CVE-2026-34831Network attack vector

Response desynchronization via multibyte path length mismatch

Published Apr 2, 2026 · Updated Apr 2, 2026

HTTP response desynchronization in affected Rack versions allows remote attackers to corrupt response framing via crafted UTF-8 URL paths. Rack::Files#fail reflects a decoded missing path into a 404 body but sets Content-Length with String#size rather than String#bytesize, understating the byte count for multibyte characters. Exploitation requires Rack::Files exposure and a keep-alive topology whose intermediary or client trusts the short length, enabling parsing inconsistencies or response desynchronization.

CVSS severity4.8
Medium
EPSS probability0.15%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

HTTP response desynchronization in affected Rack versions allows remote attackers to corrupt response framing via crafted UTF-8 URL paths. Rack::Files#fail reflects a decoded missing path into a 404 body but sets Content-Length with String#size rather than String#bytesize, understating the byte count for multibyte characters. Exploitation requires Rack::Files exposure and a keep-alive topology whose intermediary or client trusts the short length, enabling parsing inconsistencies or response desynchronization.

The record

CVE
CVE-2026-34831
Published
Apr 2, 2026
Updated
Apr 2, 2026
Vendor
Rack
Product
Rack
Classifications
CWE-130, CWE-135, T1190
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Apr 2, 2026CVE publishedPublication date reported by the CVE source.
  2. Apr 2, 2026Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=< 2.2.23
  2. Affected versionversion=>= 3.0.0.beta1, < 3.1.21
  3. Affected versionversion=>= 3.2.0, < 3.2.6

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Rack · Rackversion=< 2.2.23; version=>= 3.0.0.beta1, < 3.1.21; version=>= 3.2.0, < 3.2.6

Published CVSS scores

6.5NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N
4.8GitHub, Inc.CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 9, 2026Sep 15, 2026
Latest reporting daySep 15, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-130Improper Handling of Length Parameter Inconsistency
CWE-135Incorrect Calculation of Multi-Byte String Length
T1190Exploit Public-Facing Application

Public exploit references

No public exploit references are available in this record.

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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