# CVE-2026-34835: ruby-rack: Rack is a modular Ruby web server interface

CVE-2026-34835 is fixed in ruby-rack on 1 platform, first by ruby-rack 3.2.4-1ubuntu1.1, released April 2, 2026. It is not reported as exploited. Patch Radar priority: Routine.

- Web page: https://patchblox.com/security/cve/CVE-2026-34835
- Priority: Routine
- CVSS: 6.5 (medium)
- EPSS: 0.0020
- CISA KEV: not listed
- Exploited (vendor report): no
- Publicly disclosed: no
- Published: Apr 2, 2026

## Description

Rack is a modular Ruby web server interface. From versions 3.0.0.beta1 to before 3.1.21, and 3.2.0 to before 3.2.6, Rack::Request parses the Host header using an AUTHORITY regular expression that accepts characters not permitted in RFC-compliant hostnames, including /, ?, #, and @. Because req.host returns the full parsed value, applications that validate hosts using naive prefix or suffix checks can be bypassed. This can lead to host header poisoning in applications that use req.host, req.url, or req.base_url for link generation, redirects, or origin validation. This issue has been patched in versions 3.1.21 and 3.2.6.

## Where it's fixed

| Platform | Product | Fixed in | Fix | Released |
|---|---|---|---|---|
| Linux | ruby-rack Debian 13.5 (trixie) | 3.1.20-0+deb13u2 | [DEBIAN-CVE-2026-34835](https://patchblox.com/security/data/fix/linux/debian-cve-2026-34835.md) | Apr 2, 2026 |
| Linux | ruby-rack Debian 14 | 3.2.6-2 | [DEBIAN-CVE-2026-34835](https://patchblox.com/security/data/fix/linux/debian-cve-2026-34835.md) | Apr 2, 2026 |
| Linux | ruby-rack Ubuntu 25.10 (questing) | 3.1.16-0.1ubuntu0.3 | [ruby-rack 3.1.16-0.1ubuntu0.3](https://patchblox.com/security/data/fix/linux/ubuntu-questing-ruby-rack-3.1.16-0.1ubuntu0.3.md) | Apr 30, 2026 |
| Linux | ruby-rack Ubuntu 26.04.1 LTS (resolute) | 3.2.4-1ubuntu1.1 | [ruby-rack 3.2.4-1ubuntu1.1](https://patchblox.com/security/data/fix/linux/ubuntu-resolute-ruby-rack-3.2.4-1ubuntu1.1.md) | Apr 2, 2026 |

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Data as of 2026-11-08T00:53:09Z UTC. Source: Patchblox Patch Radar (https://patchblox.com/security/patch-radar). Patchblox finds out-of-date software on Windows, macOS and Linux and updates it through release rings: https://patchblox.com/demo

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