the affected component.

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

Triage CWE-135 against known exploitation evidence

Use CISA's Known Exploited Vulnerabilities catalog to test whether a vulnerability mapped to CWE-135, Incorrect Calculation of Multi-Byte String Length, has evidence of exploitation in the wild. Confirm the CVE-to-CWE root-cause mapping independently before attaching the example, then capture the affected product, required action, and remediation deadline. A missing KEV match is not evidence that the weakness is unexploited, and a KEV entry must not be generalized to every occurrence of this CWE.

Known Exploited Vulnerabilities Catalog — Cybersecurity and Infrastructure Security Agency

Apply precise root-cause mapping to CWE-135

Apply MITRE's full root-cause mapping guidance when using CWE-135, Incorrect Calculation of Multi-Byte String Length. Separate weakness language from attacker prerequisites and technical impact, check the entry's abstraction and vulnerability-mapping notes, and prefer the most specific Base or Variant supported by the evidence. Record the rejected alternatives and require an independent review before the mapping is used for remediation trends or program metrics.

CVE to CWE Root Cause Mapping Guidance — MITRE CWE

Validate CWE-135 with root-cause mapping checks

Apply MITRE's root-cause mapping quick tips to CWE-135, Incorrect Calculation of Multi-Byte String Length. Confirm the finding describes the causal weakness rather than an impact or attack pattern, compare the abstraction and mapping notes with plausible alternatives, and have a second reviewer challenge the selection. Preserve the evidence and reasoning so recurring defects can be measured against one consistent identifier.

CVE to CWE Root Cause Mapping Quick Tips — MITRE CWE

Frequently Asked Questions

What is CWE-135: Incorrect Calculation of Multi-Byte String Length?+

CWE-135: Incorrect Calculation of Multi-Byte String Length is a Common Weakness Enumeration (CWE) entry maintained by MITRE. The product does not correctly calculate the length of strings that can contain wide or multi-byte characters.

What are the security consequences of Incorrect Calculation of Multi-Byte String Length?+

If exploited, CWE-135 (Incorrect Calculation of Multi-Byte String Length) it can compromise Integrity, Confidentiality and Availability, leading to outcomes such as Execute Unauthorized Code or Commands, Read Memory, DoS: Crash, Exit, or Restart, DoS: Resource Consumption (CPU) and DoS: Resource Consumption (Memory).

How do you prevent or mitigate Incorrect Calculation of Multi-Byte String Length?+

Recommended mitigations for CWE-135 include: Always verify the length of the string unit character. Use length computing functions (e.g. strlen, wcslen, etc.) appropriately with their equivalent type (e.g.: byte, wchar_t, etc.)

Which programming languages are affected by Incorrect Calculation of Multi-Byte String Length?+

CWE-135 commonly affects C and C++. Note that weaknesses are often language-agnostic patterns, so secure coding practices apply broadly.

What is the difference between a CWE and a CVE?+

A CWE (Common Weakness Enumeration) like CWE-135 describes a category of software weakness — the underlying flaw type. A CVE (Common Vulnerabilities and Exposures) identifies a specific, real-world vulnerability in a particular product. In short, a CWE is the kind of mistake, and a CVE is an instance of that mistake being found in software.

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

Technical Details

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Mitigation Strategies

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Code Examples & CVEs

Demonstrative Examples

CWE Relationships

Parent Weaknesses

Source-backed guidance

Verify controls for CWE-135 with SSDF evidence

Apply Input Validation controls for Incorrect Calculation of Multi-Byte String Length

Triage CWE-135 against known exploitation evidence

Apply precise root-cause mapping to CWE-135

Validate CWE-135 with root-cause mapping checks

Frequently Asked Questions

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