# CVE-2026-34831

## Summary

- **CVE ID:** CVE-2026-34831
- **Severity:** MEDIUM
- **CVSS Score:** 4.8 (CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N)
- **CWE:** CWE-130, CWE-135
- **Published:** Apr 2, 2026
- **Last Modified:** Apr 3, 2026

## Description

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.

## Affected Products

- rack — rack (< 2.2.23)
- rack — rack (>= 3.0.0.beta1, < 3.1.21)
- rack — rack (>= 3.2.0, < 3.2.6)

## References

- [CNA](https://github.com/rack/rack/security/advisories/GHSA-q2ww-5357-x388)

## Exploitation Prediction (EPSS)

- **EPSS Score:** 0.15%
- **EPSS Percentile:** 4.2

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_Exported from OnDuty AI Vulnerability Intelligence on 2026-09-11._