CVE-2026-34831
MEDIUMDescription
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.
How to fix
TridentStack Control can deploy fixes like this automatically across your Windows, macOS, and Linux fleet. See how it works
Remediation is compiled from vendor and distribution security advisories. Always confirm against the linked source for your exact version and platform.
CVSS v3 Vector
Exploitability
Impact
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N
Exploit Intelligence
Low risk: more likely to be exploited than 4% of all known CVEs.
References
Related Vulnerabilities
Other CWE-130 vulnerabilities, ordered by exploit likelihood. View all
| CVE | Severity | CVSS | EPSS | Exploited | Fix |
|---|---|---|---|---|---|
| CVE-2025-14847 | High | 7.5 | 83% | KEV | Fix |
| CVE-2024-39614 | High | 7.5 | 29% | - | Fix |
| CVE-2021-36090 | High | 7.5 | 13% | - | Fix |
| CVE-2021-35516 | High | 7.5 | 12% | - | Fix |
| CVE-2021-35517 | High | 7.5 | 11% | - | Fix |
| CVE-2019-3862 | High | 7.3 | 8.1% | - | Fix |
Common questions
How do I fix CVE-2026-34831?
Published advisories record a fix for 5 affected products. The "How to fix" section on this page lists the fixed version and source advisory for each one, so apply the entry matching what you actually run.
Is CVE-2026-34831 being actively exploited?
Not that we know of. CVE-2026-34831 is not in the CISA Known Exploited Vulnerabilities catalog. Its EPSS score of 0.15% is the estimated probability that it will be exploited in the next 30 days. That is higher than 4% of all scored CVEs.
How severe is CVE-2026-34831?
CVE-2026-34831 has a CVSS v3 base score of 4.8, rated medium. CVSS rates the technical impact if the vulnerability is exploited, not how likely that is, so weigh it alongside the exploit-prediction score when you decide what to patch first.
What does CVE-2026-34831 affect?
Published advisories record a fix for ruby-rack (Debian), librack-ruby (Ubuntu), librack-ruby1.8 (Ubuntu), librack-ruby1.9.1 (Ubuntu), and 1 more. Only products with a sourced advisory are listed, so treat this as what we can cite rather than a complete inventory.
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This product uses NVD data but is not endorsed or certified by the NVD. EPSS scores courtesy of FIRST.org (https://www.first.org/epss). Source: CISA KEV Catalog. Data as of 2026-07-24.