Common Weakness Enumeration

CWE-135

Weakness Allowed

Incorrect Calculation of Multi-Byte String Length

Abstraction: Base · Status: Draft

API response cwe.mitre.org

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

Mapping guidance

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.

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.

Relationships and members
ChildOf: CWE-682 · View CWE-1000
Modes of introduction
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.

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

Content history
Submission: CLASP · 2006-07-19 · CWE Draft 3
Contribution: Gregory Padgett · Unitrends · 2010-01-11 · CWE 1.8

correction to Demonstrative_Example

Modification: Eric Dalci · Cigital · 2008-07-01 · CWE 1.0

updated Potential_Mitigations, Time_of_Introduction

Modification: CWE Content Team · MITRE · 2008-09-08 · CWE 1.0

updated Applicable_Platforms, Relationships, Other_Notes, Taxonomy_Mappings

Modification: CWE Content Team · MITRE · 2008-11-24 · CWE 1.1

updated Relationships, Taxonomy_Mappings

Modification: CWE Content Team · MITRE · 2009-05-27 · CWE 1.4

updated Description

Modification: CWE Content Team · MITRE · 2010-02-16 · CWE 1.8

updated Demonstrative_Examples, References

Modification: CWE Content Team · MITRE · 2011-06-01 · CWE 1.13

updated Common_Consequences, Relationships, Taxonomy_Mappings

Modification: CWE Content Team · MITRE · 2011-06-27 · CWE 2.0

updated Common_Consequences

Modification: CWE Content Team · MITRE · 2012-05-11 · CWE 2.2

updated Common_Consequences, Demonstrative_Examples, Relationships, Taxonomy_Mappings

Modification: CWE Content Team · MITRE · 2012-10-30 · CWE 2.3

updated Potential_Mitigations

Modification: CWE Content Team · MITRE · 2014-06-23 · CWE 2.7

updated Enabling_Factors_for_Exploitation, Other_Notes

Modification: CWE Content Team · MITRE · 2014-07-30 · CWE 2.8

updated Relationships, Taxonomy_Mappings

Modification: CWE Content Team · MITRE · 2017-11-08 · CWE 3.0

updated Enabling_Factors_for_Exploitation, Modes_of_Introduction, References, Taxonomy_Mappings

Modification: CWE Content Team · MITRE · 2018-03-27 · CWE 3.1

updated References

Modification: CWE Content Team · MITRE · 2019-01-03 · CWE 3.2

updated Taxonomy_Mappings

Modification: CWE Content Team · MITRE · 2021-03-15 · CWE 4.4

updated References

Modification: CWE Content Team · MITRE · 2023-01-31 · CWE 4.10

updated Description

Modification: CWE Content Team · MITRE · 2023-04-27 · CWE 4.11

updated Detection_Factors, Relationships

Modification: CWE Content Team · MITRE · 2023-06-29 · CWE 4.12

updated Mapping_Notes

Modification: CWE Content Team · MITRE · 2024-07-16 · CWE 4.15

updated Common_Consequences

Modification: CWE Content Team · MITRE · 2025-12-11 · CWE 4.19

updated Weakness_Ordinalities

3 vulnerabilities reference this CWE, most recent first.

CVE-2026-34831 (GCVE-0-2026-34831)

Vulnerability from cvelistv5 – Published: 2026-04-02 16:43 – Updated: 2026-04-03 17:48
VLAI
Title
Rack: Content-Length mismatch in Rack::Files error responses
Summary
Rack is a modular Ruby web server interface. Prior to versions 2.2.23, 3.1.21, and 3.2.6, Rack::Files#fail sets the Content-Length response header using String#size instead of String#bytesize. When the response body contains multibyte UTF-8 characters, the declared Content-Length is smaller than the number of bytes actually sent on the wire. Because Rack::Files reflects the requested path in 404 responses, an attacker can trigger this mismatch by requesting a non-existent path containing percent-encoded UTF-8 characters. This results in incorrect HTTP response framing and may cause response desynchronization in deployments that rely on the incorrect Content-Length value. This issue has been patched in versions 2.2.23, 3.1.21, and 3.2.6.
Severity
4.8 (Medium)
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N
SSVC
Exploitation: none Automatable: no Technical Impact: partial
CISA Coordinator · CISA-ADP (v2.0.3)
Decision recorded 2026-04-03 17:43 UTC
CWE
  • CWE-130 - Improper Handling of Length Parameter Inconsistency
  • CWE-135 - Incorrect Calculation of Multi-Byte String Length
Assigner
GitHub_M CNA under the mitre root
CNA scorecard C 72/100 over 7802 records in the last 180 days details
References
Impacted products
Vendor Product Version
rack rack Affected: < 2.2.23
Affected: >= 3.0.0.beta1, < 3.1.21
Affected: >= 3.2.0, < 3.2.6
Create a notification for this product.
Show details on NVD website
To clipboard 

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CVE-2026-0810 (GCVE-0-2026-0810)

Vulnerability from cvelistv5 – Published: 2026-01-26 19:36 – Updated: 2026-11-05 16:10
VLAI
Title
Gix-date: gix-date: undefined behavior due to invalid string generation
Summary
A flaw was found in gix-date. The `gix_date::parse::TimeBuf::as_str` function can generate strings containing invalid non-UTF8 characters. This issue violates the internal safety invariants of the `TimeBuf` component, leading to undefined behavior when these malformed strings are subsequently processed. This could potentially result in application instability or other unforeseen consequences.
Severity
7.1 (High)
CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:H/A:H
SSVC
Exploitation: poc Automatable: no Technical Impact: total
CISA Coordinator · CISA-ADP (v2.0.3)
Decision recorded 2026-03-04 19:05 UTC
CWE
  • CWE-135 - Incorrect Calculation of Multi-Byte String Length
Assigner
redhat CNA under the redhat root
CNA scorecard B 82/100 over 1165 records in the last 180 days details
References
Impacted products
Date Public
2025-12-29 12:00
Show details on NVD website
To clipboard 

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GHSA-Q2WW-5357-X388

Vulnerability from github – Published: 2026-04-02 20:36 – Updated: 2026-05-13 16:19
VLAI
Summary
Rack has Content-Length mismatch in Rack::Files error responses
Details

Summary

Rack::Files#fail sets the Content-Length response header using String#size instead of String#bytesize. When the response body contains multibyte UTF-8 characters, the declared Content-Length is smaller than the number of bytes actually sent on the wire.

Because Rack::Files reflects the requested path in 404 responses, an attacker can trigger this mismatch by requesting a non-existent path containing percent-encoded UTF-8 characters.

This results in incorrect HTTP response framing and may cause response desynchronization in deployments that rely on the incorrect Content-Length value.

Details

Rack::Files#fail constructs error responses using logic equivalent to:

def fail(status, body, headers = {})
  body += "\n"
  [
    status,
    {
      "content-type" => "text/plain",
      "content-length" => body.size.to_s,
      "x-cascade" => "pass"
    }.merge!(headers),
    [body]
  ]
end

Here, body.size returns the number of characters, not the number of bytes. For multibyte UTF-8 strings, this produces an incorrect Content-Length value.

Rack::Files includes the decoded request path in 404 responses. A request containing percent-encoded UTF-8 path components therefore causes the response body to contain multibyte characters, while the Content-Length header still reflects character count rather than byte count.

As a result, the server can send more bytes than declared in the response headers.

This violates HTTP message framing requirements, which define Content-Length as the number of octets in the message body.

Impact

Applications using Rack::Files may emit incorrectly framed error responses when handling requests for non-existent paths containing multibyte characters.

In some deployment topologies, particularly with keep-alive connections and intermediaries that rely on Content-Length, this mismatch may lead to response parsing inconsistencies or response desynchronization. The practical exploitability depends on the behavior of downstream proxies, clients, and connection reuse.

Even where no secondary exploitation is possible, the response is malformed and may trigger protocol errors in strict components.

Mitigation

  • Update to a patched version of Rack that computes Content-Length using String#bytesize.
  • Avoid exposing Rack::Files directly to untrusted traffic until a fix is available, if operationally feasible.
  • Where possible, place Rack behind a proxy or server that normalizes or rejects malformed backend responses.
  • Prefer closing backend connections on error paths if response framing anomalies are a concern.
Severity
4.8 (Medium)
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N
Show details on source website
To clipboard 

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    "severity": "MODERATE"
  },
  "details": "## Summary\n\n`Rack::Files#fail` sets the `Content-Length` response header using `String#size` instead of `String#bytesize`. When the response body contains multibyte UTF-8 characters, the declared `Content-Length` is smaller than the number of bytes actually sent on the wire.\n\nBecause `Rack::Files` reflects the requested path in 404 responses, an attacker can trigger this mismatch by requesting a non-existent path containing percent-encoded UTF-8 characters.\n\nThis results in incorrect HTTP response framing and may cause response desynchronization in deployments that rely on the incorrect `Content-Length` value.\n\n## Details\n\n`Rack::Files#fail` constructs error responses using logic equivalent to:\n\n```ruby\ndef fail(status, body, headers = {})\n  body += \"\\n\"\n  [\n    status,\n    {\n      \"content-type\" =\u003e \"text/plain\",\n      \"content-length\" =\u003e body.size.to_s,\n      \"x-cascade\" =\u003e \"pass\"\n    }.merge!(headers),\n    [body]\n  ]\nend\n```\n\nHere, `body.size` returns the number of characters, not the number of bytes. For multibyte UTF-8 strings, this produces an incorrect `Content-Length` value.\n\n`Rack::Files` includes the decoded request path in 404 responses. A request containing percent-encoded UTF-8 path components therefore causes the response body to contain multibyte characters, while the `Content-Length` header still reflects character count rather than byte count.\n\nAs a result, the server can send more bytes than declared in the response headers.\n\nThis violates HTTP message framing requirements, which define `Content-Length` as the number of octets in the message body.\n\n## Impact\n\nApplications using `Rack::Files` may emit incorrectly framed error responses when handling requests for non-existent paths containing multibyte characters.\n\nIn some deployment topologies, particularly with keep-alive connections and intermediaries that rely on `Content-Length`, this mismatch may lead to response parsing inconsistencies or response desynchronization. The practical exploitability depends on the behavior of downstream proxies, clients, and connection reuse.\n\nEven where no secondary exploitation is possible, the response is malformed and may trigger protocol errors in strict components.\n\n## Mitigation\n\n* Update to a patched version of Rack that computes `Content-Length` using `String#bytesize`.\n* Avoid exposing `Rack::Files` directly to untrusted traffic until a fix is available, if operationally feasible.\n* Where possible, place Rack behind a proxy or server that normalizes or rejects malformed backend responses.\n* Prefer closing backend connections on error paths if response framing anomalies are a concern.",
  "id": "GHSA-q2ww-5357-x388",
  "modified": "2026-05-13T16:19:10Z",
  "published": "2026-04-02T20:36:10Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/rack/rack/security/advisories/GHSA-q2ww-5357-x388"
    },
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-34831"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/rack/rack"
    },
    {
      "type": "WEB",
      "url": "https://github.com/rubysec/ruby-advisory-db/blob/master/gems/rack/CVE-2026-34831.yml"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N",
      "type": "CVSS_V3"
    }
  ],
  "summary": "Rack has Content-Length mismatch in Rack::Files error responses"
}
Mitigation
Implementation

Strategy: Input Validation

Always verify the length of the string unit character.

Mitigation
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.)

No CAPEC attack patterns related to this CWE.

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