What’s Vulnerable › Vulnerabilities
CVE-2026-34831: Rack: Content-Length mismatch in Rack::Files error responses
- Severity: medium (CVSS 6.5 v3.1)
- Exploited in the wild: not listed in CISA KEV
- EPSS: 0.1% probability of exploitation in the next 30 days
- Public exploit or detection template: none known
- Published: 2026-04-02 · updated 2026-07-24
- Weakness: CWE-130
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: before 2.2.23; from 3.0.0 before 3.1.21; from 3.2.0 before 3.2.6 — fixed in 2.2.23, 3.1.21, 3.2.6
- Rubygems Rack: before 2.2.23; from 3.0.0.beta1 before 3.1.21; from 3.2.0 before 3.2.6; 0.1.0, 0.2.0, 0.3.0, 0.4.0, 0.9.0, 0.9.1 (+169) — fixed in 2.2.23, 3.1.21, 3.2.6
How to fix
Upgrade to a fixed version: rack 2.2.23, 3.1.21, 3.2.6; Rubygems Rack 2.2.23, 3.1.21, 3.2.6. Check the vendor advisory for mitigations.
References
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